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Bulletin of the American Physical Society

Investigation of the measurement dynamics of a flux qubit inductively coupled to a readout dc-SQUID

Peter Groszkowski, Jay Gambetta, Frank K. Wilhelm

Published Mar 21, 2011• 0 citations
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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 back-action 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.

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Dc SQUID sensitivity for readout of entangled state quantum bits

H Tanaka, Y Sekine, S Saito +1 more

We propose a scheme for entangled state measurement at flux qubits (quantum bits) depending on the sensitivity measurement of a dc SQUID (superconducting quantum interference device). The dc SQUID is used as a flux qubit readout device. The switching current distribution of the dc SQUID is sufficiently narrow to distinguish between two ground states of three Josephson junction qubits with a large mutual inductance between a qubit and the dc SQUID. However, discrimination between two ground states becomes difficult with the smaller mutual inductance that is preferable for qubit coherence. We employed averaging to increase the effective sensitivity. However the averaging smears out the entanglement over two qubits. We found that a Bell-pair measurement can be performed with flux qubits to prove the existence of an entangled state over two qubits. The pre-ensembled measurement scheme, in which our flux qubit readouts work, is preferable to entangled state measurement.

arXiv:cond-mat/0108231•Superconductor Science and Technology•Nov 21, 2001•5 citations
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 back-action 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.
We propose a scheme for entangled state measurement at flux qubits (quantum bits) depending on the sensitivity measurement of a dc SQUID (superconducting quantum interference device). The dc SQUID is used as a flux qubit readout device. The switching current distribution of the dc SQUID is sufficiently narrow to distinguish between two ground states of three Josephson junction qubits with a large mutual inductance between a qubit and the dc SQUID. However, discrimination between two ground states becomes difficult with the smaller mutual inductance that is preferable for qubit coherence. We employed averaging to increase the effective sensitivity. However the averaging smears out the entanglement over two qubits. We found that a Bell-pair measurement can be performed with flux qubits to prove the existence of an entangled state over two qubits. The pre-ensembled measurement scheme, in which our flux qubit readouts work, is preferable to entangled state measurement.