Jingtao Fan

Photon Devil’s staircase: photon long-range repulsive interaction in lattices of coupled resonators with Rydberg atoms

The realization of strong coherent interactions between individual photons is a long-standing goal in science and engineering. In this report, based on recent experimental setups, we derive a strong photon long-range repulsive interaction, by controlling the van der Waals repulsive force between Cesium Rydberg atoms located inside different cavities in extended Jaynes-Cummings-Hubbard lattices. We also find novel quantum phases induced by this photon long-range repulsive interaction. For example, without photon hopping, a photon Devil’s staircase, induced by the breaking of long-range translation symmetry, can emerge. If photon hopping occurs, we predict a photon-floating solid phase, due to the motion of particle- and hole-like defects. More importantly, for a large chemical potential in the resonant case, the photon hopping can be frozen even if the hopping term exists. We call this new phase the photon-frozen solid phase. In experiments, these predicted phases could be detected by measuring the number of polaritons via resonance fluorescence.

Electric-field-induced interferometric resonance of a one-dimensional spin-orbit-coupled electron

The efficient control of electron spins is of crucial importance for spintronics, quantum metrology, and quantum information processing. We theoretically formulate an electric mechanism to probe the electron spin dynamics, by focusing on a one-dimensional spin-orbit-coupled nanowire quantum dot. Owing to the existence of spin-orbit coupling and a pulsed electric field, different spin-orbit states are shown to interfere with each other, generating intriguing interference-resonant patterns. We also reveal that an in-plane magnetic field does not affect the interval of any neighboring resonant peaks, but contributes a weak shift of each peak, which is sensitive to the direction of the magnetic field. We find that this proposed external-field-controlled scheme should be regarded as a new type of quantum-dot-based interferometry. This interferometry has potential applications in precise measurements of relevant experimental parameters, such as the Rashba and Dresselhaus spin-orbit-coupling strengths, as well as the Landé factor.

Creating a tunable spin squeezing via a time-dependent collective atom-photon coupling

We present an experimentally feasible method to produce a large and tunable spin squeezing when an ensemble of many four-level atoms interacts simultaneously with a single-mode photon and classical driving lasers. Our approach is to simply introduce a time-dependent collective atom-photon coupling. We show that the maximal squeezing factor measured experimentally can be well controlled by both its driving magnitude and driving frequency. In particular, when increasing the driving magnitude, the maximal squeezing factor increases and thus can be rapidly enhanced. We also demonstrate explicitly in the high-frequency approximation that this spin squeezing arises from a strong repulsive spin-spin interaction induced by the time-dependent collective atom-photon coupling. Finally, we evaluate analytically, by using current experimental parameters, the maximal squeezing factor, which can reach 40 dB. This squeezing factor is far larger than previous ones.

Quantum mixed phases of a two-dimensional polarized degenerate Fermi gas in an optical cavity

The coupling of ultracold fermions to a high-finesse optical cavity can result in novel many-body phenomena, and has attracted significant interests at present. Here we consider a realization of the Fermi-Dicke model with controllable parameters, based on a two-dimensional polarized degenerate Fermi gas coupled to an optical cavity. We analytically investigate the ground-state properties of such system under the mean-field approximation. We find the system can exhibit a rich phase diagram depending on the fermion-photon coupling strength and the atomic resonant frequency. Contrasting to the bosonic counterpart, a first-order quantum phase transition between the superradiant phase and the normal phase featuring two Fermi surfaces can occur for the weak atomic resonant frequency, and there is a unique mixed phase where this normal phase and the superradiant phase coexist. The experimental detection of our results is also discussed.

Superfluid–Mott-insulator quantum phase transition of light in a two-mode cavity array with ultrastrong coupling

In this paper we construct a new type of cavity array, in each cavity of which multiple two-level atoms interact with two independent photon modes. This system can be totally governed by a two-mode Dicke-lattice model, which includes all of the counter-rotating terms and therefore works well in the ultrastrong coupling regime achieved in recent experiments. Attributed to its special atom-photon coupling scheme, this model supports a global conserved excitation and a continuous

Phase-factor-dependent symmetries and quantum phases in a three-level cavity QED system

U(1)

Tunable two-axis spin model and spin squeezing in two cavities*

symmetry, rather than the discrete

Hidden continuous symmetry and Nambu-Goldstone mode in a two-mode Dicke model

Z_{2}

Creating a giant and tunable spin squeezing via a time-dependent collective atom-photon coupling

symmetry in the standard Dicke-lattice model. This distinct change of symmetry via adding an extra photon mode strongly impacts the nature of photon localization/delocalization behavior. Specifically, the atom-photon interaction features stable Mott-lobe structures of photons and a second-order superfluid-Mott-insulator phase transition, which share similarities with the Jaynes-Cummings-lattice and Bose-Hubbard models. More interestingly, the Mott-lobe structures predicted here depend crucially on the atom number of each site. We also show that our model can be mapped into a continuous

Superfluid-superradiant mixed phase of the interacting degenerate Fermi gas in an optical cavity

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