Photonic phase transition in circuit quantum electrodynamics lattices coupled to superconducting phase qubits

A hybrid quantum architecture was proposed to engineer a localization-delocalization phase transition of light in a two-dimension square lattices of superconducting coplanar waveguide resonators, which are interconnected by current-biased Josephson junction phase qubits. We find that the competition between the on-site repulsion and the nonlocal photonic hopping leads to the Mott insulator-superfluid transition. By using the mean-field approach and the quantum master equation, the phase boundary between these two different phases could be obtained when the dissipative effects of superconducting resonators and phase qubit are considered. The good tunability of the effective on-site repulsion and photon-hopping strengths enable quantum simulation on condensed matter physics and many-body models using such a superconducting resonator lattice system. The experimental feasibility is discussed using the currently available technology in the circuit QED.     

Efficient Atomic One-Qubit Phase Gate Realized by a Cavity QED and Identical Atoms System

We present a scheme to implement a one-qubit phase gate with a two-level atom crossing an optical cavity in which some identical atoms are trapped. One can conveniently acquire an arbitrary phase shift of the gate by properly choosing the number of atoms trapped in the cavity and the velocity of the atom crossing the cavity. The present scheme provides a very simple and efficient way for implementing one-qubit phase gate.     

Magneto-optical rotation in cavity QED with Zeeman coherence

We investigate theoretically the magneto-optical rotation in cavity QED system with atomic Zeeman coherence, which is established via coherent population trapping. Owing to Zeeman coherence, the ultranarrow transmission spectrum less than 1 MHz with gain can be achieved with a flat-top Faraday rotation angle. By controlling the parameters appropriately, the input probe components within the flat-top regime rotate with almost the same angle, and transmit through the cavity perpendicularly to the other components outside the flat-top regime. The concepts discussed here provide an important tool for perfect ultranarrow Faraday optical filter and quantum information processing.     

Cluster-state preparation in thermal cavities without single-qubit operation

A potential scheme is proposed for generating cluster states of many atoms in cavity quantum electradynamics （QED）, in which an unorthodox encoding is employed with the ground state being qubit [0〉 while two closely spaced upper states being qubit ｜1〉. Throughout the scheme the cavities can be in thermal states but axe only virtually excited. We show how to create the cluster states by performing a two-step hut no single-qubit operation. Discussion is also carried out on the experimental feasibility of our scheme.     

Multi-photon blockade and dressing of the dressed states

Resonant excitation of multi-photon transitions in one-atom cavity QED, from the ground state to an excited dressed state, induce an additional semiclassical Rabi splitting. This “dressing of the dressed states” arises from a multi-photon blockade, where, for sufficiently strong coupling, detunings, brought about by the nonlinearity of the Jaynes-Cummings model, inhibit absorption of additional photons. We show how this behavior is revealed in the spectrum and photon correlations of the quasi-elastically scattered light. Explicit results are presented for the two-photon case.     

Engineering two-mode squeezed states of cold atomic clouds with a superconducting stripline resonator

A scheme is proposed for engineering two-mode squeezed states of two separated cold atomic clouds positioned near the surface of a superconducting stripline resonator. Based on the coherent magnetic coupling between the atomic spins and a stripline resonator mode, the desired two-mode squeezed state can be produced via precisely control on the dynamics of the system. This scheme may be used to realize scalable on-chip quantum networks with cold atoms coupling to stripline resonators.     

Compton scattering in intense magnetic fields

Compton scattering in intense magnetic fields in the general frame of reference is studied with the help of the QED perturbation theory in the incoming interaction picture. A general expression for the cross section is derived which reduces naturally to the one in the electron-rest frame of reference. This expression can be approximately simplified for the scattering of a high-energy electron with a low-frequency photon. Based on this simplified expreaaion, spectrum functions, as well as power spectra of scattered photons with high energies resulting from the inverse Compton scattering are calculated which manifest clearly a feature of resonances. Project supported by the National Natural Science Foundation of China (Grant No. 19573008) and the Science Research Division of Shanghai Jiaotong University.     

Nonrelativistic calculation of the Lamb shift to order α

It is demonstrated, for the first time to our knowledge, that the Lamb-shift calculation for all states of hydrogenic atoms can be carried out to order α⁵ by applying the methods of nonrelativistic quantum electrodynamics and without using the Dirac equation and the second quantization for the electron. The extremely small deviations from the standard relativistic results were calculated for different S- and non-S-states as well.     

Conditional Generation of Multiparticle Entanglement via Cavity QED

We propose a simple scheme to not only generate GHZ states and W states of the multiparticle but also form a new category of multiparticle entangled states by letting the A-type three-level atoms simultaneously interacting with a coherent cavity field followed by the selective measurements on the cavity mode. We investigate the influence of the cavity dissipation on the generated entangled state and discuss the experimental feasibility of our scheme. It is shown that the intensity of the coherent cavity field plays an instructive role in contribution to state preparation process while the cavity decay and the detuning between the atoms and cavity mode result in the deterioration of the generated entangled state.     

Common Space of Spin and Spacetime

Given Lorentz invariance in Minkowski spacetime, we investigate a common space of spin and spacetime. To obtain a finite spinor representation of the non-compact homogeneous Lorentz group including Lorentz boosts, we introduce an indefinite inner product space (IIPS) with a normalized positive probability. In this IIPS, the common momentum and common variable of a massive fermion turn out to be “doubly strict plus-operators”. Due to this nice property, it is straightforward to show an uncertainty relation between fermion mass and proper time. Also in IIPS, the newly-defined Lagrangian operators are self-adjoint, and the fermion field equations are derivable from the Lagrangians. Finally, the nonlinear QED equations and Lagrangians are presented as an example.