Up‐And‐Coming Advances in Optical and Microwave Nonreciprocity: From Classical to Quantum Realm

Abstract

Reciprocity is a fundamental physical principle that roots in the time‐reversal symmetry of physical laws. It allows making predictions on any arbitrary complex system's response and operation and hence simplifies the analysis. However, there are many practical situations in which it is advantageous to break reciprocity, e.g., isolators preventing wave scattering back to lasers and generators, full‐duplex systems for multiplexing transmission and receiving in the same channel, nonreciprocal cavity excitation, and protection of fragile states of superconductor quantum computers from thermal noise. The most widespread approach to time‐reversal symmetry breaking and nonreciprocity based on magnetic field biasing suffers from bulkiness, cost ineffectiveness, and loss, motivating researchers and engineers to search for more practical approaches. Herein, the up‐and‐coming advances in optical nonreciprocity, including new materials (Weyl semimetals, topological insulators, metasurfaces), active structures, time‐modulation, parity‐time (PT)‐symmetry breaking, nonlinearity combined with a structural asymmetry, quantum nonlinearity, unidirectional gain and loss, chiral quantum states and valley polarization are overviewed. A general description of nonreciprocal systems is provided and the pros and cons of the mentioned approaches to nonreciprocity are discussed.

Breaking reciprocity by designed loss

“…INTRODUCTIONRecently, non-reciprocal propagation of electromagnetic field became a popular research theme. Non-reciprocity is important for quite a wide of practical tasks, from field distributors and circulators to lasing and high-precision sensing [1][2][3][4][5][6]. Current interest to realization of non-reciprocal systems, and especially isolators, is born of necessity to extent methods commonly applied for longer wavelength (radio, microwaves, etc.)…”

“…toward optical region and possibility to integrate non-reciprocal systems into photonic circuitry. Traditional realizations implementing magneto-sensitive media are either hard to realize on the basis of existing integration platforms, or difficult even to realize for optical wavelength, or both [4,5]. So, a plethora of novel schemes for breaking reciprocity has appeared in recent years.…”

“…So, a plethora of novel schemes for breaking reciprocity has appeared in recent years. There are schemes exploiting time-modulation [7,8], nonlinearities [9][10][11] and interaction with few-level systems, such as atom-like structures [4,[12][13][14], optomechanics [15], topological properties of structured media [5,16,17].Some quite interesting results were also obtained by considering loss. Recently it was shown that even common single-photon energy losses in conjunction with the usual (i.e., unitary) coupling can also be a tool for devising non-reciprocal multi-mode structures [18,19].…”

Quantum Squeezing Induced Optical Nonreciprocity

“…Similar to a commercial circulator, our proposed system can also function as a quasi-circulator with two inputs and three outputs, allowing photon flow along the direction 1 → 2 → 3 [56], see Figs. 2(a,c).…”

“…Here, we focus on the fidelity and the insertion loss. To evaluate the quasicirculator performance, we calculate the average fidelity as [17,32], where T id is the transmission matrix for an ideal three-port quasi-circulator [56], and T = T i j /Υ i , with Υ i = j T i j . We define the average insertion loss as L = −10 log[(T 12 + T 23 )/2].…”