Mentor
Blog Entries: 27

## Recent Noteworthy Physics Papers

On the heels of the measurement of the average trajectory taken in a double-slit experiment using the weak measurement technique, along comes another report on a fundamental measurement in QM, also using the weak measurement. This time, they made a "direct" measurement of the QM wavefunction itself!

J.S. Lundeen et al., "Direct measurement of the quantum wavefunction" Nature v.474, p.188 (2011).

Abstract: The wavefunction is the complex distribution used to completely describe a quantum system, and is central to quantum theory. But despite its fundamental role, it is typically introduced as an abstract element of the theory with no explicit definition. Rather, physicists come to a working understanding of the wavefunction through its use to calculate measurement outcome probabilities by way of the Born rule. At present, the wavefunction is determined through tomographic methods which estimate the wavefunction most consistent with a diverse collection of measurements. The indirectness of these methods compounds the problem of defining the wavefunction. Here we show that the wavefunction can be measured directly by the sequential measurement of two complementary variables of the system. The crux of our method is that the first measurement is performed in a gentle way through weak measurement so as not to invalidate the second. The result is that the real and imaginary components of the wavefunction appear directly on our measurement apparatus. We give an experimental example by directly measuring the transverse spatial wavefunction of a single photon, a task not previously realized by any method. We show that the concept is universal, being applicable to other degrees of freedom of the photon, such as polarization or frequency, and to other quantum systems—for example, electron spins, SQUIDs (superconducting quantum interference devices) and trapped ions. Consequently, this method gives the wavefunction a straightforward and general definition in terms of a specific set of experimental operations. We expect it to expand the range of quantum systems that can be characterized and to initiate new avenues in fundamental quantum theory.

Zz.
 Mentor Blog Entries: 27 S. Zhang et al., "Optical Precursor of a Single Photon" Phys. Rev. Lett. v.106, p.243602 (2011). Abstract: We report the direct observation of optical precursors of heralded single photons with step- and square-modulated wave packets passing through cold atoms. Using electromagnetically induced transparency and the slow-light effect, we separate the single-photon precursor, which always travels at the speed of light in vacuum, from its delayed main wave packet. In the two-level superluminal medium, our result suggests that the causality holds for a single photon. Single photons obey light speed limit in vacuum! Zz.
 Mentor Blog Entries: 27 R. Lapkiewicz et al., "Experimental non-classicality of an indivisible quantum system, Nature v.474, p.490 (2011). Abstract: In contrast to classical physics, quantum theory demands that not all properties can be simultaneously well defined; the Heisenberg uncertainty principle is a manifestation of this fact. Alternatives have been explored—notably theories relying on joint probability distributions or non-contextual hidden-variable models, in which the properties of a system are defined independently of their own measurement and any other measurements that are made. Various deep theoretical results imply that such theories are in conflict with quantum mechanics. Simpler cases demonstrating this conflict have been found and tested experimentally with pairs of quantum bits (qubits). Recently, an inequality satisfied by non-contextual hidden-variable models and violated by quantum mechanics for all states of two qubits was introduced and tested experimentally. A single three-state system (a qutrit) is the simplest system in which such a contradiction is possible; moreover, the contradiction cannot result from entanglement between subsystems, because such a three-state system is indivisible. Here we report an experiment with single photonic qutrits which provides evidence that no joint probability distribution describing the outcomes of all possible measurements—and, therefore, no non-contextual theory—can exist. Specifically, we observe a violation of the Bell-type inequality found by Klyachko, Can, Binicioğlu and Shumovsky. Our results illustrate a deep incompatibility between quantum mechanics and classical physics that cannot in any way result from entanglement. Zz.
 Mentor Blog Entries: 27 P. Adamson et al., "First Direct Observation of Muon Antineutrino Disappearance", Phys. Rev. Lett. v.107, p.021801 (2011). Abstract: This Letter reports the first direct observation of muon antineutrino disappearance. The MINOS experiment has taken data with an accelerator beam optimized for ν̅ μ production, accumulating an exposure of 1.71×1020 protons on target. In the Far Detector, 97 charged current ν̅ μ events are observed. The no-oscillation hypothesis predicts 156 events and is excluded at 6.3σ. The best fit to oscillation yields |Δm̅2|=[3.36-0.40+0.46(stat)±0.06(syst)]×10-3  eV2, sin⁡2(2θ̅ )=0.86-0.12+0.11(stat)±0.01(syst). The MINOS νμ and ν̅ μ measurements are consistent at the 2.0% confidence level, assuming identical underlying oscillation parameters. You may read a review of this at the APS Physics website and also obtain a free download of the actual paper. Zz.
 Recognitions: Science Advisor C. M. Wilson et al., "Observation of the dynamical Casimir effect in a superconducting circuit, Nature v.479, p.376 (2011). Abstract: One of the most surprising predictions of modern quantum theory is that the vacuum of space is not empty. In fact, quantum theory predicts that it teems with virtual particles flitting in and out of existence. Although initially a curiosity, it was quickly realized that these vacuum fluctuations had measurable consequences—for instance, producing the Lamb shift of atomic spectra and modifying the magnetic moment of the electron. This type of renormalization due to vacuum fluctuations is now central to our understanding of nature. However, these effects provide indirect evidence for the existence of vacuum fluctuations. From early on, it was discussed whether it might be possible to more directly observe the virtual particles that compose the quantum vacuum. Forty years ago, it was suggested that a mirror undergoing relativistic motion could convert virtual photons into directly observable real photons. The phenomenon, later termed the dynamical Casimir effect, has not been demonstrated previously. Here we observe the dynamical Casimir effect in a superconducting circuit consisting of a coplanar transmission line with a tunable electrical length. The rate of change of the electrical length can be made very fast (a substantial fraction of the speed of light) by modulating the inductance of a superconducting quantum interference device at high frequencies (>10 gigahertz). In addition to observing the creation of real photons, we detect two-mode squeezing in the emitted radiation, which is a signature of the quantum character of the generation process. So, there finally is the demonstration of the dynamical Casimir effect. Almost more astonishing than the paper itself is the fact that this paper has been accepted only 16 days after it has been received. I must be doing something completely wrong when submitting papers.

