Experiments probing the macroscopic limits of QM

[Auto-Didact]

This thread is to serve as both a compilation and ground of discussion of key experiments, both historical and planned, which attempt to probe possible macroscopic limits of QM, taking into account e.g. some particular gravitational/optical/mechanical/superconducting/etc aspect and/or phenomenon.

I will start by posting a few known and perhaps some not so well known ones:

Colella et al. 1975, Observation of Gravitationally Induced Quantum Interference

We have used a neutron interferometer to observe the quantum-mechanical phase shift of neutrons caused by their interaction with Earth's gravitational field.

Marshall et al. 2002, Towards quantum superpositions of a mirror

We propose a scheme for creating quantum superposition states involving of order 10^14 atoms via the interaction of a single photon with a tiny mirror. This mirror, mounted on a high-quality mechanical oscillator, is part of a high-finesse optical cavity which forms one arm of a Michelson interferometer. By observing the interference of the photon only, one can study the creation and decoherence of superpositions involving the mirror. All experimental requirements appear to be within reach of current technology.

http://www.nature.com/nphys/journal/v8/n5/full/nphys2262.html

One of the main challenges in physics today is to merge quantum theory and the theory of general relativity into a unified framework. Researchers are developing various approaches towards such a theory of quantum gravity, but a major hindrance is the lack of experimental evidence of quantum gravitational effects. Yet, the quantization of spacetime itself can have experimental implications: the existence of a minimal length scale is widely expected to result in a modification of the Heisenberg uncertainty relation. Here we introduce a scheme to experimentally test this conjecture by probing directly the canonical commutation relation of the centre-of-mass mode of a mechanical oscillator with a mass close to the Planck mass. Our protocol uses quantum optical control and readout of the mechanical system to probe possible deviations from the quantum commutation relation even at the Planck scale. We show that the scheme is within reach of current technology. It thus opens a feasible route for table-top experiments to explore possible quantum gravitational phenomena.

Vanner et al. 2013, Cooling-by-measurement and mechanical state tomography via pulsed optomechanics

Observing a physical quantity without disturbing it is a key capability for the control of individual quantum systems. Such back-action-evading or quantum non-demolition measurements were first introduced in the 1970s for gravitational wave detection, and now such techniques are an indispensable tool throughout quantum science. Here we perform measurements of the position of a mechanical oscillator using pulses of light with a duration much shorter than a period of mechanical motion. Utilizing this back-action-evading interaction, we demonstrate state preparation and full state tomography of the mechanical motional state. We have reconstructed states with a position uncertainty reduced to 19 pm, limited by the quantum fluctuations of the optical pulse, and we have performed ‘cooling-by-measurement’ to reduce the mechanical mode temperature from an initial 1,100 to 16 K. Future improvements to this technique will allow for quantum squeezing of mechanical motion, even from room temperature, and reconstruction of non-classical states exhibiting negative phase-space quasi-probability.

Kiesel et al. 2013, Cavity cooling of an optically levitated submicron particle

The coupling of a levitated submicron particle and an optical cavity field promises access to a unique parameter regime both for macroscopic quantum experiments and for high-precision force sensing. We report a demonstration of such controlled interactions by cavity cooling the center-of-mass motion of an optically trapped submicron particle. This paves the way for a light–matter interface that can enable room-temperature quantum experiments with mesoscopic mechanical systems.

Kaltenbaek et al. 2015, Macroscopic quantum resonators (MAQRO): 2015 Update

Do the laws of quantum physics still hold for macroscopic objects - this is at the heart of Schrodinger's cat paradox - or do gravitation or yet unknown effects set a limit for massive particles? What is the fundamental relation between quantum physics and gravity? Ground-based experiments addressing these questions may soon face limitations due to limited free-fall times and the quality of vacuum and microgravity. The proposed mission MAQRO may overcome these limitations and allow addressing those fundamental questions. MAQRO harnesses recent developments in quantum optomechanics, high-mass matter-wave interferometry as well as state-of-the-art space technology to push macroscopic quantum experiments towards their ultimate performance limits and to open new horizons for applying quantum technology in space. The main scientific goal of MAQRO is to probe the vastly unexplored "quantum-classical" transition for increasingly massive objects, testing the predictions of quantum theory for truly macroscopic objects in a size and mass regime unachievable in ground-based experiments. The hardware for the mission will largely be based on available space technology. Here, we present the MAQRO proposal submitted in response to the (M4) Cosmic Vision call of the European Space Agency for a medium-size mission opportunity with a possible launch in 2025.

 

[Feeble Wonk]

The Marshall et al experiment to demonstrate macroscopic quantum superposition of a mirror was apparently proposed in 2002. Any idea whether it was ever actually attempted?

 

[Auto-Didact]

The Marshall et al experiment to demonstrate macroscopic quantum superposition of a mirror was apparently proposed in 2002. Any idea whether it was ever actually attempted?

There has indeed been a significant amount of work towards realizing the Marshall et al. experiment since then. There are many groups around the world actively pursuing this experiment and variations thereof.

Perhaps the best known one is by one of the authors of Marshall et al., the UCSB/Leiden experimentalist Dirk Bouwmeester. Incidentally, he was also involved (first author) in the original quantum teleportation experiments in Anton Zeilinger's group back in '97. Moreover, Bouwmeester recently, in 2014, got the Spinoza Prize, effectively a 2.5 million euro grant from the NWO, to help fund this particular Marshall et al. experiment.

Here is an hour long lecture of his on the state of the experiment in 2013:


Here are some of the more recent (2008 to 2016) key publications by members of Bouwmeester's experimental group on arxiv w.r.t. this experiment:

Kleckner et al. 2008, Creating and Verifying a Quantum Superposition in a Micro-optomechanical System

Micro-optomechanical systems are central to a number of recent proposals for realizing quantum mechanical effects in relatively massive systems. Here we focus on a particular class of experiments which aim to demonstrate massive quantum superpositions, although the obtained results should be generalizable to similar experiments. We analyze in detail the effects of finite temperature on the interpretation of the experiment, and obtain a lower bound on the degree of non-classicality of the cantilever. Although it is possible to measure the quantum decoherence time when starting from finite temperature, an unambiguous demonstration of a quantum superposition requires the mechanical resonator to be in or near the ground state. This can be achieved by optical cooling of the fundamental mode, which also provides a method to measure the mean phonon number in that mode. We also calculate the rate of environmentally induced decoherence and estimate the timescale for gravitational collapse mechanisms as proposed by Penrose and Diosi. In view of recent experimental advances, practical considerations for the realization of the described experiment are discussed.

Pepper et al. 2011, Optomechanical superpositions via nested interferometry

We present a scheme for achieving macroscopic quantum superpositions in optomechanical systems by using single photon postselection and detecting them with nested interferometers. This method relieves many of the challenges associated with previous optical schemes for measuring macroscopic superpositions, and only requires the devices to be in the weak coupling regime. It requires only small improvements on currently achievable device parameters, and allows observation of decoherence on a timescale unconstrained by the system's optical decay time. Prospects for observing novel decoherence mechanisms are discussed.

Pepper et al. 2012, Macroscopic superpositions via nested interferometry: finite temperature and decoherence considerations

Recently there has been much interest in optomechanical devices for the production of macroscopic quantum states. Here we focus on a proposed scheme for achieving macroscopic superpositions via nested interferometry. We consider the effects of finite temperature on the superposition produced. We also investigate in detail the scheme's feasibility for probing various novel decoherence mechanisms.

Ghobadi et al. 2014, Opto-mechanical micro-macro entanglement

We propose to create and detect opto-mechanical entanglement by storing one component of an entangled state of light in a mechanical resonator and then retrieving it. Using micro-macro entanglement of light as recently demonstrated experimentally, one can then create opto-mechanical entangled states where the components of the superposition are macroscopically different. We apply this general approach to two-mode squeezed states where one mode has undergone a large displacement. Based on an analysis of the relevant experimental imperfections, the scheme appears feasible with current technology.

