Tuning the Gap of a Superconducting Flux Qubit

Abstract

We experimentally demonstrate the in situ tunability of the minimum energy splitting (gap) of a superconducting flux qubit by means of an additional flux loop. Pulses applied via a local control line allow us to tune the gap over a range of several GHz on a nanosecond timescale. The strong flux sensitivity of the gap (up to ∼0.7 GHz/mΦ0) opens up the possibility to create different types of tunable couplings that are effective at the degeneracy point of the qubit. We investigate the dependence of the relaxation time and the Rabi frequency on the qubit gap.PACS numbers: 03.67. Lx, 85.25.Cp Superconducting circuits are promising candidates for the implementation of scalable quantum information processing [1]. To this purpose it is important to be able to selectively couple arbitrary quantum bits. Coupling multiple quantum two-level systems to a harmonic oscillator promises to be a successful strategy to create selective quantum gates in quantum optics and atomic physics [2], and in superconducting charge [3] and phase qubits [4]. In superconducting flux qubits single-qubit rotations [5], tunable qubit couplings [6] and two-qubit quantum gates [7] have been demonstrated as well as coupling between a flux qubit and a harmonic oscillator [8,9]. To controllably couple qubits via a harmonic oscillator bus requires the ability to tune the qubits in and out of resonance.In this Letter, we demonstrate the implementation of an additional flux loop to vary the minimum energy splitting, called the gap, of a superconducting flux qubit. In principle this control allows a fast change of the qubit resonance frequency while remaining at the point where the coherence properties of the qubit are optimal, i.e. at the gap. The large coupling makes this tunable qubit a good candidate to implement different types of qubit coupling besides σ z σ z , such as σ x σ z and σ x σ x [10].Commonly, a flux qubit consists of a small inductance superconducting loop intersected by three Josephson junctions ( Fig. 1(a)) [11]. If the flux penetrating the loop is close to half a superconducting flux quantum Φ 0 /2 (mod Φ 0 ), with Φ 0 = h/2e, the two lowest energy eigenstates can be used as a qubit (Fig. 1(b)). The qubit is characterized by the gap ∆ and by the persistent current I p . The energy eigenstates are linear combinations of clockwise and counterclockwise persistent-current states.In the persistent-current basis the qubit Hamiltonian can be written aswhere ǫ = 2I p (f ǫ − 1 2 )Φ 0 is the magnetic energy bias, with f ǫ the magnetic frustration of the qubit loop. σ z and σ x * Electronic address: j.e.mooij@tudelft.nl f fα fε fα

Experimental demonstration of a robust and scalable flux qubit

“…20,21 The CJJ rf-SQUID flux qubit and related variants have reappeared in a gradiometric configuration in more recent history. 14,36,37 Here, the single junction of Fig. 1͑a͒ has been replaced by a flux biased dc SQUID of inductance L cjj that allows one to tune the critical current of the rf SQUID in situ.…”

Coherent operation of a gap-tunable flux qubit

“…F. G. Paauw et al 13 utilize a gradiometric double loop design to solve this problem. With N fluxoids (N is an odd integer) trapped in the superconducting loop of the gradiometer the qubit is pre-biased closed to its degeneracy point.…”

“…This strong crosstalk will shift the operation point of the qubit, thus causing strong dephasing and transtions to higher levels. F. G. Paauw et al 13 utilize a gradiometric double loop design to solve this …”

Jahn-Teller instability in dissipative quantum systems

“…The proposal of Bourassa et al [9] was based on a three-junction flux qubit. The recent experiment of Paauw et al [10] measured the decay time, T 1 , of a three-junction flux qubit as a function of the qubit frequency. They have a considerable range: from T 1 ∼ 10 −6 to 10 −9 s. Typical T 2 times are half of this.…”