Efficient protocol for qubit initialization with a tunable environment

Abstract

We propose an efficient qubit initialization protocol based on a dissipative environment that can be dynamically adjusted. Here, the qubit is coupled to a thermal bath through a tunable harmonic oscillator. On-demand initialization is achieved by sweeping the oscillator rapidly into resonance with the qubit. This resonant coupling with the engineered environment induces fast relaxation to the ground state of the system, and a consecutive rapid sweep back to off resonance guarantees weak excess dissipation during quantum computations. We solve the corresponding quantum dynamics using a Markovian master equation for the reduced density operator of the qubit-bath system. This allows us to optimize the parameters and the initialization protocol for the qubit. Our analytical calculations show that the ground-state occupation of our system is well protected during the fast sweeps of the environmental coupling and, consequently, we obtain an estimate for the duration of our protocol by solving the transition rates between the low-energy eigenstates with the Jacobian diagonalization method. Our results suggest that the current experimental state of the art for the initialization speed of superconducting qubits at a given fidelity can be considerably improved.

Rectification of heat currents across nonlinear quantum chains: a versatile approach beyond weak thermal contact

“…In this situation, strong symmetry breaking gives rise to a finite rectification (and thus a finite heat transfer) in contrast to the classical prediction. This finding may have relevance for recent proposals for the fast initializations of quantum bits (cooling) by frequency tuning [44].…”

Theory of quantum-circuit refrigeration by photon-assisted electron tunneling

“…Whereas fully normal-metal junctions can be used as sensitive charge sensors [27] and primary thermometers [28], normalmetal-insulator-superconductor (NIS) junctions have opened an avenue for electrically refrigerating the normal-metal electron reservoirs even below the phonon bath temperature [29]. However, quantum devices are designed to be very well isolated from dissipative electron systems owing to the requirements of long coherence time [9], and hence the benefits of the NIS junction technology in quantum-circuit initialization [30,31] are far from obvious.…”

Reservoir engineering using quantum optimal control for qubit reset

“…Another alternative is given by tunable environments [6][7][8][9][10], which provide a convenient and fast way to initialize qubits on-demand while still employing the idea of thermalization. A method utilizing such a tunable environment to efficiently prepare superconducting qubits in their ground state has recently been brought forward [11]. It exploits the indirect coupling of the qubit to a low-temperature resistive bath via two intermediate resonators [11] and uses a protocol that utilizes sequential resonances (SR) with the resistive bath.…”

“…A method utilizing such a tunable environment to efficiently prepare superconducting qubits in their ground state has recently been brought forward [11]. It exploits the indirect coupling of the qubit to a low-temperature resistive bath via two intermediate resonators [11] and uses a protocol that utilizes sequential resonances (SR) with the resistive bath. Here, we use quantum optimal control theory (QOCT) to study the efficiency of this reset protocol.…”

“…Here, we lift the limitation of constant decay rates by combining reservoir engineering with QOCT and a master equation featuring time-dependent rates. The latter are both controllable and experimentally implementable with current technologies [11]. Using Krotov's method for QOCT [37], we derive the optimal shape of the external control fields that determine the time-dependent decay rates in the master equation.…”