Quantum Magnonics
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m Magnonics Yu. M. Bunkov Russian Quantum Center, Skolkovo, Moscow, 143025 Russia e-mail: [email protected] Received January 30, 2020; revised March 29, 2020; accepted March 30, 2020
Abstract—The discovery of spin superfluidity in antiferromagnetic superfluid 3He is a remarkable discovery associated with the name of Andrey Stanislavovich Borovik-Romanov. After 30 years, quantum effects in a magnon gas (such as the magnon Bose–Einstein condensate and spin superfluidity) have become quite topical. We consider analogies between spin superfluidity and superconductivity. The results of quantum calculations using a 53-bit programmable superconducting processor have been published quite recently [1]. These results demonstrate the advantage of using the quantum algorithm of calculations with this processor over the classical algorithm for some types of calculations. We consider the possibility of constructing an analogous (in many respecys) processor based on spin superfluidity. DOI: 10.1134/S1063776120070018
1. SUPERFLUID ANTIFERROMAGNETIC 3He This article was written in the memory of my teacher Andrey Stanislavovich Borovik-Romanov. I was lucky to be acquainted with secrets of physical science by this remarkable person. The superfluidity of 3He at a temperature of 0.002 K was discovered in 1972 in the United States. In the transition of 3He to the superfluid state, its spin and orbital rotational symmetries are violated. Therefore, apart from superfluidity, 3He also exhibits the properties of an antiferromagnetic liquid crystal. This unique combination of broken symmetries has rendered 3He a perfect sample for experimental investigations of many natural phenomena (in particular, even the quantum field theory [2]). In our laboratory at the Institute for Physical Problems, we constructed a unique cooper nuclear demagnetization cryostat [3] and obtained superfluid 3He for the first time in the USSR. Even in first experiments, a peculiar spin induction signal was detected [4]. This induction signal first decay in accordance with the spatial nonuniformity of the magnetic field, but was then restored and lasted longer by several orders of magnitude than should be due to the field inhomogeneity. This spontaneous restoration of coherence of magnetization precession was the first observation of Bose condensation of quasiparticles (magnons). The effect was explained theoretically by Fomin [5] as a superfluid flow of the components of deflected magnetization to the magnetic field minimum and the formation of a magnetic domain with uniform coherent precession of magnetization. This theory was immediately confirmed by direct experiment in which the
deviated magnetization “rose” in a strong magnetic field gradient [6]. The spin superfluid current can be represented as the counterflow of two superfluid 3He components with opposite magnetic moments, which emerges due to the gradient of the phase of precession in a nonuniform field [7]. However, this effect can also be described in terms of nonequilibrium magnons [7, 8]. In this case, the
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