Extracts from the Internet

Doubly charmed baryon

The Ξ++cc baryon including two c-quarks and a u-quark was registered for the first time in the LHCb experiment. The existence of this baryon had been predicted by the quark model. It was born in pp collisions of energy 13 TeV and decayed through weak interactions along the channel Ξ++cc → Λ+cK-π+π+, Λ+c → pK-π+. The Ξ++cc baryon corresponded to the peak in the invariant-mass distribution of the number of events, the statistical significance of Ξ++cc registration exceeding 12 σ. The measured baryon mass ≈ 3621 MeV c-2 was well consistent with the theoretical calculations, and the peak for the same mass was also confirmed by the LHCb data at an energy of 8 TeV. The SELEX collaboration had already reported the registration of a baryon with two heavy quarks Ξ+cc (with quark composition [ccd]), but this result had not been confirmed in independent experiments. The reliable Ξ++cc registration by LHCb is important for verification of the complicated calculations carried out in the framework of quantum chromodynamics. Theoretically, the Ξ++cc baryon has a planetary structure, namely, the light quark u revolves at a certain distance from the compact pair of heavy c-quarks (a diquark). Russian researchers from several scientific institutions have participated in LHCb collaboration. For baryons with two heavy quarks see the review by V.V. Kiselev and A.K. Likhoded in Physics-Uspekhi 45 455 (2002). Source: arXiv:1707.01621 [hep-ex]

Satellite-based transmission of quantum-entangled photons

J.-W. Pan (University of Science and Technology of China) and his colleagues fulfilled the distribution of photon pairs in quantum entangled state from the Chinese Micius space-born satellite onto three ground-based stations. The photons were distributed among the receiving stations on Earth separated by 1203 km, and with allowance for the satellite-to-ground distances the photons covered in total from 1600 to 2400 km. Earlier, quantum entangled photons could only be transmitted at distances up to ≈ 100 km because of the loss in optical-fiber and air communication lines. On board the satellite, the photons from the laser diode split into pairs in a nonlinear crystal to form photons in polarization-entangled states with a random choice among several polarization directions. The satellite- and ground-based stations had telescopes to transmit and receive photons. After reception of photons the Bell test was performed to show that the quantum precision of photon entanglement is f ≥ 0.87 ± 0.09. In another experiment, J.-W. Pan et al. accomplished quantum teleportation of particle states to a distance of 1400 km from the high-altitude Tibet station to the same Micius satellite. And Japanese researchers under the guidance of M. Sasaki (National Institute of Information and Communications Technology, Japan) carried out quantum communication – a signal with protocol of quantum distribution of keys was transmitted from the 48-kg microsatellite SOCRATES to the Earth. These experiments are of importance for practical applications in quantum communication and quantum cryptography. Sources: Science 356 1140 (2017), arXiv:1707.00934 [quant-ph],Nature Photonics 11 502 (2017)

Retrocausality and time symmetry

In their theoretical paper, M.S. Leifer (Chapman University, USA) and M.F. Pusey (Perimeter Institute of Theoretical Physics, Canada) clarified the interrelation between the symmetry of physical processes under time reversal and the retrocausality conception which is discussed as an alternative of action at a distance in quantum mechanics. The idea of retrocausality is closely connected with the question of reality of quantum states. Within this conception, future events exert certain influence upon past events. For instance, the choice of experimental conditions in future influences the state of the measured system in the past. However, signaling from future to the past in this way is impossible. M.S. Leifer and M.F. Pusey somewhat modified and generalized H. Price’s arguments claiming that the conditions of quantum state reality and time symmetry admit the effect of retrocausality. In the framework of retrocausality conception, they formulated timelike analogs of Bell’s test and found conditions when quantum processes are symmetric under time reversal. For irreversibility in quantum mechanics see the paper by B.B. Kadomtsev in Physics-Uspekhi 46 1183 (2003). Source: Proc. R. Soc. of London A 473 20160607 (2017)

