72 research outputs found
Quantum state detection of a superconducting flux qubit using a DC-SQUID in the inductive mode
We present a readout method for superconducting flux qubits. The qubit
quantum flux state can be measured by determining the Josephson inductance of
an inductively coupled DC superconducting quantum interference device
(DC-SQUID). We determine the response function of the DC-SQUID and its
back-action on the qubit during measurement. Due to driving, the qubit energy
relaxation rate depends on the spectral density of the measurement circuit
noise at sum and difference frequencies of the qubit Larmor frequency and SQUID
driving frequency. The qubit dephasing rate is proportional to the spectral
density of circuit noise at the SQUID driving frequency. These features of the
backaction are qualitatively different from the case when the SQUID is used in
the usual switching mode. For a particular type of readout circuit with
feasible parameters we find that single shot readout of a superconducting flux
qubit is possible.Comment: 11 pages, 3 figures; submitted to Phys. Rev.
Coherent Quantum Dynamics of a Superconducting Flux Qubit
We have observed coherent time evolution between two quantum states of a
superconducting flux qubit comprising three Josephson junctions in a loop. The
superposition of the two states carrying opposite macroscopic persistent
currents is manipulated by resonant microwave pulses. Readout by means of
switching-event measurement with an attached superconducting quantum
interference device revealed quantum-state oscillations with high fidelity.
Under strong microwave driving it was possible to induce hundreds of coherent
oscillations. Pulsed operations on this first sample yielded a relaxation time
of 900 nanoseconds and a free-induction dephasing time of 20 nanoseconds. These
results are promising for future solid-state quantum computing.Comment: submitted 2 December 2002; accepted 4 February 200
Detection of a persistent-current qubit by resonant activation
We present the implementation of a new scheme to detect the quantum state of
a persistent-current qubit. It relies on the dependency of the measuring
Superconducting Quantum Interference Device (SQUID) plasma frequency on the
qubit state, which we detect by resonant activation. With a measurement pulse
of only 5ns, we observed Rabi oscillations with high visibility (65%).Comment: 4 pages, 4 figures, submitted to PRB Rapid Co
QND measurement of a superconducting qubit in the weakly projective regime
Quantum state detectors based on switching of hysteretic Josephson junctions
biased close to their critical current are simple to use but have strong
back-action. We show that the back-action of a DC-switching detector can be
considerably reduced by limiting the switching voltage and using a fast
cryogenic amplifier, such that a single readout can be completed within 25 ns
at a repetition rate of 1 MHz without loss of contrast. Based on a sequence of
two successive readouts we show that the measurement has a clear quantum
non-demolition character, with a QND fidelity of 75 %.Comment: submitted to PR
Nondestructive readout for a superconducting flux qubit
We present a new readout method for a superconducting flux qubit, based on
the measurement of the Josephson inductance of a superconducting quantum
interference device that is inductively coupled to the qubit. The intrinsic
flux detection efficiency and back-action are suitable for a fast and
nondestructive determination of the quantum state of the qubit, as needed for
readout of multiple qubits in a quantum computer. We performed spectroscopy of
a flux qubit and we measured relaxation times of the order of 80 .Comment: 4 pages, 4 figures; modified content, figures and references;
accepted for publication in Phys. Rev. Let
Parametric coupling for superconducting qubits
We propose a scheme to couple two superconducting charge or flux qubits
biased at their symmetry points with unequal energy splittings. Modulating the
coupling constant between two qubits at the sum or difference of their two
frequencies allows to bring them into resonance in the rotating frame.
Switching on and off the modulation amounts to switching on and off the
coupling which can be realized at nanosecond speed. We discuss various physical
implementations of this idea, and find that our scheme can lead to rapid
operation of a two-qubit gate.Comment: 6 page
Selective darkening of degenerate transitions for implementing quantum controlled-NOT gates
We present a theoretical analysis of the selective darkening method for
implementing quantum controlled-NOT (CNOT) gates. This method, which we
recently proposed and demonstrated, consists of driving two
transversely-coupled quantum bits (qubits) with a driving field that is
resonant with one of the two qubits. For specific relative amplitudes and
phases of the driving field felt by the two qubits, one of the two transitions
in the degenerate pair is darkened, or in other words, becomes forbidden by
effective selection rules. At these driving conditions, the evolution of the
two-qubit state realizes a CNOT gate. The gate speed is found to be limited
only by the coupling energy J, which is the fundamental speed limit for any
entangling gate. Numerical simulations show that at gate speeds corresponding
to 0.48J and 0.07J, the gate fidelity is 99% and 99.99%, respectively, and
increases further for lower gate speeds. In addition, the effect of
higher-lying energy levels and weak anharmonicity is studied, as well as the
scalability of the method to systems of multiple qubits. We conclude that in
all these respects this method is competitive with existing schemes for
creating entanglement, with the added advantages of being applicable for qubits
operating at fixed frequencies (either by design or for exploitation of
coherence sweet-spots) and having the simplicity of microwave-only operation.Comment: 25 pages, 5 figure
Decoherence of Flux Qubits Coupled to Electronic Circuits
On the way to solid-state quantum computing, overcoming decoherence is the
central issue. In this contribution, we discuss the modeling of decoherence of
a superonducting flux qubit coupled to dissipative electronic circuitry. We
discuss its impact on single qubit decoherence rates and on the performance of
two-qubit gates. These results can be used for designing decoherence-optimal
setups.Comment: 16 pages, 5 figures, to appear in Advances in Solid State Physics,
Vol. 43 (2003
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