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Recent Noteworthy Physics Papers

  1. Oct 18, 2013 #161

    ZapperZ

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    A. D. Wissner-Gross and C. E. Freer, "Causal Entropic Forces", Phys. Rev. Lett. v.110, p.168702 (2013).

    Abstract: Recent advances in fields ranging from cosmology to computer science have hinted at a possible deep connection between intelligence and entropy maximization, but no formal physical relationship between them has yet been established. Here, we explicitly propose a first step toward such a relationship in the form of a causal generalization of entropic forces that we find can cause two defining behaviors of the human “cognitive niche”—tool use and social cooperation—to spontaneously emerge in simple physical systems. Our results suggest a potentially general thermodynamic model of adaptive behavior as a nonequilibrium process in open systems.

    Read the synopsis in APS Physics.

    Zz.
     
  2. Nov 15, 2013 #162

    DennisN

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    Andreas Reiserer, Stephan Ritter, Gerhard Rempe
    Nondestructive Detection of an Optical Photon
    Science DOI: 10.1126/science.1246164 (2013)
    http://www.sciencemag.org/content/early/2013/11/13/science.1246164
    Paper on arxiv: http://arxiv-web3.library.cornell.edu/abs/1311.3625

    Abstract:
    All optical detectors to date annihilate photons upon detection, thus excluding repeated measurements. Here, we demonstrate a robust photon detection scheme which does not rely on absorption. Instead, an incoming photon is reflected off an optical resonator containing a single atom prepared in a superposition of two states. The reflection toggles the superposition phase which is then measured to trace the photon. Characterizing the device with faint laser pulses, a single-photon detection efficiency of 74% and a survival probability of 66% is achieved. The efficiency can be further increased by observing the photon repeatedly. The large single-photon nonlinearity of the experiment should enable the development of photonic quantum gates and the preparation of novel quantum states of light.
     
  3. Dec 9, 2013 #163

    Cthugha

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    Jörn Dunkel and Stefan Hilbert, "Consistent thermostatistics forbids negative absolute temperatures", Nature Physics, advanced online publication, doi:10.1038/nphys2831 (2013).

    Abstract: Over the past 60 years, a considerable number of theories and experiments have claimed the existence of negative absolute temperature in spin systems and ultracold quantum gases. This has led to speculation that ultracold gases may be dark-energy analogues and also suggests the feasibility of heat engines with efficiencies larger than one. Here, we prove that all previous negative temperature claims and their implications are invalid as they arise from the use of an entropy definition that is inconsistent both mathematically and thermodynamically. We show that the underlying conceptual deficiencies can be overcome if one adopts a microcanonical entropy functional originally derived by Gibbs. The resulting thermodynamic framework is self-consistent and implies that absolute temperature remains positive even for systems with a bounded spectrum. In addition, we propose a minimal quantum thermometer that can be implemented with available experimental techniques.

    The authors argue that there are important differences between the definitions of entropy given by Gibbs and the approximate one given by Boltzmann. They show that only the Boltzmann version gives a negative absolute temperature as sometimes reported in experiments on nuclear spin systems. The authors then argue that only the Gibbs version of entropy is fully consistent and make it clear that the negative Boltzmann temperature cannot have the meaning of a thermodynamic temperature.

    See also the news and views article in the same issue: http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys2831.html
     
  4. Jan 20, 2014 #164

    Cthugha

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    Jianyong Cen, Ping Yuan, and Simin Xue, "Observation of the Optical and Spectral Characteristics of Ball Lightning", Phys. Rev. Lett. v.112, p. 035001 (2014).

    Abstract: Ball lightning (BL) has been observed with two slitless spectrographs at a distance of 0.9 km. The BL is generated by a cloud-to-ground lightning strike. It moves horizontally during the luminous duration. The evolution of size, color, and light intensity is reported in detail. The spectral analysis indicates that the radiation from soil elements is present for the entire lifetime of the BL.

    Yes, this paper is about the mysterious natural ball lightning and it appears in a quality journal. Looks like this ball lightning was a dirty fellow. The researchers found lots of spectral lines you would expect from the elements present in the soil. Also see this focus article, which summarizes the paper.
     
