Coherent Quantum Dynamics of a Superconducting Flux Qubit

Abstract

We have observed coherent time evolution between two quantum states of a superconducting flux qubit comprising three Josephson junctions in a loop. The superposition of the two states carrying opposite macroscopic persistent currents is manipulated by resonant microwave pulses. Readout by means of switching-event measurement with an attached superconducting quantum interference device revealed quantum-state oscillations with high fidelity. Under strong microwave driving it was possible to induce hundreds of coherent oscillations. Pulsed operations on this first sample yielded a relaxation time of 900 nanoseconds and a free-induction dephasing time of 20 nanoseconds. These results are promising for future solid-state quantum computing.It is becoming clear that artificially fabricated solidstate devices of macroscopic size may, under certain conditions, behave as single quantum particles. We report on the controlled time-dependent quantum dynamics between two states of a micron-size superconducting ring containing billions of Cooper pairs (1). From a ground state in which all the Cooper pairs circulate in one direction, application of resonant microwave pulses can excite the system to a state where all pairs move oppositely, and make it oscillate coherently between these two states. Moreover, multiple pulses can be used to create quantum operation sequences. This is of strong fundamental interest because it allows experimental studies on decoherence mechanisms of the quantum behavior of a macroscopicsized object. In addition, it is of great significance in the context of quantum computing (2) because these fabricated structures are attractive for a design that can be scaled up to large numbers of quantum bits or qubits (3).Superconducting circuits with mesoscopic Josephson junctions are expected to behave according to the laws of quantum mechanics if they are separated sufficiently from external degrees of freedom, thereby reducing the decoherence. Quantum oscillations of a superconducting two-level system have been observed in the Cooper pair box qubit using the charge degree of freedom (4). An improved version of the Cooper pair box qubit showed that quantum oscillations with a high quality factor could be achieved (5). In addition, a qubit based on the phase degree of freedom in a Josephson junction was presented, consisting of a single, relatively large Josephson junction current-biased close to its critical current (6,7).Our flux qubit consists of three Josephson junctions arranged in a superconducting loop threaded by an externally applied magnetic flux near half a superconducting flux quantum Φ 0 = h/2e [(8); a one-junction flux-qubit is described in (9)]. Varying the flux bias controls the energy level separation of this effectively two-level system. At half a flux quantum, the two lowest states are symmetric and antisymmetric superpositions of two classical states with clockwise and anticlockwise circulating currents. As shown by previous microwave spectroscopy studies, the qubit can be engineered such th...

Physics: Quantum all the way

“…These states can then be monitored by looking at the interference between them. In 2003, researchers at Delft University of Technology in the Netherlands showed that a related approach, using pulses of microwaves to excite oscillations in a superposition of superconducting current states, could also be used to investigate decoherence 12 . As in Haroche's experiments, decoherence shows up as a weakening of the correlations between oscillations of the superposition states as the time between pulses lengthens.…”

Electrically Tunable Multiterminal SQUID-on-Tip

“…1a. Multi-junction devices, as well as multi-terminal configurations, have been proposed and used in the past for logic circuits 40 Here, in contrast, the extra degrees of freedom are used for phase rather than flux control of the interference pattern of the SQUID.…”

“…1a. Multi-junction devices, as well as multi-terminal configurations, have been proposed and used in the past for logic circuits 40 Here, in contrast, the extra degrees of freedom are used for phase rather than flux control of the interference pattern of the SQUID.The basic principle of the mSOT phase-biased operation can be understood by considering an ideal two-terminal/two-junction SQUID with no inductance, which is described by four terminals. The primary bias current is applied to terminal 1, while terminals 2 and 4 are used to provide the superconducting phase control currents and , respectively.…”

Nanomechanical-resonator-assisted induced transparency in a Cooper-pair box system

“…The tunnel junction of the CPB shown in figure 1 is split into two to form a SQUID loop [23,25], which allows us to control its effective Josephson energy with a small external magnetic field or magnetic flux. Two microwave currents are applied in a microwave (MW) line [45] beside the CPB to induce oscillating magnetic fields in the Josephson junction SQUID loop of the CPB qubit. We call one of them the control current with frequency ω c and amplitude E c .…”