Vortex qubit based on an annular Josephson junction containing a microshort

Abstract

We report theoretical and experimental work on the development of a vortex qubit based on a microshort in an annular Josephson junction. The microshort creates a potential barrier for the vortex, which produces a double-well potential under the application of an in-plane magnetic field; The field strength tunes the barrier height. A one-dimensional model for this system is presented, from which we calculate the vortex depinning current and attempt frequency as well as the interwell coupling. Implementation of an effective microshort is achieved via a section of insulating barrier that is locally wider in the junction plane. Using a junction with this geometry we demonstrate classical state preparation and readout. The vortex is prepared in a given potential well by sending a series of "shaker" bias current pulses through the junction. Readout is accomplished by measuring the vortex depinning current.

Nonideal quantum measurement effects on the switching-current distribution of Josephson junctions

“…The wavefunction ψ(ϕ, t) exhibits the special features of a quantum solution, for instance it can tunnel through an energy barrier [1], or stay at discrete metastable energy levels [1], or show Rabi oscillations [3], to name few examples among many [4,5,7]. These quantum effects are macroscopic, for the phase ϕ is linked to the macroscopic current γ and voltage v through the celebrated Josephson equations:…”

“…Josephson Junctions (JJ) are a well established playground for macroscopic quantum tunneling [1][2][3][4][5][6][7] and a prominent field where the basic concepts of quantum mechanics have been successfully demonstrated in a mesoscopic system, with potential technological implications in quantum computing [8]. The subject is of interest per se as an application quantum dynamics to a macroscopic object (analogous, for example, to the particle-like dynamics of fluxons described by collective coordinates [2] or to the motion of the end masses of advanced gravitational wave detectors working near the quantum limit [9]) and to validate non ideal quantum measurements models [10].…”

“…In fact one could conceive that the environment noise enters the system bearing an effective temperature that is higher than the thermodynamic temperature. When escapes can be ascribed to stochastic activation, it is difficult to discern between thermal activation [15,16] and quantum tunnel [1,[3][4][5][6][7], inasmuch several quantum effects, as resonance with level quantization [17] and Rabi oscillations [18], have been reproduced in classical activated JJ. It is therefore important to pinpoint effects that are unique for quantum systems, and could not possibly be confused with classical analogue.…”

Stochastic first passage time accelerated with CUDA

“…Besides the general interest in FPT, there is an additional reason to pay a special attention to JJ. The well established [21] noisy dynamics of Josephson junctions is of great contemporary interest [7,[22][23][24] in connection with macroscopic quantum tunneling [25][26][27][28][29][30][31][32]. The quantum behavior of JJ makes them a good candidate for the realization of a quantum bit; however, the quantum regime is only observed after the ordinary thermal activated regime has been tamed [33].…”

Effects of a resonant cavity on macroscopic quantum tunneling of fluxons in long Josephson junctions

“…This property includes (i) energy quantization and (ii) macroscopic quantum tunneling 2 (MQT). We note that, for linear LJJs, the fluxon potential for either metastable state or JVQ may be obtained 9 by using NbAlO x -Nb junctions and by implanting either one or two microresistors in the insulator layer, respectively. For application of JVQs, tuning both the decoherence time and the level of entaglement by controlling the qubit property is essential.…”