Banishing quasiparticles from Josephson-junction qubits: why and how to do it

Current-biased Josephson junctions are prime candidates for the realization of quantum bits; however, a present limitation is their coherence time. In this paper it is shown qualitatively that quasiparticles create decoherence. We can decrease the number of quasiparticles present in the junctions by two methods - reducing the creation rate with current shunts and increasing the depletion rate with normal-metal traps. Experimental data demonstrate that both methods are required to significantly reduce the number of quasiparticles and increase the system's coherence. We conclude that these methods are effective and that the design of Josephson-junction qubits must consider the role of quasiparticles.

[1]  Siyuan Han,et al.  Coherent Temporal Oscillations of Macroscopic Quantum States in a Josephson Junction , 2002, Science.

[2]  V. Shumeiko,et al.  DC-CURRENT TRANSPORT AND AC JOSEPHSON EFFECT IN QUANTUM JUNCTIONS AT LOW VOLTAGE , 1997 .

[3]  John M. Martinis,et al.  Decoherence of a superconducting qubit due to bias noise , 2003 .

[4]  Jaw-Shen Tsai,et al.  Spectroscopy of Energy-Level Splitting between Two Macroscopic Quantum States of Charge Coherently Superposed by Josephson Coupling , 1997 .

[5]  D. Schuster,et al.  Measurement of the excited-state lifetime of a microelectronic circuit. , 2003, Physical review letters.

[6]  Clarke,et al.  Experimental tests for the quantum behavior of a macroscopic degree of freedom: The phase difference across a Josephson junction. , 1987, Physical review. B, Condensed matter.

[7]  T Yamamoto,et al.  Charge echo in a cooper-pair box. , 2002, Physical review letters.

[8]  Cron,et al.  Supercurrent in atomic point contacts and andreev states , 2000, Physical review letters.

[9]  Averin,et al.  ac Josephson Effect in a Single Quantum Channel. , 1995, Physical review letters.

[10]  J. Martinis,et al.  Rabi oscillations in a large Josephson-junction qubit. , 2002, Physical review letters.

[11]  Roberto Ramos,et al.  Design for effective thermalization of junctions for quantum coherence , 2001 .

[12]  Hamiltonian approach to the transport properties of superconducting quantum point contacts. , 1996, Physical review. B, Condensed matter.

[13]  P. Joyez,et al.  Manipulating the Quantum State of an Electrical Circuit , 2002, Science.

[14]  Clarke,et al.  Energy-level quantization in the zero-voltage state of a current-biased Josephson junction. , 1985, Physical review letters.

[15]  Siyuan Han,et al.  Time-Resolved Measurement of Dissipation-Induced Decoherence in a Josephson Junction , 2001, Science.

[16]  Y. Makhlin,et al.  Quantum-state engineering with Josephson-junction devices , 2000, cond-mat/0011269.

[17]  G. Arnold.  Superconducting tunneling without the tunneling Hamiltonian. II. Subgap harmonic structure , 1987 .