Perfect intrinsic squeezing at the superradiant phase transition critical point.

Some of the most exotic properties of the quantum vacuum are predicted in ultrastrongly coupled photon-atom systems; one such property is quantum squeezing leading to suppressed quantum fluctuations of photons and atoms. This squeezing is unique because (1) it is realized in the ground state of the system and does not require external driving, and (2) the squeezing can be perfect in the sense that quantum fluctuations of certain observables are completely suppressed. Specifically, we investigate the ground state of the Dicke model, which describes atoms collectively coupled to a single photonic mode, and we found that the photon-atom fluctuation vanishes at the onset of the superradiant phase transition in the thermodynamic limit of an infinite number of atoms.

Dicke-Cooperativity-Assisted Ultrastrong Coupling Enhancement in Terahertz Metasurfaces.

A system of two-level atoms cooperatively interacting with a photonic field can be described as a single giant atom coupled to the field with interaction strength . This enhancement, known as Dicke cooperativity in quantum optics, has recently become an indispensable element in quantum information technology. Here, we extend the coupling beyond the standard light-matter interaction paradigm, enhancing Dicke cooperativity in a terahertz metasurface with meta-atoms.

Ultrastrong magnon-magnon coupling dominated by antiresonant interactions.

Exotic quantum vacuum phenomena are predicted in cavity quantum electrodynamics systems with ultrastrong light-matter interactions. Their ground states are predicted to be vacuum squeezed states with suppressed quantum fluctuations owing to antiresonant terms in the Hamiltonian. However, such predictions have not been realized because antiresonant interactions are typically negligible compared to resonant interactions in light-matter systems.

Observation of Dicke cooperativity in magnetic interactions.

The interaction of two-level atoms with a single-mode light field is an extensively studied many-body problem in quantum optics, first analyzed by Dicke in the context of superradiance. A characteristic of such systems is the cooperative enhancement of the coupling strength by a factor of N. In this study, we extended this cooperatively enhanced coupling to a solid-state system, demonstrating that it also occurs in a magnetic solid in the form of matter-matter interaction.

Superradiant Phase Transition in a Superconducting Circuit in Thermal Equilibrium.

We propose a superconducting circuit that shows a superradiant phase transition (SRPT) in thermal equilibrium. The existence of the SRPT is confirmed analytically in the limit of an infinite number of artificial atoms. We also perform a numerical diagonalization of the Hamiltonian with a finite number of atoms and observe an asymptotic behavior approaching the infinite limit as the number of atoms increases.

Entangled-photon generation in nano-to-bulk crossover regime.

We have theoretically investigated the generation of entangled photons from biexcitons in a semiconductor film with a thickness in the nano-to-bulk crossover regime. In contrast with the cases of quantum dots and bulk materials, we can highly control the generated state of entangled photons through the design of a peculiar energy structure of exciton-photon coupled modes in the thickness range between nanometers and micrometers. Owing to the enhancement of the radiative decay rate of excitons (exciton superradiance), the statistical accuracy of generated photons can be increased beyond the trade-off problem with signal intensity.

Quantum squeezing generation versus photon localization in a disordered planar microcavity.

We investigate theoretically the nonlinear dynamics induced by an intense pump field in a disordered planar microcavity. Through a self-consistent theory, we show how the generation of quantum optical noise squeezing is affected by the breaking of the in-plane translational invariance and the occurrence of photon localization. We find that the generation of single-mode Kerr squeezing for the ideal planar case can be prevented by disorder as a result of multimode nonlinear coupling, even when the other modes are in the vacuum state.