Download Operation of a quantum bit circuit based on the Cooper pair

Operation of a quantum bit circuit based on the Cooper pair box
D. Vion, A. Aassime, A. Cottet, P. Joyez, H. Pothier, C. Urbina, D. Esteve, and M.H.
Devoret
Quantronics group, Service de Physique de l’Etat Condensé, CEA-Saclay
91191 Gif-sur-Yvette, France
We have designed and operated a quantum bit circuit [1-2] based on the Cooper
pair box, a superconducting device for which quantum coherence has already been
demonstrated. In this new circuit, the box Josephson junction is replaced by two junctions
in parallel forming a loop. The advantage of this design is to provide separate ports for
qubit manipulation and readout, and to efficiently decouple the qubit from its external
environment when readout is off. The qubit manipulation is performed by applying
microwave pulses to the box gate, and the readout by measuring the current in the loop,
which is different for both qubit states. This measurement is performed by monitoring the
switching of an extra large Josephson junction inserted in the loop when a bias current
pulse is applied to it. We show that this readout strategy approaches single-shot
resolution, and we demonstrate that all qubit manipulations can be performed using Rabi
precession of the qubit state when resonant microwave pulses are applied to the gate.
Using a two-pulse sequence analogous to the Ramsey sequence in atomic physics, we
have determined the coherence time of the qubit (see Figure). This coherence time, which
corresponds to about 8000 periods of the qubit transition, is sufficiently long to envision
coupled qubit circuits.
1
(%)
0
15
P 1
P 0
10
5
0
0.0
0.1
0.2
0.3
t (µs)
0.4
0.5
Top left: scanning electron micrograph of a sample. Top right: Ramsey-like pulse sequence applied to the qubit
gate (top electrode) to determine the coherence time. The microwave frequency is close to the qubit transition
frequency. The final state is measured by projection onto the qubit eigenstates 0 and 1. Bottom: experimental
population variation of these states (dots) and theoretical fit (solid line). The amplitude of the beating
oscillations decreases exponentially because of the decoherence between the two pulses.
[1] A. Cottet, D. Vion, P . Joyez, A. Aassime, D. Esteve, and M.H. Devoret, Physica C 367, 197 (2002).
[2] D. Vion, A. Aassime, A. Cottet, P. Joyez, H. Pothier, C. Urbina, D. Esteve, et M.H. Devoret, to appear in
Science (2002).