Coherent operation of a gap-tunable flux qubit

Abstract

We replace the Josephson junction defining a three-junction flux qubit's properties with a tunable direct current superconducting quantum interference devices (DC-SQUID) in order to tune the qubit gap during the experiment. We observe different gaps as a function of the external magnetic pre-biasing field and the local magnetic field through the DC-SQUID controlled by high-bandwidth on chip control lines. The persistent current and gap behavior correspond to numerical simulation results. We set the sensitivity of the gap on the control lines during the sample design stage. With a tuning range of several GHz on a qubit dynamics timescale, we observe coherent system dynamics at the degeneracy point.PACS numbers: 03.67. Lx,85.25.Hv,85.25.Cp Superconducting flux qubits consisting of three Josephson junctions embedded in a low inductance superconducting loop 1,2 are promising candidates for quantum information processing 3 . With the standard design, the magnetic field inducing an energy difference between the ground state and the first excited state is the only parameter that is adjustable during the experiment. At the degeneracy point, it reaches its minimum value (called the gap ∆), which is related to the quantum mechanical tunnel rate. However, the gap is determined at the time of production by the inherent parameters of the three Josephson junctions. One of the main problems in realizing quantum computation schemes using flux qubits is 1/f flux noise. At the degeneracy point the qubit is decoupled from low frequency flux noise 4,5 , and achieves maximal coherence times of several µs rather than ns at off-degeneracy point, wherefore this is the optimal operation point. Implementing a quantum processor based on flux qubits requires the operation at this optimal point at all times, especially when coupling several qubits. Recently, circuit quantum electrodynamics (QED) (quantum bus, qubus) schemes have been developed for many superconducting qubits 6-10 , however standard flux qubits suffer from the fact that bringing them in resonance with such a quantum bus requires to operate them at the offdegeneracy point.With the flux qubit we present here we aim to overcome these limitations by replacing the smallest junction of the flux qubit with a low inductance DC-SQUID loop 1,2 . Varying the magnetic flux penetrating this loop is equivalent to changing the critical current of the smallest junction, thus making the gap tunable during the experiment. Using this tuning parameter we can control the coupling to a qubus by tuning into or out of the qubus frequency while operating at the optimal point. Moreover, we can implement strong transverse coupling to another qubit or to a resonator 11,12 . a) xbzhu@will.brl.ntt.co.jp b) semba@will.brl.ntt.co.jp Our qubit is shown schematically in Fig. 1. Two identical junctions with Josephson energy E J in series with a symmetric DC-SQUID, in which each junction has a Josephson energy of α 0 E J are enclosed in a superconducting loop. The effective Josephson energy of the DC-SQUID ...

Controllable coupling between a charge qubit and a spin ensemble

“…Here ε 2 (f 2 ,f s ) = I (2) p 0 (f 2 + f s /2 − 1/2), I (2) p is the maximum persistent current of the flux qubit, and 2 is the tunneling amplitude and is adjustable by the effective Josephson energy αE J 2 via the external flux f s [3].…”

“…As for the flux qubit, it is hard to control the transverse term via an external parameter. Even for a gap-tunable flux qubit with four JJs [3], we cannot switch off this transverse term. The flux qubit has a longer decoherence time, since the flux qubit works in phase regime and, and to a certain extent, is immune to background charge fluctuations.…”

Hybrid-system approach to fault-tolerant quantum communication

“…The gap ( α ) can be in situ tuned on a nanosecond time scale while keeping ε( qb ) = 0 [40]. (3) Coupling between a flux qubit and an ensemble: The coupling between a flux qubit and an ensemble can be represented by [33,38] …”

Discrete-time quantum walk with nitrogen-vacancy centers in diamond coupled to a superconducting flux qubit

“…A two-level approximation can be made to the band structure of the potential by assuming that the external flux is in the vicinity of half of the flux quantum, where the two lowest bands are closest to each other. A small overlap of the wave functions of the two levels opens the qubit gap ∆ q , which can be tuned [44][45][46] by replacing the smallest Josephson junction with two parallel junctions, a DC-SQUID. On the basis of the two counter-rotating persistent currents in the qubit loop, the qubit Hamiltonian can be written as…”