Superfluid solid state in a gas with dipole-dipole interaction
1 June 2019
States combining superfluidity and crystalline periodicity are called superfluid solid states (SSS). The possibility of their existence was predicted theoretically by A.F. Andreev and I.M. Lifshitz (Sov. Phys. JETP 29 1107 (1969))
and G.V. Chester (Phys. Rev. A 2 256 (1970)) and A.J. Leggett (Phys. Rev. Lett. 25 1543 (1970)). The condition of SSS appearance is the presence of roton minimum on the dispersion curve and quantum stabilization. Observation of SSS in solid helium-4 was reported in 2004, but the result has not been confirmed. SSS was reliably discovered for the first time in 2009 in rubidium atom gas in a periodic potential generated by the radiation field. The possibility for the roton minimum and SSS to occur was predicted also without an external periodic field. Three independent teams of researchers confirmed this prediction when they discovered SSS in Bose – Einstein condensates of gases whose atoms have large magnetic dipole moments. An absorption image of atomic wave interference was obtained in a condensate cloud upon its free expansion. This was how periodically located phase-coherent drops of Bose – Einstein condensate were observed. Two groups guided by G. Modugno (the University of Florence, Italy) and T. Pfau (the University of Stuttgart, Germany) examined 162Dy isotope. The SSS lifetime measured by them was equal to 30 ms. This time was limited by three-body losses. 3D-modelingbased on the generalized Gross – Pitaevsky equation showed good agreement between theory and experiment. F. Ferlaino (the University of Innsbruck, Austria) and her colleagues revealed SSS in the gas of 166Er and 164Dy isotopes. SSS lasts 30 ms in 166Er and 150 ms in 164Dy.
Sources: Phys. Rev. Lett. 122 153601 (2019), Phys. Rev. X 9 011051 (2019), Phys. Rev. X 9 021012 (2019)
Quantum spin ice in Ce2Zr2O7
1 June 2019
J. Gaudet (McMasterUniversity, Canada, Johns Hopkins University and NIST, USA) with colleagues investigated the properties of pyrochlore magnet Ce2Zr2O7 and showed that the state of quantum liquid is possibly attained in it in the form of spin ice. As distinct from usual substances, the spins of atoms remain disordered even at absolute zero of temperature. J. Gaudet with co-authors applied neutron scattering from a source in the Oak Ridge National Laboratory for the study of spin dynamics in powder-like Ce2Zr2O7 and also in single crystals. Discovered were doublet states coupled with Ce3+ ions and energy separated from other excitations in the crystal. It turned out that at a temperature of 60 mK the character of neutron scattering is close to that predicted for quantum spin ice.
Source: Phys. Rev. Lett. 122 187201 (2019)
Test of Bell inequality using separated qubits
1 June 2019
Although quantum entanglement of superconducting qubits connected by a long channel has already been demonstrated, the Bell inequalities for them have not been verified so far because of the difficulties in transferring states with high quantum accuracy. Such a test was only realized for qubits with local coupling. A.N. Cleland (the University of Chicago and Argonne National Laboratory, USA) with colleague has tested successfully for the first time the Bell inequalities by coupling two superconducting qubits with quantum accuracy f = 0.94 using photons through a 78-cm coplanar waveguide. The Bell inequalities were violated at a level of 9.7 σ. The applied method can be used in devices of quantum information transfer.
Source: Nature Physics, online publication of 24, 04 2019
Laser radio transmitter
1 June 2019
F. Capassoa (Harvard University, USA) and his colleagues designed a radio transmitter in which the near IR laser radiation is transformed into microwave-range radio waves. As distinct from other known methods of radio wave laser generation, radio emission appears directly in the laser working volume. Quantum-cascade GaInAs/AlInAs laser worked in the frequency comb regime. The beats between neighboring optical modes in the resonator formed space-time optical field variations. These variations affected the stimulated emission and absorption of photons that induced the electron motion and radio wave generation at a frequency of 5.5 GHz.The upper metal electrode had a cut and played the role of a dipole antenna for signal transmission to the external space. The signal could be modulated by useful information through varying the laser feed current. In the inverse regime, the device can also receive radio signals. This study paves the way for designing hybrid electron-photon devices.
Source: PNAS, online publication of 24.04.2019
ISS measurement of cosmic-ray protonspectra
1 June 2019
Using the CALET device located on board the International Space Station, the cosmic-ray proton spectrum was measured in the energy range from 50 GeV to 10 TeV. This interval includes, in particular, hardening of the spectral region, where its slope changes. The observation of the spectrum above and below this region using one and the same device is important because of the absence of systematic errors. Some parts of the spectrum were measured earlier by different space detectors (PAMELA, ATIC, etc.) that revealed the spectrum hardening effect. The CALET instrument includes a charge detector and a calorimeter array. The spectrum measured by CALET agrees with the results of AMS-2 measurements, but extends to higher energies. One can see a smooth transition of the spectralindex from γ = -2.81 ± 0.03 (without solar modulation effects) in the interval of 50 to 500 GeV to γ = -2.56 ± 0.04 at 1 to 10 TeV. Measurement of the cosmic-ray proton spectrum is important as it provides insight into the mechanisms of cosmic ray acceleration and their transport in the Galaxy.
Source: Phys. Rev. Lett. 122 181102 (2019)
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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