Weaver et al. 2015, Nested Trampoline Resonators for Optomechanics

Two major challenges in the development of optomechanical devices are achieving a low mechanical and optical loss rate and vibration isolation from the environment. We address both issues by fabricating trampoline resonators made from low pressure chemical vapor deposition (LPCVD) Si3N4 with a distributed bragg reflector (DBR) mirror. We design a nested double resonator structure with 80 dB of mechanical isolation from the mounting surface at the inner resonator frequency, and we demonstrate up to 45 dB of isolation at lower frequencies in agreement with the design. We reliably fabricate devices with mechanical quality factors of around 400,000 at room temperature. In addition these devices were used to form optical cavities with finesse up to 181,000 ±1,000. These promising parameters will enable experiments in the quantum regime with macroscopic mechanical resonators.

Buters et al. 2016, Optomechanics with a polarization non-degenerate cavity

Experiments in the field of optomechanics do not yet fully exploit the photon polarization degree of freedom. Here experimental results for an optomechanical interaction in a polarization nondegenerate system are presented and schemes are proposed for how to use this interaction to perform accurate side-band thermometry and to create novel forms of photon-phonon entanglement. The experimental system utilizes the compressive force in the mirror attached to a mechanical resonator to create a micro-mirror with two radii of curvature which leads, when combined with a second mirror, to a significant polarization splitting of the cavity modes.

 

[Auto-Didact]

Here are some more key articles, including a 2014 review of the Marshall et al. experiment. They all seem to be behind pay walls though:

Kleckner et al. 2011, Optomechanical trampoline resonators

We report on the development of optomechanical “trampoline” resonators composed of a tiny SiO2/Ta2O5 dielectric mirror on a silicon nitride micro-resonator. We observe optical finesses of up to 4 × 10^4 and mechanical quality factors as high as 9 × 10^5 in relatively massive (∼100 ng) and low frequency (10–200 kHz) devices. This results in a photon-phonon coupling efficiency considerably higher than previous Fabry-Perot-type optomechanical systems. These devices are well suited to ultra-sensitive force detection, ground-state optical cooling experiments, and demonstrations of quantum dynamics for such systems.

Pepper et al. 2014, Towards Macroscopic Superpositions via Single-photon Optomechanics

We describe and compare two proposals for creating macroscopic superpositions using single-photon optomechanical systems. The realization of the proposed experiments poses major technological challenges, which we examine. Reaching the quantum ground state is essential for both schemes. We present experimental results on optical cooling, which provides a way to reach the quantum ground state for low frequency optomechanical resonators.

Eerkens et al. 2015, Optical side-band cooling of a low frequency optomechanical system

For experimental investigations of macroscopic quantum superpositions and the possible role of gravitational effects on the reduction of the corresponding quantum wave function it is beneficial to consider large mass, low frequency optomechanical systems. We report optical side-band cooling from room temperature for a 1.5×10^−10 kg (mode mass), low frequency side-band resolved optomechanical system based on a 5 cm long Fabry-Perot cavity. By using high-quality Bragg mirrors for both the stationary and the micromechanical mirror we are able to construct an optomechanical cavity with an optical linewidth of 23 kHz. This, together with a resonator frequency of 315 kHz, makes the system operate firmly in the side-band resolved regime. With the presented optomechanical system parameters cooling close to the ground state is possible. This brings us one step closer to creating and verifying macroscopic quantum superpositions.

 

[StevieTNZ]

Here is an interesting article I've just spotted: http://www.quantumlah.org/highlight/170224_Floquet.php

 

[Auto-Didact]

MAGA gravitational wave detector based on phonon excitations in BECs:

Sabin et al. 2014, Phonon creation by gravitational waves

We show that gravitational waves create phonons in a Bose–Einstein condensate (BEC). A traveling spacetime distortion produces particle creation resonances that correspond to the dynamical Casimir effect in a BEC phononic field contained in a cavity-type trap. We propose to use this effect to detect gravitational waves. The amplitude of the wave can be estimated applying recently developed relativistic quantum metrology techniques. We provide the optimal precision bound on the estimation of the waveʼs amplitude. Finally, we show that the parameter regime required to detect gravitational waves with this technique could be, in principle, within experimental reach in a medium-term timescale.

Followup paper:

Sabin et al. 2016, Thermal noise in BEC-phononic gravitational wave detectors

Quasiparticles in a Bose-Einstein condensate are sensitive to space-time distortions. Gravitational waves can induce transformations on the state of phonons that can be observed through quantum state discrimination techniques. We show that this method is highly robust to thermal noise and depletion. We derive a bound on the strain sensitivity that shows that the detection of waves in the kHz regime is not significantly affected by temperature in a wide range of parameters that are well within current experimental reach.

Review article by the same group about gravity in quantum experiments:
Howl et al. 2016, Gravity in the Quantum Lab

At the beginning of the previous century, Newtonian mechanics fell victim to two new revolutionary theories, Quantum Mechanics (QM) and General Relativity (GR). Both theories have transformed our view of physical phenomena, with QM accurately predicting the results of experiments taking place at small length scales, and GR correctly describing observations at larger length scales. However, despite the impressive predictive power of each theory in their respective regimes, their unification still remains unresolved. Theories and proposals for their unification exist but we are lacking experimental guidance towards the true unifying theory. Probing GR at small length scales where quantum effects become relevant is particularly problematic but recently there has been a growing interest in probing the opposite regime, QM at large scales where relativistic effects are important. This is principally due to the fact that experimental techniques in quantum physics have developed rapidly in recent years with the promise of quantum technologies. Here we review recent advances in experimental and theoretical work on quantum experiments that will be able to probe relativistic effects of gravity on quantum properties, playing particular attention to the role of Quantum Field Theory in Curved Spacetime (QFTCS) in describing these experiments. Interestingly, theoretical work using QFTCS has illustrated that these quantum experiments could be used to enhance measurements of gravitational effects, such as Gravitational Waves (GWs). Furthermore, verification of such enhancements, as well as other QFTCS predictions in quantum experiments, would provide the first direct validation of this limiting case of quantum gravity, several decades after it was initially proposed.

 

[mfb]

within experimental reach in a medium-term timescale.

In other words: "We have no idea how to build it now, but it is not 10 orders of magnitude away."
Or, in this case: "We achieved the necessary temperature, the necessary number of atoms in the BEC, and the necessary lifetime individually, but achieving them at the same time will be very challenging - oh, and we have to put all this into some vibration-free environment, and we need this 1 million times or need a source that emits gravitational waves long enough for 1 million measurements".

An interesting approach, but I don't see this happening for quite some time.

Edit: Now discussed here

 

[physika]

.

Large Quantum Superpositions and Interference of Massive Nanometer-Sized Objects

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.107.020405

We propose a method to prepare and verify spatial quantum superpositions of a nanometer-sized object separated by distances of the order of its size. This method provides unprecedented bounds for objective collapse models of the wave function by merging techniques and insights from cavity quantum optomechanics and matter-wave interferometry. An analysis and simulation of the experiment is performed taking into account standard sources of decoherence. We provide an operational parameter regime using present-day and planned technology.


Quantum interference of large organic molecules

http://www.univie.ac.at/qfp/publications3/pdffiles/ncomms1263.pdf

The wave nature of matter is a key ingredient of quantum physics and yet it defies our classical intuition. First proposed by Louis de Broglie a century ago, it has since been confirmed with a variety of particles from electrons up to molecules. Here we demonstrate new high-contrast quantum experiments with large and massive tailor-made organic molecules in a near-field interferometer. Our experiments prove the quantum wave nature and delocalization of compounds composed of up to 430 atoms, with a maximal size of up to 60Å, masses up to m=6,910AMU and de Broglie wavelengths down to λdB=h/mv1pm. We show that even complex systems, with more than 1,000 internal degrees of freedom, can be prepared in quantum states that are sufficiently well isolated from their environment to avoid decoherence and to show almost perfect coherence.

