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arXiv:cond-mat/0605484 · Physical Review B

Switchable resonant coupling of flux qubits

M. Grajcar, Yu-xi Liu, Franco Nori, Alexandre M. Zagoskin

Published Nov 17, 2006• 70 citations
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Abstract

We propose a coupling scheme where two or more flux qubits with different eigenfrequencies share Josephson junctions with a coupler loop devoid of its own quantum dynamics. Switchable two-qubit coupling is realized by tuning the frequency of the ac magnetic flux through the coupler to a combination frequency of two of the qubits. The coupling allows any or all of the qubits to be simultaneously at the degeneracy point and can change sign.

DOI
10.1103/physrevb.74.172505

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We propose a coupling scheme where two or more flux qubits with different eigenfrequencies share Josephson junctions with a coupler loop devoid of its own quantum dynamics. Switchable two-qubit coupling is realized by tuning the frequency of the ac magnetic flux through the coupler to a combination frequency of two of the qubits. The coupling allows any or all of the qubits to be simultaneously at the degeneracy point and can change sign.
We realize a device allowing for tunable and switchable coupling between two frequency-degenerate superconducting resonators mediated by an artificial atom. For the latter, we utilize a persistent current flux qubit. We characterize the tunable and switchable coupling in the frequency and time domains and find that the coupling between the relevant modes can be varied in a controlled way. Specifically, the coupling can be tuned by adjusting the flux through the qubit loop or by controlling the qubit population via a microwave drive. Our measurements allow us to find parameter regimes for optimal coupler performance and quantify the tunability range.
We have realized controllable coupling between two three-junction flux qubits by inserting an additional coupler loop between them, containing three Josephson junctions. Two of these are shared with the qubit loops, providing strong qubit-coupler interaction. The third junction gives the coupler a nontrivial current-flux relation; its derivative (i.e., the susceptibility) determines the coupling strength J, which thus is tunable in situ via the coupler's flux bias. In the qubit regime, J was varied from approximately 45 (antiferromagnetic) to approximately -55 mK (ferromagnetic); in particular, J vanishes for an intermediate coupler bias. Measurements on a second sample illuminate the relation between two-qubit tunable coupling and three-qubit behavior.
The ability to non-dissipatively tune the Josephson coupling energy of Josephson junctions is a useful tool in frequency-tunable qubits. This is typically done by threading magnetic flux through two junctions connected in a loop, a geometry that exposes the qubit to magnetic environmental noise. In this paper, we show that by biasing a junction with supercurrent from a separate pair of superconducting leads coupled to the device, the Josephson energy can be tuned without the need for a flux loop. Our multiterminal device may enable the realization of a frequency-tunable qubit with greatly reduced susceptibility to flux noise.
The authors propose a qubit-qubit coupling scheme for superconducting flux quantum bits (qubits), where quantized Josephson junction resonator and microwave irradiation are utilized. The junction is used as a tunable inductance controlled by changing the bias current flowing through the junction, and thus the circuit works as a tunable resonator. This enables them to make any qubits interact with the resonator. Entanglement between two of many qubits whose level splittings satisfy some conditions is formed by microwave irradiation causing a two-photon Rabi oscillation. Since the size of the resonator can be as large as submillimeters and qubits interact with it via mutual inductance, their scheme makes it possible to construct a quantum gate involving remote qubits.
We propose a hybrid quantum circuit with both charge and flux qubits connected to a large Josephson junction that gives rise to an effective interqubit coupling controlled by the external magnetic flux. This switchable interqubit coupling can be used to transfer back and forth an arbitrary superposition state between the charge qubit and the flux qubit working at the optimal point. The proposed hybrid circuit provides a promising quantum memory because the flux qubit at the optimal point can store the tranferred quantum state for a relatively long time.