Entangling flux qubits with a bipolar dynamic inductance

Abstract

We propose a scheme to implement variable coupling between two flux qubits using the screening current response of a dc Superconducting QUantum Interference Device (SQUID). The coupling strength is adjusted by the current bias applied to the SQUID and can be varied continuously from positive to negative values, allowing cancellation of the direct mutual inductance between the qubits. We show that this variable coupling scheme permits efficient realization of universal quantum logic. The same SQUID can be used to determine the flux states of the qubits.Comment: 4 pages, 4 figure

A Tunable Coupler with ScS Quantum Point Contact to Mediate Strong Interaction Between Flux Qubits

“…The density matrix of a pure state, such as (11), has the only nonzero eigenvalue λ = 1 (aŝ ρ 2 =ρ for it and thus λ 2 = λ). The reduced density operators for either of the two qubits are defined as partial traces ofρ over basis vectors of the other qubit, that iŝ…”

“…In first papers simple systems of flux qubits were studied, with qubits located side-by-side and coupled by constant coefficients of mutual inductances M ij , determined by geometry of the relative position and self-inductances of qubits 9,10 . At the same time, new versions of coupling elements were proposed theoretically [11][12][13] which, based on dc-and rf-SQUIDs, enabled to vary the magnitude and the sign of magnetic interaction of flux qubits to be coupled by means of an external control parameter (the bias current in dc SQUID and the external magnetic flux applied to the loop of rf SQUID). These signand magnitude-tunable qubit-coupling elements were implemented in the Refs.…”

Unified approach for universal quantum gates in a coupled superconducting two-qubit system with fixed always-on coupling

“…18 Implementing universal single-qubit gates and two-qubit gates in coupled multiqubit systems can be achieved by turning off and on the coupling between qubits. [18][19][20][21] In these schemes, supplementary circuits were required to control interqubit coupling. However, rapid switching of the coupling results in two serious problems.…”

“…16,17 Thus it is imperative to implement the universal single-qubit and two-qubit gates in a multiqubit system with the minimum resource and maximum efficiency. 18 Implementing universal single-qubit gates and two-qubit gates in coupled multiqubit systems can be achieved by turning off and on the coupling between qubits. [18][19][20][21] In these schemes, supplementary circuits were required to control interqubit coupling.…”

“…This approach can be used to construct scalable quantum information processing with simpler and fewer hardware resources when it is combined with other schemes. [18][19][20][21][22]24 In this approach, each single-qubit gate is realized via two controlled two-qubit gates which we call ͉0͘-controlled and ͉1͘-controlled twoqubit gates, respectively. The ͉0͘ ͉͑1͒͘-controlled gate implements a desired gate in one of the two coupled qubits when the other is in ͉0͘ ͉͑1͒͘.…”