A strict experimental test of macroscopic realism in a superconducting flux qubit

https://www.nature.com/articles/ncomms13253

Macroscopic realism is the name for a class of modifications to quantum theory that allow macroscopic objects to be described in a measurement-independent manner, while largely preserving a fully quantum mechanical description of the microscopic world. Objective collapse theories are examples which aim to solve the quantum measurement problem through modified dynamical laws. Whether such theories describe nature, however, is not known. Here we describe and implement an experimental protocol capable of constraining theories of this class, that is more noise tolerant and conceptually transparent than the original Leggett–Garg test. We implement the protocol in a superconducting flux qubit, and rule out (by ∼84 s.d.) those theories which would deny coherent superpositions of 170 nA currents over a ∼10 ns timescale. Further, we address the ‘clumsiness loophole’ by determining classical disturbance with control experiments. Our results constitute strong evidence for the superposition of states of nontrivial macroscopic distinctness.


Experiments testing macroscopic quantum superpositions must be slow

https://www.nature.com/articles/srep22777

We consider a thought experiment where the preparation of a macroscopically massive or charged particle in a quantum superposition and the associated dynamics of a distant test particle apparently allow for superluminal communication. We give a solution to the paradox which is based on the following fundamental principle: any local experiment, discriminating a coherent superposition from an incoherent statistical mixture, necessarily requires a minimum time proportional to the mass (or charge) of the system. For a charged particle, we consider two examples of such experiments, and show that they are both consistent with the previous limitation. In the first, the measurement requires to accelerate the charge, that can entangle with the emitted photons. In the second, the limitation can be ascribed to the quantum vacuum fluctuations of the electromagnetic field. On the other hand, when applied to massive particles our result provides an indirect evidence for the existence of gravitational vacuum fluctuations and for the possibility of entangling a particle with quantum gravitational radiation.

 

[Auto-Didact]

Gerlich et al. 2011, Quantum interference of large organic molecules

The wave nature of matter is a key ingredient of quantum physics and yet it defies our classical intuition. First proposed by Louis de Broglie a century ago, it has since been confirmed with a variety of particles from electrons up to molecules. Here we demonstrate new high-contrast quantum experiments with large and massive tailor-made organic molecules in a near-field interferometer. Our experiments prove the quantum wave nature and delocalization of compounds composed of up to 430 atoms, with a maximal size of up to 60 Å, masses up to m=6,910 AMU and de Broglie wavelengths down to λdB=h/mv≃1 pm. We show that even complex systems, with more than 1,000 internal degrees of freedom, can be prepared in quantum states that are sufficiently well isolated from their environment to avoid decoherence and to show almost perfect coherence.

Emary et al. 2013, Leggett-Garg Inequalities

In contrast to the spatial Bell's inequalities, which probe entanglement between spatially-separated systems, the Leggett-Garg inequalities test the correlations of a single system measured at different times. Violation of a genuine Leggett-Garg test implies either the absence of a realistic description of the system or the impossibility of measuring the system without disturbing it. Quantum mechanics violates the inequalities on both accounts and the original motivation for these inequalities was as a test for quantum coherence in macroscopic systems. The last few years has seen a number of experimental tests and violations of these inequalities in a variety of microscopic systems such as superconducting qubits, nuclear spins, and photons. In this article, we provide an introduction to the Leggett-Garg inequalities and review these latest experimental developments. We discuss important topics such as the significance of the non-invasive measurability assumption, the clumsiness loophole, and the role of weak measurements. Also covered are some recent theoretical proposals for the application of Leggett-Garg inequalities in quantum transport, quantum biology and nano-mechanical systems.

Lychkovskiy 2015, Large quantum superpositions of a nanoparticle immersed in superfluid helium

Preparing and detecting spatially extended quantum superpositions of a massive object comprises an important fundamental test of quantum theory. These quantum states are extremely fragile and tend to quickly decay into incoherent mixtures due to the environmental decoherence. Experimental setups considered up to date address this threat in a conceptually straightforward way -- by eliminating the environment, i.e. by isolating an object in a sufficiently high vacuum. We show that another option exists: decoherence is suppressed in the presence of a strongly interacting environment if this environment is superfluid. Indeed, as long as an object immersed in a pure superfluid at zero temperature moves with a velocity below the critical one, it does not create, absorb or scatter any excitations of the superfluid. Hence, in this idealized situations the decoherence is absent. In reality the decoherence will be present due to thermal excitations of the superfluid and impurities contaminating the superfluid. We examine various decoherence channels in the superfluid 4He. It is shown that the total decoherence time can be as large as tens of seconds for a 10⁶ amu nanoparticle delocalized over 300 nm in helium at 1 mK. Performing interference experiments in superfluid helium can provide certain practical advantages compared to conventional schemes, e.g. compensation of gravity by the buoyancy force and effective cooling.

Hu et al. 2016, Strictly nonclassical behavior of a mesoscopic system

We experimentally demonstrate the strictly nonclassical behavior in a many-atom system using a recently derived criterion [E. Kot et al., Phys. Rev. Lett. 108, 233601 (2012)] that explicitly does not make use of quantum mechanics. We thereby show that the magnetic moment distribution measured by McConnell et al. [R. McConnell et al., Nature 519, 439 (2015)] in a system with a total mass of 2.6×10⁵ atomic mass units is inconsistent with classical physics. Notably, the strictly nonclassical behavior affects an area in phase space 10³ times larger than the Planck quantum ℏ.

Naeij et al. 2016, Double-Slit Interference Pattern for a Macroscopic Quantum System

In this study, we solve analytically the Schrodinger equation for a macroscopic quantum oscillator as a central system coupled to two environmental micro-oscillating particles. Then, the double-slit interference patterns are investigated in two limiting cases, considering the limits of uncertainty in the position probability distribution. Moreover, we analyze the interference patterns based on a recent proposal called stochastic electrodynamics with spin. Our results show that when the quantum character of the macro-system is decreased, the diffraction pattern becomes more similar to a classical one. We also show that, depending on the size of the slits, the predictions of quantum approach could be apparently different with those of the aforementioned stochastic description.

Yin et al. 2016, Bringing quantum mechanics to life: from Schrödinger's cat to Schrödinger's microbe

The question whether quantum mechanics is complete and the nature of the transition between quantum mechanics and classical mechanics have intrigued physicists for decades. There have been many experimental breakthroughs in creating larger and larger quantum superposition and entangled states since Erwin Schr\"odinger proposed his famous thought experiment of putting a cat in a superposition of both alive and dead states in 1935. Remarkably, recent developments in quantum optomechanics and electromechanics may lead to the realization of quantum superposition of living microbes soon. Recent evidence also suggests that quantum coherence may play an important role in several biological processes. In this review, we first give a brief introduction to basic concepts in quantum mechanics and the Schr\"odinger's cat thought experiment. We then review developments in creating quantum superposition and entangled states and the realization of quantum teleportation. Non-trivial quantum effects in photosynthetic light harvesting and avian magnetoreception are also discussed. At last, we review recent proposals to realize quantum superposition, entanglement and state teleportation of microorganisms, such as viruses and bacteria.

