Efficient protocol for qubit initialization with a tunable environment

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. The ability to accurately reset qubits is one of the fundamental criteria in implementing quantum computers. Jani Tuorila and co-workers from Aalto University (Finland) propose a qubit initialization protocol using a dynamically adjustable environment. The scheme differs from the conventional methods which reach for a given qubit state with microwave cooling or projective measurements of the qubit. The protocol relies on enhancing the dissipation to the qubit ground state by tuning an engineered environment on resonance with the qubit. The new theoretical study suggests that the experimental benchmark for the initialization speed at a given precision can be increased with this method almost by an order of magnitude. Such an improvement would present a significant step towards meeting the stringent operational conditions of a working largescale quantum computer.