Quantum States of Light at Thermal Equilibrium Can Enable High-precision Sensing Technology

One of the most interesting developments in quantum optics has been the generation of “squeezed light”, or light characterized by a quantum state such that one of the conjugate variables (for example, an electric field or magnetic field) describing the light field has fluctuations below the standard quantum limit at the expense of higher fluctuations in the other. Due to this interesting property, squeezed light has found applications in optical communication, quantum computing, and notably, precision measurements in gravitational wave detection.

There is, however, a caveat: the generation of squeezed light requires the system to be driven to a non-equilibrium state, which limits the degree of squeezing in actual experiments due to unexpected noise. Moreover, the squeezed states obtained are not stable. Thus, one is led to ask if it is possible to obtain squeezed states at thermal equilibrium.

It turns out that a light-matter coupled system can show such equilibrium squeezing in its ground state, a phenomenon known as “intrinsic squeezing”. Such squeezed states are inherently stable and noise resilient, with the degree of squeezing governed by the light-matter coupling strength. A perfect squeezing, characterized by vanishing of fluctuations in one variable, is reached at the ideal limit of infinite coupling.

But is perfect squeezing necessarily ideal? In our study, we theoretically show that this is, in fact, not the case. We consider the standard Dicke model, which describes coupling between a photonic field and an atomic ensemble. We first numerically carry out a search for the optimal photon-atom two-mode basis that maximizes the observable squeezing in its ground state. We find that perfect squeezing can emerge upon the onset of a curious phenomenon called “superradiant phase transition”, which is characterized by the spontaneous appearance of coherence due to strong light-matter coupling. We then analytically confirm that the squeezing in the ground state is the ideal one, making it beneficial for future applications.

We believe that our results, once experimentally observed, can bring about a paradigm shift in quantum optics, opening doors to a revolution in noise-robust quantum sensing and quantum information

Kenji Hayashida, Takuma Makihara, Nicolas Marquez Peraca, Diego Fallas Padilla, Han Pu, Junichiro Kono, and Motoaki Bamba
“Perfect Intrinsic Squeezing at the Superradiant Phase Transition Critical Point”
Scientific Reports 13, 2526 (2023)