Recognitions:
Gold Member
 Quote by Cthugha So, there finally is the demonstration of the dynamical Casimir effect. Almost more astonishing than the paper itself is the fact that this paper has been accepted only 16 days after it has been received. I must be doing something completely wrong when submitting papers.
It wasn't really. It has been available on the arXiv for quite a while and Nature even published a news-story about it a couple of months or so ago (some of the results have already been shown at conferences).

Recognitions:
 Quote by Cthugha So, there finally is the demonstration of the dynamical Casimir effect. Almost more astonishing than the paper itself is the fact that this paper has been accepted only 16 days after it has been received. I must be doing something completely wrong when submitting papers.
Also, Nature has a process in which a paper is formally rejected but the authors invited to resubmit. The submission date stated in the accepted version is not necessarily the date of first submission to that journal.

Recognitions:
 Quote by atyy Also, Nature has a process in which a paper is formally rejected but the authors invited to resubmit. The submission date stated in the accepted version is not necessarily the date of first submission to that journal.
Yes, I am aware of that, but even for these cases where the resubmission date is taken as the submission date the editorial timescales are usually much longer. I suppose the editors sent it out for review almost immediately and the referees answered almost instantly and suggested publication.

However, another paper I just recently noticed. I hope September still qualifies as recent.

Mackillo Kira et al., "Quantum spectroscopy with Schrödinger-cat states, Nature Physics v.7, p.799 (2011).

Abstract: Laser-spectroscopic techniques that exploit light–matter entanglement promise access to many-body configurations. Their practical implementation, however, is hindered by the large number of coupled states involved. Here, we introduce a scheme to deal with this complexity by combining quantitative experiments with theoretical analysis. We analyse the absorption properties of semiconductor quantum wells and present a converging cluster-expansion transformation that robustly projects a large set of quantitative classical measurements onto the true quantum responses. Classical and quantum sources are shown to yield significantly different results; Schrödinger-cat states can enhance the signal by an order of magnitude. Moreover, squeezing of the source can help to individually control and characterize excitons, biexcitons and electron–hole complexes.

