On Monday, December 12, the seminar of Aleksandr Krasnok took place, who presented his work Quantum nonreciprocity with nonlinearity and Weyl semimetals.
The recording of the seminar is available on our YouTube channel.
Quantum Nonreciprocity with Nonlinearity and Weyl semimetals
The emerging ﬁeld of quantum computing has been rapidly growing and has shown interesting opportunities to overcome the limitations of classical computers for many currently unfeasible problems . A key technology required for quantum computation devices is unidirectional signal propagation and routing, whereby electromagnetic radiation propagates asymmetrically between two points . This eﬀect is particularly important to protect JJ-qubits from reﬂections and noise originating in the readout. However, most modern nonreciprocal components are realized based on the magneto-optical effect in ferrite materials . These devices are barely tunable, bulky, and incompatible with planar technologies, including transmission-line quantum circuits (circuit QED). Although isolators based on other approaches, e.g., two-dimensional magnetic materials and topological isolators/semimetals, cold atoms, have been extensively investigated recently, they are still limited in many aspects and unsuitable for superconducting circuit QED systems. Another recently extensively developed approach based on time-modulated isolators requires precise control over the modulation phase and connection to the external modulation generators and hence is also subject to thermal and apparate noise.
In this talk, I will present our recent results on isolators suitable for quantum systems. We will discuss the isolation effect obtained by a suitable combination of quantum nonlinearities and symmetry breaking . By an example of a two-qubit system, we show that the presence of the dark state  is crucial to establishing strong nonreciprocity in this class of systems. Such two-qubit devices have been implemented as systems with an asymmetric dependence on the direction of the input ﬁeld, allowing them to act like unidirectional devices in circuit QED. Then we will discuss how the circuit with Lorentz-type qubits can be turned into Fano-type qubits with the asymmetric spectral response, which improves the isolator figure of merit, power bandwidth, and efficiency. Finally, we will discuss a novel approach to tunable isolation based on twisted bilayered Weyl semimetals. The approach enables highly efficient tuning of both direction and value of isolation with the relative rotation of Weyl semimetals.
 E. Gibney, “Hello quantum world! Google publishes landmark quantum supremacy claim,” Nature, vol. 574, no. 7779, pp. 461–462, Oct. 2019, doi: 10.1038/d41586-019-03213-z.
 S. V. Kutsaev, A. Krasnok, S. N. Romanenko, A. Y. Smirnov, K. Taletski, and V. P. Yakovlev, “Up‐And‐Coming Advances in Optical and Microwave Nonreciprocity: From Classical to Quantum Realm,” Adv. Photonics Res., vol. 2, no. 3, p. 2000104, Mar. 2021, doi: 10.1002/adpr.202000104.
 D. M. Pozar, Microwave engineering, 4th Edition. John Wiley & Sons, Inc., 2011.
 N. Nefedkin, M. Cotrufo, A. Krasnok, and A. Alù, “Dark-State Induced Quantum Nonreciprocity,” Adv. Quantum Technol., vol. 5, no. 3, p. 2100112, Mar. 2022, doi: 10.1002/qute.202100112.
 N. Nefedkin, A. Alù, and A. Krasnok, “Quantum Embedded Superstates,” Adv. Quantum Technol., vol. 4, no. 6, p. 2000121, Jun. 2021, doi: 10.1002/qute.202000121.
Prof. Alex Krasnok earned his Ph.D. from ITMO University (2013). After spending two years (2016-2018) as a research scientist at The University of Texas at Austin, and three years with CUNY Advanced Science Research Center (New York) as a Research Assistant Professor and Founding Director of Photonic Core Facility, he joined Florida International University in 2021 as a tenure-track Assistant Professor. Prof. Krasnok’s current research interests are in quantum engineering, nanophotonics and quantum optics, with particular emphasis on cross-disciplinary research. He has made significant contributions in the areas of extreme scattering engineering, nanoantennas, metasurfaces, optics of 2D transition-metal dichalcogenides, and low-loss dielectric nanostructures. Alex has authored or co-authored five books and book chapters, four patents, and more than 150 papers. He has earned several research awards, including the gold medal of Nobel Laureate Zhores Alferov’s Foundation (2016) and the Early-Career Award in Nanophotonics (2021).