Bistable Josephson cell as a single microwave photon sensor

The dissipative dynamics of a bistable Josephson cell in a transmission line resonator in a weakly dissipative regime was studied theoretically and numerically. The bistable cell consists of a quantronium qubit and a nonlinear bifurcation amplifier. Under the rotating wave approximation, the problem of the system evolution in a boson thermostat is reduced to the Pauli equation. Using the master equation, we have numerically studied the dissipative dynamics of the system. It was shown that the nonlinear bifurcation amplifier can determine the qubit states and the system can be used as a detector of single microwave photons.

Practical one-step synthesis of multipartite entangled states on superconducting circuits

Quantum entanglement is an important resource for quantum information processing tasks. However, realistic multipartite entangled state production is very difficult. In this paper, we propose an efficient single-step scheme for generating many body Greenberger–Horne–Zeilinger (GHZ) states on superconducting circuits by using a superconducting transmission-line resonator (TLR) interact with [Formula: see text] superconducting transmon qubits. The distinct merit of our proposal is that it does not require the qubit-resonator coupling strengths to be the same, which is usually impractical experimentally, and thus is one of the main reasons for entanglement generation infidelity in previous single-step schemes. The removing of the uniform interaction requirement is achieved by modulating the qubits splitting frequencies with ac microwave fields, which results in tunable individual qubit-resonator coupling strength, and thus effective uniform qubit–qubit interaction Hamiltonian can be obtained. Since microwave control is conventional nowadays, our proposal can be directly tested experimentally, which makes previous multipartite entangled states generation schemes more efficient.

The study of entropy in a transmission line resonator interacting with a capacitively coupled Cooper pair box

In the present work, we theoretically study the entropy of a Cooper pair box coupled to a one-dimensional transmission line resonator. The proposed scheme uses the Jaynes-Cummings model in presence of losses to study the time avolution of the system entropy. The coupling of these two subsystems provides an important indication of the influence of Cooper pair box on the evolution of system entropy. Decaying CPB allows us to control variation of entropy at certain times.