  5. Jan 30, 2014 #165
    M. W. Ray, E. Ruokokoski, S. Kandel, M. Möttönen & D. S. Hall,"Observation of Dirac monopoles in a synthetic magnetic field", Nature 505, 657-660 (2014),
    doi:10.1038/nature12954

    Magnetic monopoles—particles that behave as isolated north or south magnetic poles—have been the subject of speculation since the first detailed observations of magnetism several hundred years ago1. Numerous theoretical investigations and hitherto unsuccessful experimental searches2 have followed Dirac’s 1931 development of a theory of monopoles consistent with both quantum mechanics and the gauge invariance of the electromagnetic field3. The existence of even a single Dirac magnetic monopole would have far-reaching physical consequences, most famously explaining the quantization of electric charge3, 4. Although analogues of magnetic monopoles have been found in exotic spin ices5, 6 and other systems7, 8, 9, there has been no direct experimental observation of Dirac monopoles within a medium described by a quantum field, such as superfluid helium-3 (refs 10, 11, 12, 13). Here we demonstrate the controlled creation14 of Dirac monopoles in the synthetic magnetic field produced by a spinor Bose–Einstein condensate. Monopoles are identified, in both experiments and matching numerical simulations, at the termini of vortex lines within the condensate. By directly imaging such a vortex line, the presence of a monopole may be discerned from the experimental data alone. These real-space images provide conclusive and long-awaited experimental evidence of the existence of Dirac monopoles. Our result provides an unprecedented opportunity to observe and manipulate these quantum mechanical entities in a controlled environment.


    Haven't been active in these forums for a while, but a colleague brought this paper to my attention and I felt like sharing to see if it might interest someone :D
     
  6. Jan 31, 2014 #166

    ZapperZ

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    A review of this paper can be found at PhysicsWorld:

    http://physicsworld.com/cws/article/news/2014/jan/30/magnetic-monopoles-seen-in-the-lab

    Note that both this, and the earlier claim of magnetic monopole physics observation in spin-ice, are both condensed matter systems. This is just another example where the so-called "applied" field of physics is actually providing insight on fundamental physics.

    Zz.
     
  7. Apr 8, 2014 #167

    ZapperZ

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    Y-C. Lee, M-H. Hsieh, S.T. Flammia, and R-K. Lee "Local PT Symmetry Violates the No-Signaling Principle", Phys. Rev. Lett. 112, 130404 (2014).

    Abstract: Bender et al. [Phys. Rev. Lett. 80, 5243 (1998)] have developed PT-symmetric quantum theory as an extension of quantum theory to non-Hermitian Hamiltonians. We show that when this model has a local PT symmetry acting on composite systems, it violates the nonsignaling principle of relativity. Since the case of global PT symmetry is known to reduce to standard quantum mechanics A. Mostafazadeh [J. Math. Phys. 43, 205 (2001)], this shows that the PT-symmetric theory is either a trivial extension or likely false as a fundamental theory.

    A review of this can be found at APS Physics.

    Zz.
     
  8. Apr 24, 2014 #168

    ZapperZ

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    T. Grover, et al., "Emergent Space-Time Supersymmetry at the Boundary of a Topological Phase" Science v.344, p.280 (2014).

    Abstract: In contrast to ordinary symmetries, supersymmetry (SUSY) interchanges bosons and fermions. Originally proposed as a symmetry of our universe, it still awaits experimental verification. Here, we theoretically show that SUSY emerges naturally in condensed matter systems known as topological superconductors. We argue that the quantum phase transitions at the boundary of topological superconductors in both two and three dimensions display SUSY when probed at long distances and times. Experimental consequences include exact relations between quantities measured in disparate experiments and, in some cases, exact knowledge of the universal critical exponents. The topological surface states themselves may be interpreted as arising from spontaneously broken SUSY, indicating a deep relation between topological phases and SUSY.

    This is another example where a so-called "applied" field of physics (condensed matter) is providing insight into fundamental physics. Condensed matter systems had already discovered analogs of Majorana fermions and magnetic monopoles.

    Zz.
     
  9. May 6, 2014 #169

    Cthugha

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    T. Peng, et al., "Delayed-Choice Quantum Eraser with Thermal Light" Phys. Rev. Lett. 112, 180401 (2014).

    Abstract: We report a random delayed-choice quantum eraser experiment. In a Young’s double-slit interferometer, the which-slit information is learned from the photon-number fluctuation correlation of thermal light. The reappeared interference indicates that the which-slit information of a photon, or wave packet, can be “erased” by a second photon or wave packet, even after the annihilation of the first. Different from an entangled photon pair, the jointly measured two photons, or wave packets, are just two randomly distributed and randomly created photons of a thermal source that fall into the coincidence time window. The experimental observation can be explained as a nonlocal interference phenomenon in which a random photon or wave packet pair, interferes with the pair itself at distance.

