Observation of macroscopic Landau – Zener transitions in a superconducting device

– A two-level system traversing a level anticrossing has a small probability to make a so-called Landau–Zener (LZ) transition between its energy bands, in deviation from simple adiabatic evolution. This effect takes on renewed relevance due to the observation of quantum coherence in superconducting qubits (macroscopic “Schrödinger cat” devices). We report an observation of LZ transitions in an Al three-junction qubit coupled to a Nb resonant tank circuit. In analogy to their classical counterparts, qubits are effectively two-level systems, with a time-dependent bias enabling one-qubit gate operations. Besides their computational use, this makes them suitable for studying Landau–Zener (LZ) transitions [1,2] (see below eq. (1)). One prominent qubit is a superconducting loop with low inductance L, interrupted by three Josephson junctions (a 3JJ qubit) [3]. Its Josephson energy, UJ = ∑3 j=1EJj(φj), depends on the phase differences φj across the junctions. In a small loop, due to magnetic flux quantization, only two φj ’s are independent. The two minima in UJ(φ1, φ2) correspond to the qubit states ψL and ψR, carrying opposite supercurrents around the loop. These become degenerate in the presence of an external magnetic flux Φe = 1 2 Φ0 (Φ0 ≡ h/2e is the flux quantum). The potential UJ is sketched in fig. 1a. (∗) E-mail: [email protected]