Ultrastrong coupling between light and matter has, in the past decade, transitioned from a theoretical idea to an experimental reality. It is a new regime of quantum light–matter interaction, which goes beyond weak and strong coupling to make the coupling strength comparable to the transition frequencies in the system. The achievement of weak and strong coupling has led to increased control of quantum systems and to applications such as lasers, quantum sensing, and quantum information processing. Here we review the theory of quantum systems with ultrastrong coupling, discussing entangled ground states with virtual excitations, new avenues for nonlinear optics, and connections to several important physical models. We also overview the multitude of experimental setups, including superconducting circuits, organic molecules, semiconductor polaritons, and optomechanical systems, that have now achieved ultrastrong coupling. We conclude by discussing the many potential applications that these achievements enable in physics and chemistry.
Ultrastrong coupling (USC) can be achieved by coupling many dipoles to light, or by using degrees of freedom whose coupling is not bounded by the smallness of the fine-structure constant.
The highest light–matter coupling strengths have been measured in experiments with Landau polaritons in semiconductor systems and in setups with superconducting quantum circuits.
With USC, standard approximations break down, allowing processes that do not conserve the number of excitations in the system, leading to a ground state that contains virtual excitations.
Potential applications of USC include fast and protected quantum information processing, nonlinear optics, modified chemical reactions and the enhancement of various quantum phenomena.
Now that USC has been reached in several systems, it is time to experimentally explore the new phenomena predicted for this regime and to find their useful applications.
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Numerical simulations were performed using QuTiP215. The authors thank Y.-X. Liu, X. Gu, O. Di Stefano and A. Ridolfo for useful discussions. The authors also thank A. Settineri, M. Cirio and S. Ahmed for technical assistance with some of the figures. A.F.K. acknowledges partial support from a JSPS Postdoctoral Fellowship for Overseas Researchers (P15750). A.M. and F.N. acknowledge support from the Sir John Templeton Foundation. S.D.L. acknowledges support from a Royal Society research fellowship and thanks F.N. for his hospitality at RIKEN during the course of this work. F.N. also acknowledges support from the MURI Center for Dynamic Magneto-Optics via the Air Force Office of Scientific Research (AFOSR) award No. FA9550-14-1-0040, the Army Research Office (ARO) under grant No. W911NF-18-1-0358, the Asian Office of Aerospace Research and Development (AOARD) grant No. FA2386-18-1-4045, the Japan Science and Technology Agency (JST) through the Q-LEAP program, the ImPACT program, and CREST grant No. JPMJCR1676, the Japan Society for the Promotion of Science (JSPS) through the JSPS-RFBR grant No. 17-52-50023 and the JSPS-FWO Grant No. VS.059.18N, and the RIKEN-AIST Challenge Research Fund.
The authors declare no competing interests.
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Frisk Kockum, A., Miranowicz, A., De Liberato, S. et al. Ultrastrong coupling between light and matter. Nat Rev Phys 1, 19–40 (2019). https://doi.org/10.1038/s42254-018-0006-2
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