Qubit-based quantum Maxwell demon

In their experiment, N. Cottet (Sorbonne University, France) with colleagues realized for the first time the conception of Maxwell demon operating in quantum regime with quantum superpositions of microwave photons and from measurements of its state observed the information-to-work conversion. The role of demon was played by a microwave cavity that traced the state of the related superconducting qubit. The microwave pulses passed through the cavity, and it could only do work (transfer energy to photons under stimulated emission) if the qubit was in the lowest energy state. On the contrary, if the qubit was excited, the cavity frequency was shifted, and the cavity blocked the pulses. Thus, the Maxwell demon randomly let pulses pass, the occupation numbers of the Fock states of the cavity itself varying. The quantum tomography method was used to reconstruct the cavity density matrix with respect to the photon occupation numbers. It provided information on the demon memory, which has its energy cost. As was theoretically expected, the increase in the entropy corresponding to the demon memory indemned for the decrease in the system entropy. For the quantum information see the book of B.B. Kadomtsev “Dynamics and information” and the review in Physics-Uspekhi 37 425 (1994). Source: Proc. Nat. Ac. Sci. 1704827114 (2017)

Negative effective mass and quantum measurements

If the coordinates and momenta of a positive-effective-mass (meff) system are determined relative to the corresponding variables of another system with meff<0, the difference of these quantities will be commutative quantum variables. These can be measured simultaneously as distinct from absolute coordinates and momenta for which the measurement accuracy is limited to the Heisenberg uncertainty principle. The researchers from the Niels Bohr Institute of the University of Copenhagen guided by E. Polzik were the first to demonstrate this method of measurements in their experiment. Two optically coupled oscillators were used, namely, a 0.5-mm dielectric membrane with a large mechanical Q factor and an ensemble of ≈ 109 cesium atoms in a magnetic field within an optical cell which served as a spin oscillator. The ensemble could be transferred to the state with spins directed opposite to the magnetic field, and in this case the spin oscillator had meff<0. A laser beam passed sequentially through the ensemble of particles and lighted up the membrane, thus initiating optomechanical coupling between them. And the membrane oscillation spectrum was measured by the reflection of pulses from another laser. The inverse influence exerted by the measurement process was partially compensated when meff<0, which increased the measurement precision by 1.8 dB. This method can be applied in measuring devices operating close to the standard quantum limit. Source: Nature 547 191 (2017)

Quantum Zeno and anti-Zeno effects

In their experiment with a superconducting qubit, P.M. Harrington, J.T. Monroe, and K.W. Murch (Washington University in St-Louis) demonstrated both the quantum Zeno effect slowing down the quantum transition between qubit levels owing to a sequence of frequent measurements and the anti-Zeno effect accelerating the transition. The anti-Zeno effect in the experiment with a unit qubit was observed for the first time. The superconducting qubit was placed into an electromagnetic cavity, and the states of the qubit were determined and the time before their decay was measured from the shifts of the test pulse phases in the cavity. Given this, the quantum Zeno effect showed up according to the theory. Then the experimental conditions were changed: the qubit was affected by electromagnetic noise with a specially structured spectrum. The presence of noise changes the number of final states accessible for quantum transition. The decrease in the number of states suppresses the transition, whereas the increase leads to its acceleration. Either Zeno or anti-Zeno effect occurred in the experiment depending on the difference between the cavity eigenfrequency and the average noise frequency. The experiment was repeated when “quasi-measurements” were performed that randomized the Berry phase to misphase the state of the system, but the information on the results of measurements was not transferred to the external world. The Zeno and anti-Zeno effects were observed in this case, too. Source: Phys. Rev. Lett. 118 240401 (2017)

Higgs mode in a two-dimensional antiferromagnet

Near a quantum critical point, instabilities (order parameter fluctuations) occur in crystals whose properties resemble those of the Higgs field in the Standard Model of elementary particles. The Higgs excited modes near the quantum critical point have already been observed in three-dimensional crystals. However, in 2D systems they are typically strongly unstable and decay rapidly into pairs of Nambu-Goldstone modes, for which reason they could not be identified. T. Hong (Oak Ridge National Laboratory, USA) with colleagues used the neutron scattering method to observe for the first time the Higgs mode near the quantum critical point in the two-dimensional antiferromagnet C9H18N2CuBr4 in which this mode is stable. From the viewpoint of the theory the antiferromagnet structure can be described by a set of lattices with spins S=1/2 in the lattice sites. The experiment showed good agreement of the Higgs mode properties with this model, and the numerical values of the Hamiltonian system parameters were measured. Source: Nature Physics 13 638 (2017)

