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.
Highlights
The conventional passive initialization protocol relies on the relaxation of the qubit to a thermal state determined by the residual coupling to the environment
In the more refined topological quantum error correction codes,[6] the logical error can be suppressed with stabilizing measurements, which increase the thresholds up to 10−2 for the physical qubit operations and lead to improved protection of quantum information during the computation
The superconducting qubit with the transition energy ħωq is coupled to a bosonic heat bath through two LC resonators
Summary
Preparation of a qubit into a well-defined initial state is one of the key requirements for any quantum computational algorithm.[1, 2] The conventional passive initialization protocol relies on the relaxation of the qubit to a thermal state determined by the residual coupling to the environment. The codes initiate from a predetermined state for the physical qubits and are being constantly executed during a computation They have strict requirements for the initialization and gate error thresholds for individual qubits, of the order of 10−5 for the conventional concatenated codes.[4, 5] In the more refined topological quantum error correction codes,[6] the logical error can be suppressed with stabilizing measurements, which increase the thresholds up to 10−2 for the physical qubit operations and lead to improved protection of quantum information during the computation. Initialization time is an issue in large-scale quantum computing
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