Radio-frequency method for investigation of quantum properties of superconducting
structures
E. Il’ichev,
*
N. Oukhanski, Th. Wagner, and H.-G. Meyer
Institute for Physical High Technology, P.O. Box 100239, D-07702 Jena, Germany
A. Yu. Smirnov
D-Wave Systems Inc., 320-1985 W. Broadway, Vancouver, B.C., V6J 4Y3, Canada
M. Grajcar
Department of Solid State Physics, Comenius University, SK-84248 Bratislava, Slovakia
A. Izmalkov
Institute for Physical High Technology, P.O. Box 100239, D-07702 Jena, Germany; Moscow Engineering
Physics Institute (State University), Kashirskoe sh. 31, Moscow, 115409, Russia
D. Born
Institute for Physical High Technology, P.O. Box 100239, D-07702 Jena, Germany; Friedrich Schiller
University, Institute of Solid State Physics, D-07743 Jena, Germany
W. Krech
Friedrich Schiller University, Institute of Solid State Physics, D-07743 Jena, Germany
A. Zagoskin
D-Wave Systems Inc., 320-1985 W. Broadway, Vancouver, B.C., V6J 4Y3, Canada; Physics and Astronomy
Dept., The University of British Columbia, 6224 Agricultural Rd., Vancouver, B.C., V6T 1Z1 Canada
͑Submitted January 28, 2004͒
Fiz. Nizk. Temp. 30, 823–833 ͑July–August 2004͒
We implement the impedance measurement technique ͑IMT͒ for characterization of interferometer-
type superconducting qubits. In the framework of this method, the interferometer loop is
inductively coupled to a high-quality tank circuit. We show that the IMT is a powerful tool for
studying the response of an externally controlled two-level system to different types of
excitations. Conclusive information about the qubits is obtained from a readout of the tank
properties. © 2004 American Institute of Physics. ͓DOI: 10.1063/1.1789933͔
1. INTRODUCTION
Quantum effects in mesoscopic superconducting circuits
of small Josephson junctions have attracted renewed atten-
tion. It was clearly demonstrated that Josephson devices can
behave like single microscopic particles if they are suffi-
ciently isolated from the environment. Therefore, ideas de-
veloped in atomic and molecular physics can be used for
description of artificially fabricated circuits of macroscopic
size. These concepts are stimulated further by the prospect of
a promising way to realize quantum bits ͑qubits͒ for quantum
information processing.
Qubits are two-level quantum systems with externally
controlled parameters. Generally, two kinds of such devices
with small-size Josephson junctions have been developed.
One approach is based on the charge degree of freedom;
basic states of this kind of qubit are distinguished by the
number of Cooper pairs on a specially designed island. The
alternative realization utilizes the phase of a Josephson junc-
tion ͑or the flux in a ring geometry͒, which is conjugate to
the charge degree of freedom. Due to macroscopic the size of
superconducting qubits, they are extremely sensitive to ex-
ternal disturbances. Thus the back-action of a detector should
be as small as possible. Many different detectors have been
suggested in the literature ͑see Ref. 1 and references therein͒.
In this paper we review our results obtained on super-
conducting qubits by the impedance measurement technique
͑IMT͒. Below we shall discuss several quantum effects in-
cluding macroscopic quantum tunneling, Landau–Zener
transitions, Rabi oscillations, and direct resonant spectros-
copy of the qubit energy levels. Finally, we present our very
recent results of investigation of two coupled qubits.
2. MACROSCOPIC QUANTUM TUNNELING
For the flux qubits the Josephson energy dominates over
the charge energy, E
J
ӷE
C
. It was predicted that such sys-
tems should exhibit various quantum-mechanical effects, in-
cluding macroscopic quantum tunneling ͑MQT͒ of the flux.
2
Indeed, the predicted effects have been observed
experimentally.
3–6
In this Section we briefly discuss the main
properties of the flux qubits and demonstrate that the IMT
technique is a powerful tool for the investigation of the
MQT.
LOW TEMPERATURE PHYSICS VOLUME 30, NUMBERS 7–8 JULY–AUGUST 2004
6201063-777X/2004/30(7–8)/9/$26.00 © 2004 American Institute of Physics