Extracts from the Internet


Heat transport through a quantum valve

The team of researchers from Aalto University (Finland) headed by J. Pekola designed a device which can become a platform for the study of some quantum thermodynamic phenomena. The device consists of two metal resistors (heat reservoirs) coupled with two identical microwave resonators. Between the resonators is a superconducting transmon-type qubit showing capacitive coupling with them. The possibility of photon propagation from one resistor to the other through the qubit depends on its quantum state, i.e., the qubit plays the role of quantum valve. Since the photons transport heat, the heat transport between resistors-reservoirs can be examined at the quantum level. The transport efficiency depends on the ratio of resonator frequencies to the set of qubit eigenfrequencies and on the strength of couplings between the qubit, the resonators, and the reservoirs. The results are qualitatively dissimilar for different coupling regimes. Investigated in the experiment was the transferred power at different temperatures depending on the magnitude of the magnetic flux that affects the oscillation frequency in the transmon qubit. The device operation was shown to agree with the theoretical calculations. Such a valve can be of use, in particular, for designing quantum heatengines. Source: Nature Physics, online publication of July 9, 2018

Quantum entangled state of 18 qubits

Creation of quantum entanglement of an increasingly large number of particles and a coherent control of their state is one of the central subjects of research in quantum informatics for it allows realization of more powerful quantum algorithms. In their experiment, X.-L. Wang (the University of Science and Technology of China) with colleagues obtained for the first time an entangled state of 18 qubits. Entanglement over several degrees of freedom, referred to as hyper-entanglement, was obtained on trajectories, polarization, and orbital angular momentum of six photons. An entangled state of 14 ions (one degree of freedom per ion) and a 10-qubit entangled state of 5 photons (two degrees of freedom per photon) were created earlier. X.-L. Wang et al. used the method of parametric down conversion in a nonlinear crystal to create at first three pairs of photons in entangled states. The photons passed through splitters and united in a common single-mode optical fiber becoming hyper-entangled and having 262144 states in toto. Measurement of the states of an individual qubit and a verification of general entanglement is the problem no less complicated than the creation of entanglement. These measurements were conducted in three stages: using Mach – Zander interferometers, measurement of polarization, and measurement of the orbital angular momentum with the help of splitters, prisms, and interferometers. This was how the state of each qubit was individually controlled. The device had 30 interferometers and 48 one-photon detectors. The statistical significance of entanglement of 18 qubits exceeded 13 σ. Source: Phys. Rev. Lett. 120 260502 (2018)

High thermal conductivity of BAs crystals

High thermal conductivity is needed for cooling microelectronic devices. Although possessing a record thermal conductivity, the diamondis not relevant for this purpose because its thermal expansion coefficient differs strongly from the corresponding coefficient of microelectronic elements. The theoretical calculations performed in 2003 by L. Lindsay, D.A. Broido, and T.L. Reinecke pointed out that boron arsenide BAs single crystals must have high thermal conductivity owing to certain properties of their phonon spectrum. L. Shi (the University of Texas at Austin, USA) and his colleagues synthesized top-quality 4 × 2 × 1 mm3 BAs crystals and confirmed their high thermal conductivity up to 1000 W m-1 K-1, which is much higher than, for example, that of copper (400 W m-1 K-1). The measurements were carried out by local pulsed laser heating and by contact heating; the stationary temperature distribution about the surface was also measured. The calculations show the important role of four-phonon interactions in BAs thermal conductivity. The allowance for these interactions lowers thermal conductivity compared to the three-phonon approximation, but the calculated value remains a little higher than the experimentally measured one. In another independent experiment, Y. Hu (the University of California, Los Angeles, USA) and his colleagues, too, synthesized high-quality BAs single crystals and measured their room-temperature thermal conductivity which proved to equal 1300 W m-1 K-1. The spectroscopic data indicate that BAs shows a long free path of phonons and strong high-order anharmonicity in the four-phonon process. Source: Science, online publication of July 5, 2018, Science, online publication of July 5, 2018

Collective spin modes in a gas

Collective effects in quantum gases are due to atomic interaction. S. Lepoutre (Paris-13 University, France) with colleagues investigated the collective behavior of chromium atoms in a trap in the state of Bose – Einstein condensate. The bonds of atoms through their spins and orbital angular momenta was reached with the help of the gradient of the magnetic field generated by several solenoids. The earlier unobserved collective spin oscillations of a spin gas were revealed. The atomic states were measured using Stern – Gerlach separation upon trap potential switch-off. Initially, the radio frequency pulse was used to turn all the spins perpendicular to the external magnetic field so that they began precessing. If the atoms did not interact, then in an inhomogeneous magnetic field in different places they would precess independently at different frequencies around nonparallel axes. But in the given experiment, the precession axes kept the initial position and it was only the oscillation amplitude that depended on position, which was indicative of a collective effect associated with spontaneous generation of the trapped magnon mode. The experimental results are well consistent with the theoretical study involving the solution of the hydrodynamic equations and 3D Gross – Pitaevskii equation. The experiment showed that in spite of their rarefied character, the spin gases can have collective excitations typical of solid-state ferromagnets and ferromagnetic liquids. Source: Phys. Rev. Lett. 121 013201 (2018)

