Single-Shot Readout of Macroscopic Quantum Superposition State in a Superconducting Flux Qubit

Among many candidates toward realization of quan-tum computation, superconducting qubits based onJosephson junctions take on a position of increasing im-portance [1]. Although their coherence time still needto be substantially improved, solid state qubits suchas semiconductor qubits or superconductor qubits havethe distinct merit of scalability due to mature state ofnanometer-scale fabrication technology. Recently, sev-eral groups have achieved a fairly long coherence time,and gate operation for single qubits has become availableby the application of custom made resonant microwavepulses, which control Rabi oscillations [2, 3, 4, 5, 6, 7].Rotational gate operation together with two-qubit con-trolled NOT operation has also been successfully demon-strated [8]. As regards the qubit readout, averaging pro-cesses are commonly used and a way to overcome imper-fect readout visibility has been a topic of debate.In this paper, we report the observation of the single-shot readout of a superconducting flux qubit com-prising three Josephson junctions in a loop [9]. Al-though, singlequbit operationand entanglementbetweentwo qubits have been demonstrated in superconductingchargequbits, fabricated Josephsoncircuits exhibit staticand dynamic charge noise due to background chargedimpurities. In contrast, in flux qubits there is relativelylittle magnetic background noise. Superconducting fluxqubits therefore have the potential advantages of a longercoherence time and greater stability.Figure 1(a) shows a scanning electron microscope im-age of a sample; a superconducting qubit (inside loopwith three Josephsonjunctions ofcriticalcurrent I