 

[Auto-Didact]

Fray et al. 2004, Atomic Interferometer with Amplitude Gratings of Light and its Applications to Atom Based Tests of the Equivalence Principle

We have developed a matter wave interferometer based on the diffraction of atoms from effective absorption gratings of light. In a setup with cold rubidium atoms in an atomic fountain the interferometer has been used to carry out tests of the equivalence principle on an atomic basis. The gravitational acceleration of the two isotopes 85Rb and 87Rb was compared, yielding a difference $\Delta g/g =(1.2 \pm 1.7)×10^{-7}$. We also perform a differential free fall measurement of atoms in two different hyperfine states, and obtained a result of $\Delta g/g =(0.4 \pm 1.2)×10^{-7}$.

Touboul et al. 2012, The MICROSCOPE experiment, ready for the in-orbit test of the equivalence principle

Deviations from standard general relativity are being intensively tested in various aspects. The MICROSCOPE space mission, which has recently been approved to be launched in 2016, aims at testing the universality of free fall with an accuracy better than $10^{−15}$. The instrument has been developed and most of the sub-systems have been tested to the level required for the detection of accelerations lower than one tenth of a femto-g. Two concentric test masses are electrostatically levitated inside the same silica structure and controlled on the same trajectory at better than 0.1 Å. Any dissymmetry in the measured electrostatic pressures shall be analysed with respect to the Earth's gravity field. The nearly 300 kg heavy dedicated satellite is defined to provide a very steady environment to the experiment and a fine control of its attitude and of its drag-free motion along the orbit. Both the evaluations of the performance of the instrument and the satellite demonstrate the expected test accuracy. The detailed description of the experiment and the major driving parameters of the instrument, the satellite and the data processing are provided in this paper.

Schlippert et al. 2014, Quantum Test of the Universality of Free Fall

We simultaneously measure the gravitationally-induced phase shift in two Raman-type matter-wave interferometers operated with laser-cooled ensembles of 87Rb and 39K atoms. Our measurement yields an Eotvos ratio of $η_{Rb,K}=(0.3±5.4)×10^{−7}$. We briefly estimate possible bias effects and present strategies for future improvements.

Altschul et al. 2014, Quantum Tests of the Einstein Equivalence Principle with the STE-QUEST Space Mission

We present in detail the scientific objectives in fundamental physics of the Space-Time Explorer and QUantum Equivalence Space Test (STE-QUEST) space mission. STE-QUEST was pre-selected by the European Space Agency together with four other missions for the cosmic vision M3 launch opportunity planned around 2024. It carries out tests of different aspects of the Einstein Equivalence Principle using atomic clocks, matter wave interferometry and long distance time/frequency links, providing fascinating science at the interface between quantum mechanics and gravitation that cannot be achieved, at that level of precision, in ground experiments. We especially emphasize the specific strong interest of performing equivalence principle tests in the quantum regime, i.e. using quantum atomic wave interferometry. Although STE-QUEST was finally not selected in early 2014 because of budgetary and technological reasons, its science case was very highly rated. Our aim is to expose that science to a large audience in order to allow future projects and proposals to take advantage of the STE-QUEST experience.

Will 2014, The Confrontation between General Relativity and Experiment

The status of experimental tests of general relativity and of theoretical frameworks for analyzing them are reviewed and updated. Einstein's equivalence principle (EEP) is well supported by experiments such as the Eotvos experiment, tests of local Lorentz invariance and clock experiments. Ongoing tests of EEP and of the inverse square law are searching for new interactions arising from unification or quantum gravity. Tests of general relativity at the post-Newtonian level have reached high precision, including the light deflection, the Shapiro time delay, the perihelion advance of Mercury, the Nordtvedt effect in lunar motion, and frame-dragging. Gravitational-wave damping has been detected in an amount that agrees with general relativity to better than half a percent using the Hulse-Taylor binary pulsar, and a growing family of other binary pulsar systems is yielding new tests, especially of strong-field effects. Current and future tests of relativity will center on strong gravity and gravitational waves.

Zych et al. 2015, Quantum formulation of the Einstein Equivalence Principle

Validity of just a few physical conditions comprising the Einstein Equivalence Principle (EEP) suffices to ensure that gravity can be understood as space-time geometry. EEP is therefore subject to an ongoing experimental verification, with present day tests reaching the regime where quantum mechanics becomes relevant. Here we show that the classical formulation of the EEP does not apply in such a regime. The EEP requires equivalence between the total rest mass-energy of a system, the mass-energy that constitutes its inertia, and the mass-energy that constitutes its weight. In quantum mechanics internal energy is given by a Hamiltonian operator describing dynamics of internal degrees of freedom. We therefore introduce a quantum formulation of the EEP -- equivalence between the rest, inertial and gravitational internal energy operators. We show that the validity of the classical EEP does not imply the validity of its quantum formulation, which thus requires an independent experimental verification. We reanalyse some already completed experiments with respect to the quantum EEP and discuss to which extent they allow testing its various aspects.

Orlando et al. 2016, A test of the equivalence principle(s) for quantum superpositions

We propose an experimental test of the quantum equivalence principle introduced by Zych and Brukner (arXiv:1502.00971), which generalises the Einstein equivalence principle to superpositions of internal energy states. We consider a harmonically trapped $\mathrm{spin} \mbox{-} \tfrac{1}{2}$ atom in the presence of both gravity and an external magnetic field and show that when the external magnetic field is suddenly switched off, various violations of the equivalence principle would manifest as otherwise forbidden transitions. Performing such an experiment would put bounds on the various phenomenological violating parameters. We further demonstrate that the classical weak equivalence principle can be tested by suddenly putting the apparatus into free fall, effectively 'switching off' gravity.

Rosi et al. 2017, Quantum test of the equivalence principle for atoms in superpositions of internal energy eigenstates

The Einstein Equivalence Principle (EEP) has a central role in the understanding of gravity and space-time. In its weak form, or Weak Equivalence Principle (WEP), it directly implies equivalence between inertial and gravitational mass. Verifying this principle in a regime where the relevant properties of the test body must be described by quantum theory has profound implications. Here we report on a novel WEP test for atoms. A Bragg atom interferometer in a gravity gradiometer configuration compares the free fall of rubidium atoms prepared in two hyperfine states and in their coherent superposition. The use of the superposition state allows testing genuine quantum aspects of EEP with no classical analogue, which have remained completely unexplored so far. In addition, we measure the Eotvos ratio of atoms in two hyperfine levels with relative uncertainty in the low $10^{−9}$, improving previous results by almost two orders of magnitude.

 

[houlahound]

Thanks for sources, not thanks for destroying my free time.

 

[Auto-Didact]

Bialynicki-Birula et al. 1976, Nonlinear wave mechanics

Nonlinear wave mechanics is constructed, based on Schrödinger-type equation with nonlinearity −bψ ln | ψ |2. This nonlinearity is selected by assuming the factorization of wavefunctions for composed systems. Its most attractive features are: existence of the lower energy bound and validity of Planck's relation E =h̵ω. In any number of dimensions, soliton-like solutions (gaussons) of our equation exist and move in slowly varying fields like classical particles. The Born interpretation of the wavefunction is consistent with logarithmic nonlinearity and we tentatively estimate the order of magnitude of the universal constant b.

Gähler et al. 1981, Neutron optical tests of nonlinear wave mechanics

We analyze the free-space propagation of matter waves with a view to placing an upper limit on the strength of possible nonlinear terms in the Schrödinger equation. Such additional terms of the form ψF(|ψ|^2) were introduced by Bialynicki-Birula and Mycielski in order to counteract the spreading of wave packets, thereby allowing solutions which behave macroscopically like classical particles. For the particularly interesting case of a logarithmic nonlinearity of the form F = −b ln |ψ|^2 , we find that the free-space propagation of slow neutrons places a very stringent upper limit on the magnitude of b. Precise measurements of Fresnel diffraction with slow neutrons do not give any evidence for nonlinear effects and allow us to deduce an upper limit for b < 3.3×10^−15 eV about 3 orders of magnitude smaller than the lower bound proposed by the above authors.