Microlasers

With decreasing laser size the optical loss increases, and therefore a higher pumping power is needed. A. Fernandez-Bravo (Lawrence Berkeley National Laboratory, USA) with colleagues created microlasers several µm in size that have the excitation threshold upon a minimum pumping power of 14 kW cm-2. A microlaser is a polystyrene microsphere 5 µm in diameter. Its surface is covered with thulium Tm3+ nanoparticles, the electron transitions in which are connected with the microsphere modes. Pumping was realized with a 1064-nm-wavelength laser and wavelengths of nearly 807 nm dominated in the radiation spectrum. Owing to a complete internal reflection the light inside the microsphere can circulate thousands of times forming intensity nodes in the vicinity of which lasing is generated upon transitions 1D23F4 and 1G43H6 in thulium. The microlasers created earlier needed a higher pumping power and hence operated in pulsed regime only. The new microlaser, on the contrary, demonstrated continuous 5-hour operation. The property of the new laser important for practical application such as diagnosticsis its capacity of operating in biological media. The experiment described demonstrated laser operation in blood serum. Source: Nature Nanotechnology 13 572 (2018)

Stereoscopic Wigner Time Delay in molecule ionization

Attosecond metrology allows investigation of some fine properties of the photoelectric effect, in particular, measurement of the Wigner time delay between the light pulse and the electron escape. As distinct from atoms, molecules have a more complicated electronic structure, which makes difficulties for interpreting the results of measurements at photoeffect on molecules. The new experiment conducted under the guidance of U. Keller (Swiss Federal Institute of Technology Zurich) examined the photoeffect on the CO molecule and measured for the first time the stereoscopic Wigner time delay characterizing the escape time depending on the electron position in a molecule. A beam of molecules was exposed to attosecond UV laser pulses. IR pulses synchronized with exciting UV pulses were used to measure the direction and the instant of escape. Through reconstruction this provided information on the electron wave function. The electrons emitted with higher energy were located for the most part closer to the oxygen atom and lower-energy electrons were closer to carbon. The stereoscopic Wigner ionization time delay was determined as the difference between the delays in these two cases. The result depends strongly on the molecule orientation relative to the laser pulse polarization plane. In the case of orthogonal orientation, measurements are in good agreement with the theoretical calculations while in the parallel case, although the qualitative run of the dependence corresponds to the expected one, a certain difference is observed at low electron energies, which is possibly due to an incomplete allowance for higher electron levels of the molecule. Source: Science 360 1326 (2018)

Universal microcavity

The resonant light-matter interaction is a promising area of research because the radiative properties of materials are modified in an electromagnetic field, which paves an additional way for their study. A resonator is normally designed for only one particular substance. Fit for the study of different substances in different regimes is a universal tunable microcavity fabricated at the Moscow Engineering Physics Institute under the guidance of Yu.P. Rakovich.The resonator consists of plane and convex mirrors separated by several hundreds of nanometers. One of the mirrors can be moved in three directions to choose the most convenient approximately plane-parallel region for an accurate control of distances with the help Z-piezopositioner with nanometer precision. This half-wave Fabry – Perotcavity can operate in IR, visible, and UV ranges. Light comes to the cavity through a confocal lens system and is registered by a CCD matrix and a spectrometer. This universal tool is convenient for the study of chemical and biological properties of objects placed in the wave field both in the weak and strong coupling regimes. In the former case the radiation effect on the substance in the cavity is weakand in the latter case the properties of substances and the course of reactions is strongly modified by radiation because of the coupling with the cavity modes. Source: Review of Scientific Instruments 89 053105 (2018)

Quantum magnetometer

Researchers from Switzerland, Russia, and Finland applied quantum phase estimation algorithms underlying the protocols of quantum information processing to measure the magnetic field using a superconducting transmon qubit (SQUID with an additional loop and a resonator). Modified versions of Kitaev algorithm and Fourier phase retrieval algorithm with which one can overcome the problem of phase 2 π-periodicity were applied. The qubit can be thought of as an “artificial atom” with a set of quantum levels. It is sensitive to the external magnetic field because it has a large intrinsic magnetic moment and can therefore serve as a magnetometer. The magnetic field is determined through measurement of oscillation frequency in the qubit proportional to the field magnitude. Both the algorithms make it possible to attain the sensitivity of 19.3-29.3 pT Hz-1/2 exceeding the classical level of shot noise and to approach the limit imposed by the Heisenberg uncertainty principle, the accuracy being mainly restricted by decoherence. It is also possible to increase substantially the sensitivity of such type of devices in future. Russian scientists from MIPT (Moscow Institute of Physics and Technology) and ITP (L.D. Landau Institute for Theoretical Physics) are taking part in this research. Source: npj Quantum Information 4 29 (2018)

The “missing baryons” are found

The amount of baryon gas in the Universe is rather reliably predicted by the theory of primary nucleosynthesis and is calculated from observations of fluctuations of relic radiation. It is however only 10 % of all baryons that are observed in galaxies and 60 % in the intergalactic space while the remaining 30 % have been invisible. The missing baryons were assumed to be present in the intergalactic filamentary structures forming the so-called warm-hot intergalactic medium with the gas temperature of 105-107 K enriched with heavy elements flowing from galaxies. This picture was obtained by R. Cen, J.P. Ostriker et al. and was confirmed by the numerical simulations performed by J.M. Shull et al. The “missing baryons” are difficult to find because they are ionized. Only weak and ambiguous evidence of their existence has up to now been obtained in observations. F. Nicastro (National Institute of Astrophysics, Italy and Harvard Smithsonian Center of Astrophysics, USA) and his colleagues investigated the oxygen (O VII) absorption lines in the quasar X-ray spectrum at a redshift z>0.4. The observations were carried out by the spectrometer of cosmic telescope XMM-Newton. The data obtained imply that a large amount of hot gas with oxygen admixture is present in intergalactic space on the line of sight. The constancy and shape of the absorption spectrum practically exclude the possibility that the gas is connected with the quasar itself or with its host galaxy. Thus, the conclusion suggests itself that the missing 30 % of the Universe baryons have been revealed. Source: Nature 558 406 (2018)

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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.

Permanent editor is Yu.N. Eroshenko.

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