Quasiparticle transitions in charge-phase qubits probed by rf oscillations

Abstract

We investigated transitions in an Al charge-phase qubit of SQUID-configuration which was inductively coupled to an rf tank circuit that made it possible to read out the state by measuring the Josephson inductance of the qubit. Depending on the flux and charge bias and on the amplitude of the rf-oscillations, we probed either the ground state or a dynamic change of the qubit states which we attributed to stochastic single quasiparticle tunneling onto and off the qubit island, involving an exchange of energy with the qubit. Within the scope of this model, a selection rule for quasiparticle-induced transitions in the qubit is discussed.

Current-phase relation and Josephson inductance in a superconducting Cooper-pair transistor

“…6 However, in superconducting Cooper pair transistors, where Coulomb energy plays an important role, the results on ͑derivatives of͒ CPRs have remained few and unpersuasive. [25][26][27] Closest to our work is the L J measurement of Ref. 27 but their result was distinctly affected by a mixing of the ground and excited states due to nonequilibrium quasiparticles.…”

Progress in the development of a single Cooper-pair box qubit

“…Another source of decoherence in a conventional SCB comes from the tunneling of quasiparticles from the ground plane to the island and back again, due to the presence of a non-equilibrium population of quasiparticles in the ground electrodes [12][13][14][15][16][17][18]. This effect, known as quasiparticle poisoning, can be easily observed in the average charge due to the presence of a "short step" in the Coulomb staircase, or directly in the time domain when measuring the quantum capacitance.…”

“…Embedding charge qubits in electrical resonators led to an extraordinary series of experiments in circuit quantum electrodynamics involving the interaction between microwave photons and the quantum states of the SCB [11]. Improved understanding of decoherence mechanisms such as quasiparticle poisoning [12][13][14][15][16][17][18] and background charge fluctuations [19] has led to optimized architectures, such as the transmon [20,21], showing a high degree of immunity to these problems. In what follows, we describe the progress in the development of the SCB at the Jet Propulsion Laboratory.…”

Energy relaxation of a superconducting charge qubit via Andreev processes

“…A large number of recent experimental studies 1,2,3,4,5,6,7,8,9 indicates the presence of quasiparticles in superconducting single-charge devices at low temperatures. The operation of these devices, of which the best known is Cooper-pair box qubit, requires 2e-periodic dependence of the charge of the box on its gate voltage, and thus, an introduction of an unpaired electron(quasiparticle) in the Cooper-pair box (CPB) is a significant problem.…”