Bassi et al. 2013, Models of wave-function collapse, underlying theories, and experimental tests.

Quantum mechanics is an extremely successful theory that agrees with every experimental test. However, the principle of linear superposition, a central tenet of the theory, apparently contradicts a commonplace observation: macroscopic objects are never found in a linear superposition of position states. Moreover, the theory does not explain why during a quantum measurement, deterministic evolution is replaced by probabilistic evolution, whose random outcomes obey the Born probability rule. In this article a review is given of an experimentally falsifiable phenomenological proposal, known as continuous spontaneous collapse: a stochastic nonlinear modification of the Schrödinger equation, which resolves these problems, while giving the same experimental results as quantum theory in the microscopic regime. Two underlying theories for this phenomenology are reviewed: trace dynamics and gravity-induced collapse. As the macroscopic scale is approached, predictions of this proposal begin to differ appreciably from those of quantum theory and are being confronted by ongoing laboratory experiments that include molecular interferometry and optomechanics. These experiments, which test the validity of linear superposition for large systems, are reviewed here, and their technical challenges, current results, and future prospects summarized. It is likely that over the next two decades or so, these experiments can verify or rule out the proposed stochastic modification of quantum theory.

Curceanu et al. 2015, X-rays help to unfuzzy the concept of measurement

In the last decades huge theoretical effort was devoted to the development of consistent theoretical models, aiming to solve the so-called "measurement problem", to which John Bell dedicated part of his thoughts. Among these, the Dynamical Reduction Models possesses the unique characteristic to be experimentally testable, thus enabling to set experimental upper bounds on the reduction rate parameter λ characterizing these models. Analysing the X-ray spectrum emitted by an isolated slab of Germanium, we set the most stringent limit on the parameter λ up to date.

Arndt et al. 2014, Testing the limits of quantum mechanical superpositions.

Quantum physics has intrigued scientists and philosophers alike, because it challenges our notions of reality and locality — concepts that we have grown to rely on in our macroscopic world. It is an intriguing open question whether the linearity of quantum mechanics extends into the macroscopic domain. Scientific progress over the past decades inspires hope that this debate may be settled by table-top experiments.

Cotter et al. 2017, In search of multipath interference using large molecules

The superposition principle is fundamental to the quantum description of both light and matter. Recently, a number of experiments have sought to directly test this principle using coherent light, single photons, and nuclear spin states. We extend these experiments to massive particles for the first time. We compare the interference patterns arising from a beam of large dye molecules diffracting at single, double, and triple slit material masks to place limits on any high-order, or multipath, contributions. We observe an upper bound of less than one particle in a hundred deviating from the expectations of quantum mechanics over a broad range of transverse momenta and de Broglie wavelength.

 

[Auto-Didact]

Koduru Joshi et al. 2017, Space QUEST mission proposal: Experimentally testing decoherence due to gravity

Models of quantum systems on curved space-times lack sufficient experimental verification. Some speculative theories suggest that quantum properties, such as entanglement, may exhibit entirely different behavior to purely classical systems. By measuring this effect or lack thereof, we can test the hypotheses behind several such models. For instance, as predicted by Ralph and coworkers [T C Ralph, G J Milburn, and T Downes, Phys. Rev. A, 79(2):22121, 2009, T C Ralph and J Pienaar, New Journal of Physics, 16(8):85008, 2014], a bipartite entangled system could decohere if each particle traversed through a different gravitational field gradient. We propose to study this effect in a ground to space uplink scenario. We extend the above theoretical predictions of Ralph and coworkers and discuss the scientific consequences of detecting/failing to detect the predicted gravitational decoherence. We present a detailed mission design of the European Space Agency's (ESA) Space QUEST (Space - Quantum Entanglement Space Test) mission, and study the feasibility of the mission schema.

Rätzel et al. 2017, Testing small scale gravitational wave detectors with dynamical mass distributions

The recent discovery of gravitational waves by LIGO created renewed interest in the investigation of alternative gravitational detector designs, such as small scale resonant detectors. In this article, it is shown how proposed small scale detectors can be tested by generating dynamical gravitational near fields with appropriate distributions of moving masses. This opens up the possibility to evaluate detector proposals very early in the development phase and may help to progress quickly in their development.

Rätzel et al. 2017, Frequency spectrum of an optical resonator in a curved spacetime

The effect of gravity and proper acceleration on the frequency spectrum of an optical resonator - both rigid or deformable - is considered in the framework of general relativity. The optical resonator is modeled either as a rod of matter connecting two mirrors or as a dielectric rod whose ends function as mirrors. Explicit expressions for the frequency spectrum are derived for the case that it is only perturbed slightly. For a deformable resonator, the perturbation of the frequency spectrum depends on the speed of sound in the rod supporting the mirrors. A connection is found to a relativistic concept of rigidity when the speed of sound approaches the speed of light. In contrast, the corresponding result for the assumption of Born rigidity is recovered when the speed of sound becomes infinite. The results presented in this article can be used as the basis for the description of optical and opto-mechanical systems in a curved spacetime. We apply our results to the examples of a uniformly accelerating resonator and an optical resonator in the gravitational field of a small moving sphere. Our approach is not limited to weak gravitational fields, which we exemplify by its application to the fictitious situation of an optical resonator falling into a black hole.

Hartley et al. 2017, Analogue simulation of gravitational waves in a 3+1 dimensional Bose-Einstein condensate

The recent detections of gravitational waves (GWs) by the LIGO and Virgo collaborations have opened the field of GW astronomy, intensifying interest in GWs and other possible detectors sensitive in different frequency ranges. Although strong GW producing events are rare and currently unpredictable, GWs can in principle be simulated in analogue systems at will in the lab. Simulation of GWs in a manifestly quantum system would allow for the study of the interaction of quantum phenomena with GWs. Such predicted interaction is exploited in a recently proposed Bose-Einstein condensate (BEC) based GW detector. In this paper, we show how to manipulate a BEC to mimic the effect of a passing GW. By simultaneously varying the external potential applied to the BEC, and an external magnetic field near a Feshbach resonance, we show that the resulting change in speed of sound can directly reproduce a GW metric. We also show how to simulate a metric used in the recently proposed BEC based GW detector, to provide an environment for testing the proposed metrology scheme of the detector. Explicit expressions for simulations of various GW sources are given. This result is also useful to generally test the interaction of quantum phenomena with GWs in a curved spacetime analogue experiment.

 

[StevieTNZ]

https://phys.org/news/2018-01-groun...e=menu&utm_medium=link&utm_campaign=item-menu

An interesting experiment, which will produce interesting results.

 

[microsansfil]

https://phys.org/news/2018-01-groun...e=menu&utm_medium=link&utm_campaign=item-menu

An interesting experiment, which will produce interesting results.

Interesting experiment.

1/

In other words, why do everyday objects such as cars, trees and people not behave in a quantum mechanical manner and exist in two places at once?

Probably what we (humain being) perceive is not what is in itself.

For example we perceive this chair of green colors which leads us to say that this chair is green. Which leads us to think that color (first-person experience) is an intrinsic property of light.

However, there is another interpretation.

Color is not actually a property of light or of objects that reflect light. It is a sensation that arise within the brain.
http://www.esalq.usp.br/departamentos/leb/aulas/lce1302/visao_aves.pdf

Tommy Edison, who has been blind since birth, talks about describing colors to blind people.

I guess one of the things you guys are the most curious about is color.
How does it work for me ? What is it ?
I don't Know
So, beng blind since birth, I've never seen color
I don't have any concept of what it is
I mean, I've never seen anything
But there's this whole part of vocabulary, of language that doesn't mean anything to me.
Over the years, people have tried and tried to explain color to me, and i just don't understand it
I think the best way to show you, is to try and explain to someone who'snever heard before, what the ocean sounds like. Or what the birds sound like.
...

it seems to me, therefore, that in order to build a rational understanding of macroscopic / microscopic interaction, all the factors must be taken into account, belonging to the different scientific fields.

