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

We have performed spectroscopy measurements on two coupled flux qubits. The qubits are coupled inductively, which results in a sigma(z)(1)sigma(z)(2) interaction. By applying microwave radiation, we observe resonances due to transitions from the ground state to the first two excited states. From the position of these resonances as a function of the applied magnetic field, we observe the coupling of the qubits. The coupling strength agrees with calculations of the mutual inductance.