    The delayed choice quantum eraser is a topic which comes up quite often in this forum. It is frequently seen that people tend to assume strange things like "changing the past" in order to explain the experiment. Here is a completely classical version of the experiment using just classical light which shows that there is absolutely nothing mystical going on and everything can be explained in terms of standard physics.
     
  10. Sep 5, 2014 #170

    Andy Resnick

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    G. B. Lemos, V. Borish, G. D. Cole, S. Ramelow, R. Lapkiewicz, and A. Zeilinger, "Quantum imaging with undetected photons," Nature 512, 409-412 (2014).

    "Information is central to quantum mechanics. In particular, quantum interference occurs only if there exists no information to distinguish between the superposed states. The mere possibility of obtaining information that could distinguish between overlapping states inhibits quantum interference1, 2. Here we introduce and experimentally demonstrate a quantum imaging concept based on induced coherence without induced emission3, 4. Our experiment uses two separate down-conversion nonlinear crystals (numbered NL1 and NL2), each illuminated by the same pump laser, creating one pair of photons (denoted idler and signal). If the photon pair is created in NL1, one photon (the idler) passes through the object to be imaged and is overlapped with the idler amplitude created in NL2, its source thus being undefined. Interference of the signal amplitudes coming from the two crystals then reveals the image of the object. The photons that pass through the imaged object (idler photons from NL1) are never detected, while we obtain images exclusively with the signal photons (from NL1 and NL2), which do not interact with the object. Our experiment is fundamentally different from previous quantum imaging techniques, such as interaction-free imaging5 or ghost imaging6, 7, 8, 9, because now the photons used to illuminate the object do not have to be detected at all and no coincidence detection is necessary. This enables the probe wavelength to be chosen in a range for which suitable detectors are not available. To illustrate this, we show images of objects that are either opaque or invisible to the detected photons. Our experiment is a prototype in quantum information—knowledge can be extracted by, and about, a photon that is never detected."

    The last few sentences of the abstract are particularly intriguing.
     
  11. Sep 5, 2014 #171

    DennisN

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    Yes, thanks for the info! :smile:
    I add links here to the paper mentioned above:
    Arxiv link: Quantum Imaging with Undetected Photons: http://arxiv.org/abs/1401.4318
    Gabriela B. Lemos, Victoria Borish, Garrett D. Cole, Sven Ramelow, Radek Lapkiewicz, Anton Zeilinger (Submitted on 17 Jan 2014 (v1), last revised 27 Jan 2014 (this version, v2))
    Nature link: http://www.nature.com/nature/journal/v512/n7515/full/nature13586.html
     
  12. Jan 21, 2015 #172

    ZapperZ

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    C. Robens et al., "Ideal Negative Measurements in Quantum Walks Disprove Theories Based on Classical Trajectories" Phys. Rev. X 5, 011003 (2015).

    Abstract: We report on a stringent test of the nonclassicality of the motion of a massive quantum particle, which propagates on a discrete lattice. Measuring temporal correlations of the position of single atoms performing a quantum walk, we observe a 6σ violation of the Leggett-Garg inequality. Our results rigorously excludes (i.e., falsifies) any explanation of quantum transport based on classical, well-defined trajectories. We use so-called ideal negative measurements—an essential requisite for any genuine Leggett-Garg test—to acquire information about the atom’s position, yet avoiding any direct interaction with it. The interaction-free measurement is based on a novel atom transport system, which allows us to directly probe the absence rather than the presence of atoms at a chosen lattice site. Beyond the fundamental aspect of this test, we demonstrate the application of the Leggett-Garg correlation function as a witness of quantum superposition. Here, we employ the witness to discriminate different types of walks spanning from merely classical to wholly quantum dynamics.

    Don't miss a very interesting review of this work here. That link also provides a link to a free copy to the paper.

    Zz.
     
  13. Feb 6, 2015 #173

    ZapperZ

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    L. Tao et al., "Silicene field-effect transistors operating at room temperature", Nature Nanotechnology http://dx.doi.org/10.1038/NNANO.2014.325[/URL] (2015).