2/

If the experiment proves that quantum mechanics can be applied to larger-scale systems, it could make creating quantum technologies for use in space easier, with satellites being used to transmit quantum information rather than relying on fibres on the ground or under the sea."

Experiments to transmit quantum information using satellite already exist : https://arxiv.org/pdf/1712.09722.pdf

Best regards
Patrick

 

[mfb]

https://phys.org/news/2018-01-groun...e=menu&utm_medium=link&utm_campaign=item-menu

I wonder if the author has heard of decoherence. The article doesn't mention the most important concept related to this kind of research, not even once.

 

[Auto-Didact]

I wonder if the author has heard of decoherence. The article doesn't mention the most important concept related to this kind of research, not even once.

The phys.org article is a copypaste from the university's website, unfortunately neither of them link to a paper. It of course goes without saying that these news outlets specifically post clickbait articles to garner more interest and generate hype. It is very clear that the goal of the experiment is of purely scientific interest and that all the mentioned potential spinoffs and technologies are afterthoughts used to justify the experiment to the public; its understandable but still somewhat a shame that stating such justifications for science are today part of academic guidelines when writing a paper, but hey, in a capitalist society fundamental science funding just isn't cheap.

I have not found any actual papers yet regarding the proposed experiment, a quick search of the papers of one of the researchers involved (Mauro Paternostro) reveals nothing about Project TEQ, but it does show he is involved in the MAQRO experiment proposal I linked to earlier in this thread. I'm pretty confident that he knows the distinction between environmental and intrinsic decoherence, seeing there are multiple review articles on the subject.

 

[DrClaude]

More information on Project TEQ can be found at http://cordis.europa.eu/project/rcn/211916_en.html

Microscopic systems can be prepared in quantum configurations with no classical counterpart. Such a possibility seems precluded when the 'complexity' of the system grows towards the macroscopic domain: so far we have no evidence of non-classical behavior of the macroscopic world. Why is it so? How is quantumness lost as we abandon the microscopic domain? These questions, which remain to date largely unanswered, address interesting and challenging goals of modern research in physics, and serve the overarching goal of this project. TEQ will establish the large-scale limit of quantum mechanics by pursuing a novel research programme that aims at surpassing the current approach based on matter-wave interferometry. Specifically, the TEQ Consortium will
1) Trap an ad hoc manufactured nanocrystal in a radio-frequency ion trap, cooling it by optical parametric feedback, so as to let it operate in ultra-low noise environments.
2) Determine quantitatively all the major sources of decoherence affecting the nanocrystal, and control them experimentally so as to prepare high-quality quantum states of its motional degrees of freedom.
3) Analyse the light scattered by the nanocrystal to test the quantum predictions for the motion of the particle against those of spontaneous collapse and non-standard decoherence mechanisms, and thus pinpoint/rule-out key quantum-spoiling effects, beyond all the studies performed so far.
This roadmap will enable the test of quantum effects for systems whose mass is orders of magnitude larger than that employed in the most successful quantum experiments to date, thus closing the gap with the macroscopic world. Moreover, it will entail significant technological impact: the device that will be built will exhibit exquisite sensitivity to frequency and displacements, thus embodying a significant contribution of explicit technological nature to the design of quantum empowered metrological sensors.

 

[Auto-Didact]

More information on Project TEQ can be found at http://cordis.europa.eu/project/rcn/211916_en.html

Thanks. This is very exciting stuff. Does the reported timetable there (until 12/31/2021) refer to funding time or to projected time until the measurements are finished, before data analysis is done?

I'm asking because to me a mere four years seems like a ridiculously short time to finish an experiment of this magnitude, at least in comparison with the ongoing efforts of groups carrying out the Marshall et al. experiment. If TEQ is just 5 years away from having significant results, this may actually be finished before Bouwmeester's decades long ongoing attempt.

It would definitely be nice though, especially seeing it may just fall within Penrose's lifespan, who is of course the theorist whose ideas and work have resulted in most of these experiments looking for deviations from QM for ≥ Planck mass systems in superposition.

 

[f95toli]

Thanks. This is very exciting stuff. Does the reported timetable there (until 12/31/2021) refer to funding time or to projected time until the measurements are finished, before data analysis is done?

It is the funding time; they have to finish all their activities by then and then submit their final report within a few months (I think it is 6 months) after this date.
Note that there is nothing that says that they will actually finish by then; most of the time you get funding for three or four years and if you have an experiment that takes longer than that to complete you just have to fund it from two (or more) separate projects. They might also have some funding from elsewhere (not for the same activities but for e.g. salaries for scientist working on the projects) and can of course use existing facilities/ setups for the work . This is not the kind of work someone would start from scratch and expect to complete in 4 years.

The projects should have a website up and running soon (it is a requirement for this type of project, part of the mandatory "impact" workpackage)

 

[StevieTNZ]

https://phys.org/news/2018-04-entanglement-near-macroscopic.html

"In work recently published in Nature, a team led by Prof. Mika Sillanpää at Aalto University in Finland has shown that entanglement of massive objects can be generated and detected.

The researchers managed to bring the motions of two individual vibrating drumheads—fabricated from metallic aluminium on a silicon chip—into an entangled quantum state. The macroscopic objects in the experiment are truly massive compared to the atomic scale—the circular drumheads have a diametre similar to the width of a thin human hair."

 

[Mentz114]

https://phys.org/news/2018-04-entanglement-near-macroscopic.html

"In work recently published in Nature, a team led by Prof. Mika Sillanpää at Aalto University in Finland has shown that entanglement of massive objects can be generated and detected.

The researchers managed to bring the motions of two individual vibrating drumheads—fabricated from metallic aluminium on a silicon chip—into an entangled quantum state. The macroscopic objects in the experiment are truly massive compared to the atomic scale—the circular drumheads have a diametre similar to the width of a thin human hair."

It is interesting to get two entangled macroscopic waves. The publication is available here
( I think it is the same one. Same subject matter )
(https://arxiv.org/abs/1711.01640)

 

[StevieTNZ]

An addition:

http://iopscience.iop.org/article/10.1088/1367-2630/aabb8d/meta

https://phys.org/news/2018-05-quant...e=menu&utm_medium=link&utm_campaign=item-menu

From the phys.org article:

Researchers have studied how a 'drumstick' made of light could make a microscopic 'drum' vibrate and stand still at the same time.

A team of researchers from the UK and Australia have made a key step towards understanding the boundary between the quantum world and our everyday classical world.

Quantum mechanics is truly weird. Objects can behave like both particles and waves, and can be both here and there at the same time, defying our common sense. Such counterintuitive behaviour is typically confined to the microscopic realm and the question "why don't we see such behaviour in everyday objects?" challenges many scientists today.

Now, a team of researchers have developed a new technique to generate this type of quantum behaviour in the motion of a tiny drum just visible to the naked eye. The details of their research are published today in New Journal of Physics.

 

[physika]

.

...I would have preferred a somewhat "larger" object



Nonclassicality of the Harmonic-Oscillator Coherent State Persisting up to the Macroscopic Domain

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.210402

Can the most “classical-like” of all quantum states, namely the Schrödinger coherent state of a harmonic oscillator, exhibit nonclassical behavior? We find that for an oscillating object initially in a coherent state, merely by observing at various instants which spatial region the object is in, the Leggett-Garg inequality (LGI) can be violated through a genuine negative result measurement, thereby repudiating the everyday notion of macrorealism. This violation thus reveals an unnoticed nonclassicality of the very state which epitomizes classicality within the quantum description. It is found that for any given mass and oscillator frequency, a significant quantum violation of LGI can be obtained by suitably choosing the initial peak momentum of the coherent state wave packet. It thus opens up potentially the simplest way (without coupling with any ancillary quantum system or using nonlinearity) for testing whether various recently engineered and sought after macroscopic oscillators, such as feedback cooled thermal trapped nanocrystals of ∼10⁶–10⁹  amu mass, are indeed bona fide nonclassical objects.