One of these papers where the supplementary is longer than the paper itself. In experiments one would like to have different light sources to probe the system of interest. For example many systems behave differently when excited with laser light, thermal light or photon number states. However, some of the really interesting states one could use for excitation cannot be realized reliably, most cannot be realized at all. However, Glauber showed (and got the Nobel prize for that) that every possible state of the light field can be described by a superposition of coherent states weighted with a quasi-probability distribution, the so-called Glauber-Sudarshan representation. So in principle one could just measure the system response to coherent states and then calculate the response to some other kind of excitation light field if the Glauber-Sudarshan representation of that state is known. Unfortunately, these weighting function often behaves so badly for non-classical states that the integrals one has to solve cannot be evaluated. This paper introduces a method to transform the measure the experimental response to coherent excitation into a well behaved function. In this framework the integrals can be evaluated.
In summary the authors present a quantum light source emulator and apply it to a many body system.

See also the News and Views article on this one written by Carlo Piermarocchi: http://www.nature.com/nphys/journal/...nphys2107.html.
 Mentor Blog Entries: 27 M. Fridman et al., "Demonstration of temporal cloaking", Nature v.481, p.62 (2012). Abstract:Recent research has uncovered a remarkable ability to manipulate and control electromagnetic fields to produce effects such as perfect imaging and spatial cloaking. To achieve spatial cloaking, the index of refraction is manipulated to flow light from a probe around an object in such a way that a ‘hole’ in space is created, and the object remains hidden. Alternatively, it may be desirable to cloak the occurrence of an event over a finite time period, and the idea of temporal cloaking has been proposed in which the dispersion of the material is manipulated in time, producing a ‘time hole’ in the probe beam to hide the occurrence of the event from the observer. This approach is based on accelerating the front part of a probe light beam and slowing down its rear part to create a well controlled temporal gap—inside which an event occurs—such that the probe beam is not modified in any way by the event. The probe beam is then restored to its original form by the reverse manipulation of the dispersion. Here we present an experimental demonstration of temporal cloaking in an optical fibre-based system by applying concepts from the space–time duality between diffraction and dispersive broadening. We characterize the performance of our temporal cloak by detecting the spectral modification of a probe beam due to an optical interaction and show that the amplitude of the event (at the picosecond timescale) is reduced by more than an order of magnitude when the cloak is turned on. These results are a significant step towards the development of full spatio-temporal cloaking. Also read the News and Views article in the same issue of Nature. Zz.
 Parthiban Santhanam et al., "Thermoelectrically Pumped Light-Emitting Diodes Operating above Unity Efficiency", Phys. Rev. Lett. 108, 097403 (2012) Abstract: A heated semiconductor light-emitting diode at low forward bias voltage V
 Almost forgot this one :) Brendan McMonigal et al., "Alcubierre warp drive: On the matter of matter", Phys. Rev. D 85, 064024 (2012) Abstract: The Alcubierre warp drive allows a spaceship to travel at an arbitrarily large global velocity by deforming the spacetime in a bubble around the spaceship. Little is known about the interactions between massive particles and the Alcubierre warp drive, or the effects of an accelerating or decelerating warp bubble. We examine geodesics representative of the paths of null and massive particles with a range of initial velocities from -c to c interacting with an Alcubierre warp bubble traveling at a range of globally subluminal and superluminal velocities on both constant and variable velocity paths. The key results for null particles match what would be expected of massive test particles as they approach ±c. The increase in energy for massive and null particles is calculated in terms of vs, the global ship velocity, and vp, the initial velocity of the particle with respect to the rest frame of the origin/destination of the ship. Particles with positive vp obtain extremely high energy and velocity and become “time locked” for the duration of their time in the bubble, experiencing very little proper time between entering and eventually leaving the bubble. When interacting with an accelerating bubble, any particles within the bubble at the time receive a velocity boost that increases or decreases the magnitude of their velocity if the particle is moving toward the front or rear of the bubble, respectively. If the bubble is decelerating, the opposite effect is observed. Thus Eulerian matter is unaffected by bubble accelerations/decelerations. The magnitude of the velocity boosts scales with the magnitude of the bubble acceleration/deceleration. arXiv.org