    Abstract: [i]Free-standing silicene, a silicon analogue of graphene, has a buckled honeycomb lattice[URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref1[/URL] and, because of its Dirac bandstructure[URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref2[/URL], [URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref3[/URL] combined with its sensitive surface, offers the potential for a widely tunable two-dimensional monolayer, where external fields and interface interactions can be exploited to influence fundamental properties such as bandgap[URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref4[/URL] and band character[URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref5[/URL] for future nanoelectronic devices[URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref6[/URL], [URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref7[/URL]. The quantum spin Hall effect[URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref3[/URL], chiral superconductivity[URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref8[/URL], giant magnetoresistance[URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref9[/URL] and various exotic field-dependent states[URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref7[/URL] have been predicted in monolayer silicene. Despite recent progress regarding the epitaxial synthesis of silicene[URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref8[/URL], [URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref9[/URL], [URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref10[/URL] and investigation of its electronic properties[URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref11[/URL], [URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref13[/URL], [URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref14[/URL], [URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref15[/URL], to date there has been no report of experimental silicene devices because of its air stability issue[URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref16[/URL]. Here, we report a silicene field-effect transistor, corroborating theoretical expectations regarding its ambipolar Dirac charge transport[URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref17[/URL], with a measured room-temperature mobility of ∼100 cm2 V–1 s–1 attributed to acoustic phonon-limited transport[URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref18[/URL] and grain boundary scattering. These results are enabled by a growth–transfer–fabrication process that we have devised—silicene encapsulated delamination with native electrodes. This approach addresses a major challenge for material preservation of silicene during transfer and device fabrication and is applicable to other air-sensitive two-dimensional materials such as germanene[URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref2[/URL], [URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref3[/URL], [URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref4[/URL] and phosphorene[URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref19[/URL], [URL]http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2014.325.html#ref20[/URL]. Silicene's allotropic affinity with bulk silicon and its low-temperature synthesis compared with graphene or alternative two-dimensional semiconductors suggest a more direct integration with ubiquitous semiconductor technology.[/i]

    This is the first proof-of-principle demonstration of a transistor made with this material. Read a [URL='http://www.nature.com/news/graphene-s-cousin-silicene-makes-transistor-debut-1.16839']review of this work here[/URL].

    Zz.
     
    Last edited by a moderator: May 7, 2017
  14. Feb 19, 2015 #174
    Uhmm . . . ZZ it Seems like the link is not working anymore. :(
     
    Last edited by a moderator: May 7, 2017
  15. Mar 3, 2015 #175

    ZapperZ

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    X-L. Wang et al. "Quantum teleportation of multiple degrees of freedom of a single photon" Nature 518, 516 (2015).

    Abstract: Quantum teleportationhttp://www.nature.com/nature/journal/v518/n7540/full/nature14246.html#ref1 provides a ‘disembodied’ way to transfer quantum states from one object to another at a distant location, assisted by previously shared entangled states and a classical communication channel. As well as being of fundamental interest, teleportation has been recognized as an important element in long-distance quantum communicationhttp://www.nature.com/nature/journal/v518/n7540/full/nature14246.html#ref2, distributed quantum networkshttp://www.nature.com/nature/journal/v518/n7540/full/nature14246.html#ref3 and measurement-based quantum computationhttp://www.nature.com/nature/journal/v518/n7540/full/nature14246.html#ref4, http://www.nature.com/nature/journal/v518/n7540/full/nature14246.html#ref5. There have been numerous demonstrations of teleportation in different physical systems such as photonshttp://www.nature.com/nature/journal/v518/n7540/full/nature14246.html#ref6, http://www.nature.com/nature/journal/v518/n7540/full/nature14246.html#ref7, http://www.nature.com/nature/journal/v518/n7540/full/nature14246.html#ref8, atomshttp://www.nature.com/nature/journal/v518/n7540/full/nature14246.html#ref9, ionshttp://www.nature.com/nature/journal/v518/n7540/full/nature14246.html#ref10, http://www.nature.com/nature/journal/v518/n7540/full/nature14246.html#ref11, electronshttp://www.nature.com/nature/journal/v518/n7540/full/nature14246.html#ref12 and superconducting circuitshttp://www.nature.com/nature/journal/v518/n7540/full/nature14246.html#ref13. All the previous experiments were limited to the teleportation of one degree of freedom only. However, a single quantum particle can naturally possess various degrees of freedom—internal and external—and with coherent coupling among them. A fundamental open challenge is to teleport multiple degrees of freedom simultaneously, which is necessary to describe a quantum particle fully and, therefore, to teleport it intact. Here we demonstrate quantum teleportation of the composite quantum states of a single photon encoded in both spin and orbital angular momentum. We use photon pairs entangled in both degrees of freedom (that is, hyper-entangled) as the quantum channel for teleportation, and develop a method to project and discriminate hyper-entangled Bell states by exploiting probabilistic quantum non-demolition measurement, which can be extended to more degrees of freedom. We verify the teleportation for both spin–orbit product states and hybrid entangled states, and achieve a teleportation fidelity ranging from 0.57 to 0.68, above the classical limit. Our work is a step towards the teleportation of more complex quantum systems, and demonstrates an increase in our technical control of scalable quantum technologies.

