Extracts from the Internet

Theory of high-temperature superconductivity

The Bardeen – Cooper – Schrieffer (BCS) theory explains successfully superconductivity of simple metals, but it fails to describe high-temperature superconductors – cuprates. P.W. Phillips, L. Yeo and E.W. Huang (University of Illinois at Urbana Champaign, USA) formulated a new theory [1] explaining cuprate superconductivity at the microlevel. Cuprates belong to doped Mott insulators. The authors described them using the Hatsugai – Kohmoto model which is a modification of the Hubbard model and found new exact solutions. The mechanism playing the role of Cooper pairing in BCS theory was identified and an instability corresponding to the superconducting state was found. The ratio of the transition temperature to the energy gap is shown to be higher and the superfluid component density to be lower than in BCS theory. The model also explains theenhanced low-frequency radiation absorption by cuprates observed in the experiment. For the discussion of some modelsof high-temperature superconductors see [2-4]. [1] Phillips P W, Yeo L, Huang E W Nature Physics, online publication on July 27 [2] Izyumov Yu A, Plakida N M, Skryabin Yu N Sov. Phys. Usp. 32 1060 (1989); UFN 159 621 (1989) [3] Maksimov E G Phys. Usp. 43 965 (2000); UFN 170 1033 (2000) [4] Val’kov V V et al. Phys. Usp. 64, accepted (2020) ; UFN 191, accepted (2020)

Magnetic solitons

G. Lamporesi (University of Trento and Trento Institute forFundamental Physics and Applications, Italy)and his colleagues investigated magnetic solitonsin Bose – Einstein condensate [5]. 23Na atoms in a hybrid trap were transferred to a mixture of components with opposite spin directions using microwave pulses. Moving magnetic solitons in which magnetization phase changed twice by π were created at the trap edge through potential perturbation.The dissipationless soliton dynamics, including their oscillations, was investigated.Pair collisions of solitons with the same or opposite magnetization was observed. The results of measurements are consistent with the theoretical description presented in paper [6]. An independent observation of magnetic solitons was also reported in [7]. For solitons in ultracold gases see [8]. [5] Farolfi A et al. Phys. Rev. Lett. 125 030401 (2020); [6] Qu C, Pitaevskii L P, Stringari S Phys. Rev. Lett. 116 160402 (2016) [7] Chai X et al. Phys. Rev. Lett. 125 030402 (2020) [8] Pitaevskii L P Phys. Usp. 59 1028 (2016); UFN 186 1127 (2016)

Local character of Aharonov – Bohm effect

The Aharonov – Bohm effect is typically considered as an example of a nonlocal phenomenon: the charge acquires phase by enclosing a solenoid, where the solenoid’s electromagnetic field is zero. However, C. Marletto and V. Verdal (the Universityof Oxford (Great Britain), the National University of Singapore and the Institute for Scientific Exchange (Italy)) have found [9] that in a self-consistent quantum-mechanical consideration the phase gain has a local character, that is, the phase is gained as the particle moves from point to point. Developing the approach proposed in [10] they showed that both the charge motion and the electromagnetic field of the solenoid should be quantized. A quantized field will not already be zero outside the solenoid (its expectation mean alone is zero), and therefore the charge interacts with photons and gains the quantum phase gradually. The idea is suggested to experimentally verify the conclusion drawn in the work through quantum tomography of the charge state. [9] Marletto C, Vedral V Phys. Rev. Lett. 125 040401 (2020) [10] Vaidman L Phys. Rev. A 86 040101(R) (2012)

Reverse quantum evolution

Time reversalof quantum evolution has already been demonstrated in an experiment with a simple quantum computer [11]. In their theoretical work [12], A.V. Lebedev (MIPT) and V.M. Vinokur (Argonne National Laboratory and the University of Chicago, USA) described the method for bringing on a reverse time evolution of the system even without knowing its initial state that can be a mixed quantum state. To this end, the second system described by the same Hamiltonian should be created and certain operations on the auxiliary system should be performed simultaneously with quantum operations on the investigated system. System thermalization at the end of the evolution induces a reverse sequence of quantum transitions and transfers the system to a state with the same density matrix as the one at the outset. [11] Lesovik G B, Sadovskyy I A, Suslov M V, Lebedev A V, Vinokur V M Sci. Rep. 9 4396 (2019) [12] Lebedev A V, Vinokur V M Communications Physics 3 129 (2020)

Recovery of quantum information

The researchers B. Yan and N.A. Sinitsyn (Los-Alamos National Laboratory, USA) developed a new method for recovery of quantum information damaged by measuring [13]. To this end one should know certain correlators taken at different instants of time and provoke a reverse evolution of the system. A partial simulation of this algorithm on a 5-qubit quantum IBM processor showed that when it is implemented, decoherence remains at a low level. Owing to the fact that a local damage of quantum information does not lead to a catastrophic increase in damage (in the quantum region the “butterfly effect” is absent), the damaged quantum information is to a great extent accessible to recovery. [13] Yan B, Sinitsyn N A Phys. Rev. Lett. 125 040605 (2020)

Is a primordial black hole in the solar system?

In the motion of icy bodies, anomalies have recently been noticed at a distance of 300-1000 a.u. from the Sun that can be explained by the presence of the 9th planet with a mass of 5 to 15 Earth masses. The OGLE telescope simultaneously observes microlensing events that may have been caused by compact objects with masses of 0.5 to 20 Earth masses. These objects may be free floating planets in the interstellar space. J. Scholtz (Durham University, Great Britain) and J. Unwin (University of Illinois at Chicago, USA) suggested an alternative hypothesis [14] that in both cases the objects are primordial black holes (PBH) rather than planets. The authors showed that the capture of PBH in the solar system, if they exist in the amount given by OGLE, is as probable as the capture of a planet. In this case, at the edge of the solar system it may be a black hole rather than the 9th planet. Such a PBH can be revealed by annihilation of dark matter particles that must form a dense cluster around a PBH. For PBH see [15]. [14] Scholtz J, Unwin J Phys. Rev. Lett. 125 051103 (2020); [15] Dolgov A D Phys. Usp. 61 115 (2018); UFN 188 121 (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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