 Mentor Blog Entries: 27 F. Buscemi, "All Entangled Quantum States Are Nonlocal", Phys. Rev. Lett. v.108, p. 200401 (2012). Abstract: Departing from the usual paradigm of local operations and classical communication adopted in entanglement theory, we study here the interconversion of quantum states by means of local operations and shared randomness. A set of necessary and sufficient conditions for the existence of such a transformation between two given quantum states is given in terms of the payoff they yield in a suitable class of nonlocal games. It is shown that, as a consequence of our result, such a class of nonlocal games is able to witness quantum entanglement, however weak, and reveal nonlocality in any entangled quantum state. An example illustrating this fact is provided. Also see this Viewpoint article, where you can have a free access to download the actual paper. Zz.
 Recognitions: Science Advisor Bernhard Wittmann, Sven Ramelow, Fabian Steinlechner, Nathan K Langford, Nicolas Brunner, Howard M Wiseman, Rupert Ursin and Anton Zeilinger, "Loophole-free Einstein–Podolsky–Rosen experiment via quantum steering", New J. Phys. 14, 053030 (2012). Abstract: Tests of the predictions of quantum mechanics for entangled systems have provided increasing evidence against local realistic theories. However, there remains the crucial challenge of simultaneously closing all major loopholes—the locality, freedom-of-choice and detection loopholes—in a single experiment. An important sub-class of local realistic theories can be tested with the concept of 'steering'. The term 'steering' was introduced by Schrödinger in 1935 for the fact that entanglement would seem to allow an experimenter to remotely steer the state of a distant system as in the Einstein–Podolsky–Rosen (EPR) argument. Einstein called this 'spooky action at a distance'. EPR-steering has recently been rigorously formulated as a quantum information task opening it up to new experimental tests. Here, we present the first loophole-free demonstration of EPR-steering by violating three-setting quadratic steering inequality, tested with polarization-entangled photons shared between two distant laboratories. Our experiment demonstrates this effect while simultaneously closing all loopholes: both the locality loophole and a specific form of the freedom-of-choice loophole are closed by having a large separation of the parties and using fast quantum random number generators, and the fair-sampling loophole is closed by having high overall detection efficiency. Thereby, we exclude—for the first time loophole-free—an important class of local realistic theories considered by EPR. Besides its foundational importance, loophole-free steering also allows the distribution of quantum entanglement secure event in the presence of an untrusted party. The paper can be downloaded for free here and as different EPR experiments and their loopholes are discussed here quite often, I thought it would be a good idea to link it here.
 Mentor Blog Entries: 27 E. Kot et al., "Breakdown of the Classical Description of a Local System", Phys. Rev. Lett., v.08, p.233601 (2012). Abstract: We provide a straightforward demonstration of a fundamental difference between classical and quantum mechanics for a single local system: namely, the absence of a joint probability distribution of the position x and momentum p. Elaborating on a recently reported criterion by Bednorz and Belzig [ Phys. Rev. A 83 052113 (2011)] we derive a simple criterion that must be fulfilled for any joint probability distribution in classical physics. We demonstrate the violation of this criterion using the homodyne measurement of a single photon state, thus proving a straightforward signature of the breakdown of a classical description of the underlying state. Most importantly, the criterion used does not rely on quantum mechanics and can thus be used to demonstrate nonclassicality of systems not immediately apparent to exhibit quantum behavior. The criterion is directly applicable to any system described by the continuous canonical variables x and p, such as a mechanical or an electrical oscillator and a collective spin of a large ensemble. Zz.