    See a review of this work at PhysicsWorld. Strangely enough, there is also free access to the actual paper, as of now (not sure if this is part of Nature's open access paper, or if this is available only for a limited time). So get it while you can!

    Zz.
     
    Last edited by a moderator: May 7, 2017
  16. May 29, 2015 #176

    Andy Resnick

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    A. G. Manning et. al., "Wheeler's delayed-choice gedanken experiment with a single atom" Nature Physics (2015) doi:10.1038/nphys3343

    Abstract:
    The wave–particle dual nature of light and matter and the fact that the choice of measurement determines which one of these two seemingly incompatible behaviours we observe are examples of the counterintuitive features of quantum mechanics. They are illustrated by Wheeler’s famous ‘delayed-choice’ experimenthttp://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3343.html#ref1, recently demonstrated in a single-photon experimenthttp://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3343.html#ref2. Here, we use a single ultracold metastable helium atom in a Mach–Zehnder interferometer to create an atomic analogue of Wheeler’s original proposal. Our experiment confirms Bohr’s view that it does not make sense to ascribe the wave or particle behaviour to a massive particle before the measurement takes placehttp://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3343.html#ref1. This result is encouraging for current work towards entanglement and Bell’s theorem tests in macroscopic systems of massive particleshttp://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3343.html#ref3.

    A delayed-choice experiment using massive particles.
     
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  17. Aug 12, 2015 #177

    ZapperZ

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    Did you read the very FIRST post in this thread?

    Zz.
     
  18. Aug 13, 2015 #178

    ZapperZ

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    Then you should always read the first page of any thread.

    Zz.
     
  19. Aug 14, 2015 #179

    mfb

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    We shouldn't forget the pentaquarks:

    Observation of J/ψp Resonances Consistent with Pentaquark States in Λ0b → J/ψKp Decays
    R. Aaij et al. (LHCb Collaboration)
    Phys. Rev. Lett. 115, 072001
     
  20. Aug 16, 2015 #180

    DennisN

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    What's the matter with antimatter? Well...

    S. Ulmer et al., High-precision comparison of the antiproton-to-proton charge-to-mass ratio, Nature 524, 196–199 (13 August 2015) doi:10.1038/nature14861
    Paper available for free here: http://www.nature.com/nature/journal/v524/n7564/full/nature14861.html

    Abstract:
    Invariance under the charge, parity, time-reversal (CPT) transformation is one of the fundamental symmetries of the standard model of particle physics. This CPT invariance implies that the fundamental properties of antiparticles and their matter-conjugates are identical, apart from signs. There is a deep link between CPT invariance and Lorentz symmetry—that is, the laws of nature seem to be invariant under the symmetry transformation of spacetime—although it is model dependent. A number of high-precision CPT and Lorentz invariance tests—using a co-magnetometer, a torsion pendulum and a maser, among others—have been performed, but only a few direct high-precision CPT tests that compare the fundamental properties of matter and antimatter are available. Here we report high-precision cyclotron frequency comparisons of a single antiproton and a negatively charged hydrogen ion (H) carried out in a Penning trap system. From 13,000 frequency measurements we compare the charge-to-mass ratio for the antiproton nature14861-m1.jpg to that for the proton nature14861-m2.jpg and obtain nature14861-m3.jpg . The measurements were performed at cyclotron frequencies of 29.6 megahertz, so our result shows that the CPT theorem holds at the atto-electronvolt scale. Our precision of 69 parts per trillion exceeds the energy resolution of previous antiproton-to-proton mass comparisons as well as the respective figure of merit of the standard model extension by a factor of four. In addition, we give a limit on sidereal variations in the measured ratio of <720 parts per trillion. By following the arguments of ref. 11, our result can be interpreted as a stringent test of the weak equivalence principle of general relativity using baryonic antimatter, and it sets a new limit on the gravitational anomaly parameter of nature14861-m4.jpg < 8.7 × 10−7.
     
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