Nanjing University has made important progress in the non-reciprocal field of on-chip integrated optics

Lorentz reciprocity theorem derived from the Maxwell equation is a basic physical law generally followed by the electromagnetic system. The theorem points out the time inversion symmetry or reversibility of electromagnetic wave propagation, that is, the propagation path of an electromagnetic wave in the ordinary linear medium is reversible. The study of non-reciprocity is not only of basic scientific significance; but also of wide application value. Optical devices based on non-reciprocity, such as optical isolators and optical circulators, only allow light to pass in one direction and isolate backscattered light. They are not only used in laser protection, optical communication, and optical information processing; but also indispensable functional units in many special quantum information processing protocols such as non-traditional quantum computing, quantum measurement, and quantum network.

Although bulk optical nonreciprocal devices based on magneto-optical effect have been widely used in various fields, how to realize all-optical nonreciprocal devices integrated on-chip is still a challenge. The lack of an optical isolator and circulator that can be integrated on the chip is one of the main factors that limit the integration of photonic chips, and also limits the integration of lidar and laser gyroscope. On-chip integrated optical nonreciprocal devices are also very important for integrated optical quantum information processing. Xia Keyu's research group and international collaborators put forward an on-chip integrated all-optical controlled optical isolation method and nonreciprocal photonic transistor.

This achievement creatively puts forward a theoretical scheme of inducing optical non-reciprocity by using unidirectional compressed cavity mode. The optical nonreciprocal system shown in Figure 1 consists of two lithium niobate-based nonlinear ring microcavities and two coupled waveguides. The pump light is incident from port 3. Under the condition of phase matching, the nonlinear parametric down conversion occurs in the RB cavity to produce a counterclockwise compressed cavity mode, but the clockwise mode is still an ordinary cavity mode. The forward signal light forms a clockwise ordinary cavity mode in the RA cavity and is coupled with the counterclockwise compression cavity mode in the RB cavity. However, for the reverse signal light, two ordinary cavity modes in the system are coupled. Compared with the forward and reverse incidence of signal light, the coupling strength of ordinary cavity mode and compressed cavity mode is greater than that of two ordinary cavity modes. Compared with weak coupling, strong coupling forms mode splitting, as shown in Fig. 2 (a). And the equivalent compressed cavity mode frequency is less than the ordinary cavity mode frequency, resulting in mode frequency drift, as shown in Fig. 2 (b). The resulting optical nonreciprocity can achieve optical isolation with isolation greater than 40 dB and a three-port quasi circulator with a fidelity greater than 98%. If the squeezed vacuum field matched with the squeezed cavity mode is injected into the cavity, the noise introduced by the pump will be eliminated, so as to realize the single-photon isolator and circulator (). Moreover, the switching weak pump light can control the transmission of forwarding strong signal light from ports 1 to 2, but can not control the reverse signal light. The proposed scheme can realize the non-reciprocal photonic transistor with control gain G greater than 1. The all-optical controlled optical nonreciprocal device reported in this work adopts lithium niobate optical microcavity with simple structure. It can be used for nonreciprocal regulation of classical coherent light and a single photon, which opens up a new way to realize integrated nonreciprocal quantum information processing.

Fig. 1 photon  photon chiral coupling is induced by squeezing cavity mode of classical optically pumped lithium niobate microcavity to realize non-reciprocal optical transmission. (a) Photon photon coupling with forward signal light incident and quantum compression modulation; (b) When the reverse signal light is incident, the cavity mode interaction is not modulated by quantum squeezing.

Fig. 2 (a, b) transmission spectrum of optical isolator and optical circulator, represented by red curve, blue curve and green curve; (C, d) gain of photonic transistor.

Professor Xia Keyu of the school of modern technology of Nanjing University, in cooperation with Professor Xiao Min of the University of Arkansas and Professor Franco nori of the Japanese Institute of science and chemistry, published the latest research results of integrated optical nonreciprocal devices under the title of "quantum squeezing induced optical nonreciprocity" in the authoritative journal of Physics Physical Review Letters on February 23, 2022. Tang Lei, a doctoral student from the school of modern engineering and Applied Sciences of Nanjing University, is the first author of the paper, and Professor Xia Keyu is the corresponding author. Tang Jiangshan and Chen Mingyuan of Nanjing University and international collaborators, Professor Xiao Min and Professor Franco nori, have made important contributions to this work. This work has been supported by the key R & D program of the Ministry of science and technology, the National Natural Science Foundation of China, the "innovation and entrepreneurship talents" and "innovation and entrepreneurship team" program of Jiangsu Province, and the excellent research program of Nanjing University.

Link:

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.083604