.

 

[StevieTNZ]

https://www.quantamagazine.org/real...e-the-boundary-of-the-quantum-world-20180625/

with the relevant papers mentioned, in Nature:

1. https://www.nature.com/articles/s41586-018-0036-z
2. https://www.nature.com/articles/s41586-018-0038-x

 

[mfb]

https://www.nature.com/articles/s41586-018-0036-z

Interesting detail in the abstract:

each composed of about 10¹² atoms

Despite the formatting, the 12 is not meant as reference, but as exponent.

 

[DrClaude]

Despite the formatting, the 12 is not meant as reference, but as exponent.

That's Wikipedia-esque. "We did the experiment using 10^{[citation needed]} atoms."

 

[physika]

.

http://www.materials.ox.ac.uk/peoplepages/ares.htmlPutting the mechanics into quantum mechanics: creating superpositions of motion using vibrating carbon nanotubes
Dr E. A. Laird / Dr N. Ares / Professor G. A . D. Briggs

The quantum mechanics of microscopic objects such as atoms and spins is well established. But what about larger objects? Can we verify true quantum behaviour for these?

As a first step to answering this question, we plan to create and measure quantum superpositions of nanoscale mechanical devices. Although tiny by everyday standards, even the smallest fabricated device contains thousands of atoms. We will make use of suspended vibrating carbon nanotubes. These possesses many attractive features for creating mechanical quantum superpositions, including low mass, large quantum level spacing, and comparatively large zero-point motion. Our goal is to carry out a foundational test of quantum mechanics – the Leggett-Garg test – that falsifies the hypothesis of classical behaviour in this device. This project will focus on creating and probing so-called “macroscopically distinct” superpositions, such as a superposition of zero and ten phonons in the same device. These challenging experiments on tiny devices are the first step on a long road to discovering whether quantum mechanics applies to macroscopic objects..

 

[Auto-Didact]

2017/2018 key developments of the Marshall experiment by Bouwmeester's group:

Weaver et al. 2017, Coherent Optomechanical State Transfer between Disparate Mechanical Resonators

Hybrid quantum systems have been developed with various mechanical, optical and microwave harmonic oscillators. The coupling produces a rich library of interactions including two mode squeezing, swapping interactions, back-action evasion and thermal control. In a multimode mechanical system, coupling resonators of different scales (both in frequency and mass) leverages the advantages of each resonance. For example: a high frequency, easily manipulated resonator could be entangled with a low frequency massive object for tests of gravitational decoherence. Here we demonstrate coherent optomechanical state swapping between two spatially and frequency separated resonators with a mass ratio of 4. We find that, by using two laser beams far detuned from an optical cavity resonance, efficient state transfer is possible through a process very similar to STIRAP (Stimulated Raman Adiabatic Passage) in atomic physics. Although the demonstration is classical, the same technique can be used to generate entanglement between oscillators in the quantum regime.

Tsaturyan et al. 2017, Ultracoherent nanomechanical resonators via soft clamping and dissipation dilution

The small mass and high coherence of nanomechanical resonators render them the ultimate mechanical probe, with applications that range from protein mass spectrometry and magnetic resonance force microscopy to quantum optomechanics. A notorious challenge in these experiments is the thermomechanical noise related to the dissipation through internal or external loss channels. Here we introduce a novel approach to define the nanomechanical modes, which simultaneously provides a strong spatial confinement, full isolation from the substrate and dilution of the resonator material's intrinsic dissipation by five orders of magnitude. It is based on a phononic bandgap structure that localizes the mode but does not impose the boundary conditions of a rigid clamp. The reduced curvature in the highly tensioned silicon nitride resonator enables a mechanical $Q > 10^8$ at $1 \rm {MHz}$ to yield the highest mechanical Qf products ($>10^{14} \rm {Hz}$) yet reported at room temperature.

The corresponding coherence times approach those of optically trapped dielectric particles. Extrapolation to $4.2 \rm {K}$ predicts quanta per milliseconds heating rates, similar to those of trapped ions.

Sonar et al. 2018, Squeezing Enhances Quantum Synchronization

It is desirable to observe synchronization of quantum systems in the quantum regime, defined by low number of excitations and a highly non-classical steady state of the self-sustained oscillator. Several existing proposals of observing synchronization in the quantum regime suffer from the fact that the noise statistics overwhelms synchronization in this regime. Here we resolve this issue by driving a self-sustained oscillator with a squeezing Hamiltonian instead of a harmonic drive and analyze this system in the classical and quantum regime. We demonstrate that strong entrainment is possible for small values of squeezing, and in this regime the states are non-classical. Furthermore, we show that the quality of synchronization measured by the FWHM of the power spectrum is enhanced with squeezing.

Weaver et al. 2018, Phonon Interferometry for Measuring Quantum Decoherence

Experimental observation of the decoherence of macroscopic objects is of fundamental importance to the study of quantum collapse models and the quantum to classical transition. Optomechanics is a promising field for the study of such models because of its fine control and readout of mechanical motion. Nevertheless, it is challenging to monitor a mechanical superposition state for long enough to investigate this transition. We present a scheme for entangling two mechanical resonators in spatial superposition states such that all quantum information is stored in the mechanical resonators. The scheme is general and applies to any optomechanical system with multiple mechanical modes. By analytic and numeric modeling, we show that the scheme is resilient to experimental imperfections such as incomplete pre-cooling, faulty postselection and inefficient optomechanical coupling. This proposed procedure overcomes limitations of previously proposed schemes that have so far hindered the study of macroscopic quantum dynamics.

Sonar et al. 2018, Strong Thermo-mechanical Squeezing in a far detuned Membrane-in-the-middle System

We demonstrate $8.5 \rm {dB}$ thermal squeezing of a membrane oscillator using the dynamical backaction effect and electrostatic feedback in an optomechanical membrane-in-the-middle setup. We show that strong squeezing can be obtained even in the far detuning regime of a sideband-resolved system. By using the dielectrophoretic force of a metallic needle kept in close proximity to the membrane, we implement the one-quadrature active feedback scheme to prevent the divergence of the amplified quadrature and surpass the $3 \rm {dB}$ limit of mechanical squeezing. We also discuss different regions of the sideband spectrum where strong squeezing can be obtained. Although the demonstration here is classical, this technique is equally applicable to prepare the mechanical oscillator in a quantum squeezed state.

Rossi et al. 2018, Measurement-based quantum control of mechanical motion

Controlling a quantum system based on the observation of its dynamics is inevitably complicated by the backaction of the measurement process. Efficient measurements, however, maximize the amount of information gained per disturbance incurred. Real-time feedback then enables both canceling the measurement's backaction and controlling the evolution of the quantum state. While such measurement-based quantum control has been demonstrated in the clean settings of cavity and circuit quantum electrodynamics, its application to motional degrees of freedom has remained elusive. Here we show measurement-based quantum control of the motion of a millimetre-sized membrane resonator. An optomechanical transducer resolves the zero-point motion of the soft-clamped resonator in a fraction of its millisecond coherence time, with an overall measurement efficiency close to unity. We use this position record to feedback-cool a resonator mode to its quantum ground state (residual thermal occupation $n = 0.29 \pm 0.03$), $9 \rm {dB}$ below the quantum backaction limit of sideband cooling, and six orders of magnitude below the equilibrium occupation of its thermal environment. This realizes a long-standing goal in the field, and adds position and momentum to the degrees of freedom amenable to measurement-based quantum control, with potential applications in quantum information processing and gravitational wave detectors.