 Mentor Blog Entries: 27 J.R. Williams et al., "Unconventional Josephson Effect in Hybrid Superconductor-Topological Insulator Devices", Phys. Rev. Lett. 109, 056803 (2012) Abstract: We report on transport properties of Josephson junctions in hybrid superconducting-topological insulator devices, which show two striking departures from the common Josephson junction behavior: a characteristic energy that scales inversely with the width of the junction, and a low characteristic magnetic field for suppressing supercurrent. To explain these effects, we propose a phenomenological model which expands on the existing theory for topological insulator Josephson junctions. Also see a review of this work at APS Physics where you will have a free download to the paper. Zz.
 Mentor Blog Entries: 27 J. Yin et al., "Quantum teleportation and entanglement distribution over 100-kilometre free-space channels" Nature v.488, p.185 (2012). Abstract: Transferring an unknown quantum state over arbitrary distances is essential for large-scale quantum communication and distributed quantum networks. It can be achieved with the help of long-distance quantum teleportation1, 2 and entanglement distribution. The latter is also important for fundamental tests of the laws of quantum mechanics3, 4. Although quantum teleportation5, 6 and entanglement distribution7, 8, 9 over moderate distances have been realized using optical fibre links, the huge photon loss and decoherence in fibres necessitate the use of quantum repeaters10 for larger distances. However, the practical realization of quantum repeaters remains experimentally challenging11. Free-space channels, first used for quantum key distribution12, 13, offer a more promising approach because photon loss and decoherence are almost negligible in the atmosphere. Furthermore, by using satellites, ultra-long-distance quantum communication and tests of quantum foundations could be achieved on a global scale. Previous experiments have achieved free-space distribution of entangled photon pairs over distances of 600 metres (ref. 14) and 13 kilometres (ref. 15), and transfer of triggered single photons over a 144-kilometre one-link free-space channel16. Most recently, following a modified scheme17, free-space quantum teleportation over 16 kilometres was demonstrated18 with a single pair of entangled photons. Here we report quantum teleportation of independent qubits over a 97-kilometre one-link free-space channel with multi-photon entanglement. An average fidelity of 80.4 ± 0.9 per cent is achieved for six distinct states. Furthermore, we demonstrate entanglement distribution over a two-link channel, in which the entangled photons are separated by 101.8 kilometres. Violation of the Clauser–Horne–Shimony–Holt inequality4 is observed without the locality loophole. Besides being of fundamental interest, our results represent an important step towards a global quantum network. Moreover, the high-frequency and high-accuracy acquiring, pointing and tracking technique developed in our experiment can be directly used for future satellite-based quantum communication and large-scale tests of quantum foundations. Zz.
 J. M. Hill and B. J. Cox, "Einstein's special relativity beyond the speed of light" Proc. R. Soc. A published ahead of print October 3, 2012, (2012) Abstract: We propose here two new transformations between inertial frames that apply for relative velocities greater than the speed of light, and that are complementary to the Lorentz transformation, giving rise to the Einstein special theory of relativity that applies to relative velocities less than the speed of light. The new transformations arise from the same mathematical framework as the Lorentz transformation, displaying singular behaviour when the relative velocity approaches the speed of light and generating the same addition law for velocities, but, most importantly, do not involve the need to introduce imaginary masses or complicated physics to provide well-defined expressions. Making use of the dependence on relative velocity of the Lorentz transformation, the paper provides an elementary derivation of the new transformations between inertial frames for relative velocities v in excess of the speed of light c, and further we suggest two possible criteria from which one might infer one set of transformations as physically more likely than the other. If the energy–momentum equations are to be invariant under the new transformations, then the mass and energy are given, respectively, by the formulae $m=(p_\infty/c)[(v/c)^2-1]^{-1/2}$ and $\mathcal{E}=mc^2$ where $p_\infty$ denotes the limiting momentum for infinite relative velocity. If, however, the requirement of invariance is removed, then we may propose new mass and energy equations, and an example having finite non-zero mass in the limit of infinite relative velocity is given. In this highly controversial topic, our particular purpose is not to enter into the merits of existing theories, but rather to present a succinct and carefully reasoned account of a new aspect of Einstein's theory of special relativity, which properly allows for faster than light motion. I'll post the full citation when it officially gets published.