Observation of individual-atom diffusion

A. Widera (University of Kaiserslautern and Graduate School Material Science in Mainz, Germany) and his colleagues observed the diffusion of individual 133Cs atoms in rarefied ultracold gas of 87Rb atoms in an optical trap. The 133Cs atoms were directed to the trap where they impinged on 87Rb atoms and gradually slowing down found themselves in thermal equilibrium with them. Laser excitation of 133Cs atoms and registration of the fluorescent light coming from them were used to determine the spatial shifts of the atoms in different time intervals after the beginning of motion, which made it possible to find the statistical distribution of atoms. It had a bimodal form as part of 133Cs atoms failed to collide before the observation. The entrapped 87Rb gas was nonuniform, which allowed a simultaneous examination of many realizations with different Knudsen numbers from Kn≈ 1 in the trap center to large Kn at the edge. It turned out that within a wide range of Kn after only few collisions with 87Rb atoms the ensemble of 133Cs atoms is already well described by the Langevin equation with a velocity-dependent friction factor. Source: Phys. Rev. Lett. 118 263401 (2017)

Tokamak with lithium-coated walls

D.P. Boyle (Princeton Plasma Physics Laboratory, USA) with colleagues performed a tokamak type experiment in which the chamber inner walls were lithium coated. This permitted the plasma temperature near the walls to be heightened compared to ordinary tokamaks in which divertors are used to remove the external layers of plasma filament. In the new experiment, a lithium layer ≈ 75-100 nm thick was sputtered onto the inner walls of a spherical tokamak. The plasma was diagnosed, i.e., its temperature profile was measured by the method of Thomson scattering of optical photons by ions during an electric charge in plasma. The results of measurements showed for the first time that in the presence of lithium the temperature profile along the chamber cross section flattens, i.e., the temperature levels up in the central nucleus and at the walls. This effect was predicted in 2003 by S.I. Krasheninnikov, L.E. Zakharov, and G.V. Pereverzev as being due to the fact that lithium bonds the hydrogen atoms into lithium hydride. This reduces their reverse flux and decreases the temperature gradient and plasma turbulence. In future, this effect may lead to an increase of the energy output in tokamaks. For the development of tokamaks see B.B. Kadomtsev’s paper in Physics-Uspekhi 39 419 (1996). Source: Phys. Rev. Lett. 119 015001 (2017)

Binary supermassive black hole

The radio galaxy 0402+379 which contains two supermassive black holes (SMBH) rotating around each other was observed with VLBA radio telescopes of the National Radio Astronomical Observatory NRAO. The binary SMBH can have been formed upon merge of two galaxies. Such merges are typical episodes of evolution of most galaxies, especially in dense clusters, but it is only few galaxies with binary SMBHs that are known to date. The two SMBHs located near the radiation brightness peaks (cores) are spaced by nearly 7.3 pc. Their orbit is the closest among the orbits of those binary SMBHs that were spatially resolved. Galaxy 0402+379 was observed at frequencies of 5, 8, 15 and 22 GHz; the frequency dependence of the visible distance between the cores was revealed and the evidence of the proper motion of the cores was possibly obtained. The former effect, which was predicted by A. Konigl and A.P. Lobanov, is associated with the structure of jets and magnetic fields at their origin. The latter effect shows that it was possibly for the first time that the proper orbital motion of SMBH in the binary was observed in this experiment. The period of their revolution – 30 thousand years was estimated by the velocity of their motion, the summed SMBH mass amounting to ≈ 15 × 109M. Source: The Astrophysical Journal 843 14 (2017)

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The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) — the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics.

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