De Wit et al. 2018, Vibration isolation with high thermal conductance for a cryogen-free dilution refrigerator

We present the design and implementation of a mechanical low-pass filter vibration isolation used to reduce the vibrational noise in a cryogen-free dilution refrigerator operated at $10 \rm {mK}$, intended for scanning probe techniques. We discuss the design guidelines necessary to meet the competing requirements of having a low mechanical stiffness in combination with a high thermal conductance. We demonstrate the effectiveness of our approach by measuring the vibrational noise levels of an ultrasoft mechanical resonator positioned above a SQUID. Starting from a cryostat base temperature of $8 \rm {mK}$, the vibration isolation can be cooled to $10.5 \rm {mK}$, with a cooling power of $113 \rm {μW}$ at $100 \rm {mK}$. We use the low vibrations and low temperature to demonstrate an effective cantilever temperature of less than $20 \rm {mK}$. This results in a force sensitivity of less than $500 \rm {zN}/\sqrt {\rm {Hz}}$, and an integrated frequency noise as low as $0.4 \rm {mHz}$ in a $1 \rm {Hz}$ measurement bandwidth.

 

[StevieTNZ]

https://phys.org/news/2018-11-probi...e=menu&utm_medium=link&utm_campaign=item-menu

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.121.220404

Over the past few decades, experimental tests of Bell-type inequalities have been at the forefront of understanding quantum mechanics and its implications. These strong bounds on specific measurements on a physical system originate from some of the most fundamental concepts of classical physics—in particular that properties of an object are well-defined independent of measurements (realism) and only affected by local interactions (locality). The violation of these bounds unambiguously shows that the measured system does not behave classically, void of any assumption on the validity of quantum theory. It has also found applications in quantum technologies for certifying the suitability of devices for generating quantum randomness, distributing secret keys and for quantum computing. Here we report on the violation of a Bell inequality involving a massive, macroscopic mechanical system. We create light-matter entanglement between the vibrational motion of two silicon optomechanical oscillators, each comprising approx. 1010 atoms, and two optical modes. This state allows us to violate a Bell inequality by more than 4 standard deviations, directly confirming the nonclassical behavior of our optomechanical system under the fair sampling assumption.

 

[Auto-Didact]

Howl et al. 2018, Exploring the unification of quantum theory and general relativity with a Bose-Einstein condensate

Despite almost a century worth of study, it is still unclear how general relativity (GR) and quantum theory (QT) should be unified into a consistent theory. The conventional approach is to retain the foundational principles of QT, such as the superposition principle, and modify GR. This is referred to as 'quantizing gravity', resulting in a theory of 'quantum gravity'. The opposite approach is 'gravitizing QT' where we attempt to keep the principles of GR, such as the equivalence principle, and consider how this leads to modifications of QT. What we are most lacking in understanding which route to take, if either, is experimental guidance. Here we consider using a Bose-Einstein condensate (BEC) to search for clues. In particular, we study how a single BEC in a superposition of two locations could test a gravitizing QT proposal where wavefunction collapse emerges from a unified theory as an objective process, resolving the measurement problem of QT. Such a modification to QT due to general relativistic principles is testable at the Planck mass scale, which is much closer to experiments than the Planck length scale where quantum, general relativistic effects are traditionally anticipated in quantum gravity theories. Furthermore, experimental tests of this proposal should be simpler to perform than recently suggested experiments that would test the quantizing gravity approach in the Newtonian gravity limit by searching for entanglement between two massive systems that are both in a superposition of two locations.

General method for computing the evolution of a quantum scalar field in curved spacetime. Applications to small perturbations and cosmology

We develop a method for computing the evolution of a quantum real Klein-Gordon field in a region of spacetime. The method is both of general applicability and particularly useful in certain important problems, such as the study of confined quantum fields under small perturbations. Instead of computing the evolution of the initial state of the field in time, we define Bogoliubov transformations between spatial hypersurfaces at different times, and then obtain a differential equation and a formal integral expression for these time-dependent Bogoliubov transformations. In the case of quantum fields confined inside cavities, the method allows to easily make quantitative precise predictions on their behavior under small perturbations of the background geometry and/or the boundary conditions, by yielding a simple recipe for computing the resonances of the field with the perturbation and their amplitudes. Therefore, the method provides a crucial tool in the recently growing research area of confined quantum fields in table-top experiments. We also prove its utility in addressing other problems, such as particle creation in a cosmological expansion, thus giving also an example of its more general applicability.

 

[StevieTNZ]

https://phys.org/news/2018-12-quantum-superposition-revivals.html

Physicists have proposed an entirely new way to test the quantum superposition principle—the idea that a quantum object can exist in multiple states at the same time. The new test is based on examining the quantum rotation of a macroscopic object—specifically, a nanoscale rotor, which is considered macroscopic despite its tiny size.
...
The physicists, led by Klaus Hornberger at the University of Duisburg-Essen, Germany, have published a paper on the proposed test in a recent issue of the New Journal of Physics.

http://iopscience.iop.org/article/10.1088/1367-2630/aaece4

Whether quantum physics is universally valid is an open question with far-reaching implications. Intense research is therefore invested into testing the quantum superposition principle with ever heavier and more complex objects. Here we propose a radically new, experimentally viable route towards studies at the quantum-to-classical borderline by probing the orientational quantum revivals of a nanoscale rigid rotor. The proposed interference experiment testifies a macroscopic superposition of all possible orientations. It requires no diffraction grating, uses only a single levitated particle, and works with moderate motional temperatures under realistic environmental conditions. The first exploitation of quantum rotations of a massive object opens the door to new tests of quantum physics with submicron particles and to quantum gyroscopic torque sensors, holding the potential to improve state-of-the-art devices by many orders of magnitude.

 

[Auto-Didact]

This one is pretty groundbreaking!
Marletto et al. 2018, Entanglement between living bacteria and quantized light witnessed by Rabi splitting

We model recent experiments on living sulphur bacteria interacting with quantised light, using the Dicke model. Our analysis shows that the strong coupling between the bacteria and the light, when both are treated quantum-mechanically, indicates that in those experiments there is entanglement between the bacteria (modelled as dipoles) and the quantised light (modelled as a single quantum harmonic oscillator). The existence of lower polariton branch due to the vacuum Rabi splitting, measured in those experiments for a range of different parameters, ensures the negativity of energy (with respect to the lowest energy of separable states), thus acting as an entanglement witness.

 

[StevieTNZ]

https://journals.aps.org/pra/abstract/10.1103/PhysRevA.99.032125

We show violations of Leggett-Garg inequalities to be possible for single-mode cat states evolving dynamically in the presence of a nonlinear quantum interaction arising from, for instance, a Kerr medium. In order to prove the results, we derive a generalized version of the Leggett-Garg inequality involving different cat states at different times. The violations demonstrate failure of the premise of macrorealism as defined by Leggett and Garg, provided extra assumptions associated with experimental tests are valid. With the additional assumption of stationarity, violations of the Leggett-Garg inequality are predicted for the multicomponent cat states observed in the Bose-Einstein condensate and superconducting circuit experiments of Greiner et al. [Nature (London) 419, 51 (2002)] and Kirchmair et al. [Nature (London) 495, 205 (2013)]. The violations demonstrate a mesoscopic quantum coherence, by negating that the system can be in a classical mixture of mesoscopically distinct coherent states. Higher orders of nonlinearity are also studied and shown to give strong violation of Leggett-Garg inequalities. Loopholes for testing macrorealism are discussed.

Pre-print version available here: https://arxiv.org/abs/1812.11114