Dissipative quantum state preparation

Short answer: Dissipative state preparation uses a controlled reservoir, such as resonator photon loss or auxiliary qubits, to steer a system into a target state instead of building it with gates alone. A 2013 theory paper proposed it for two superconducting qubits, and a 2023 Science experiment used up to 49 superconducting qubits to prepare low-energy states of the transverse-field Ising model, with ground-state fidelity 0.86 for 18 qubits at the critical point. These are research results.

How does dissipation stabilize a quantum state?

Instead of relying solely on unitary gates to create a state, this method couples the system to an engineered reservoir. The reservoir removes excitations or drives transitions toward the target state. In superconducting qubit setups, this often involves combining photon loss in a resonator with microwave drives. This combination creates an effective decay mechanism that prepares a maximally entangled state. The state is then maintained as the steady state of the driven, dissipative evolution.

What experimental platforms have been used?

The 2013 paper proposes and numerically studies a scheme for circuit quantum electrodynamics (QED) setups. One study presented a scheme for two superconducting qubits, specifically transmons. The setup involves coupling the internal levels of two transmons to a resonator. Microwave fields provide virtual couplings that combine into an effective two-photon drive. Another study utilized up to 49 superconducting qubits to prepare low-energy states of the transverse-field Ising model. These experiments coupled the system to dissipative auxiliary qubits.

How robust is the preparation against imperfections?

The authors of the reservoir-engineering paper say the approach has improved robustness to decoherence. One study investigated the effects of potential experimental imperfections on a two-qubit entanglement scheme. The authors report that, in their analysis, the scheme gives high fidelity with the target state despite small imperfections. Another study highlighted that established methods for dissipative state preparation typically rely on resolving resonances. This reliance can limit target state fidelity due to competition between the stabilization mechanism and uncontrolled dissipation. A newer framework combining static dispersive couplings with strong parametric driving aims to realize high-fidelity and fast entanglement stabilization without such constraints. This approach allows arbitrary state preparation within a fixed parity manifold.

What many-body states can be prepared?

Engineered dissipative reservoirs can steer many-body quantum systems toward correlated steady states. Using up to 49 superconducting qubits, researchers prepared low-energy states of the transverse-field Ising model. In one dimension, long-range quantum correlations were observed. A ground-state fidelity of 0.86 was measured for 18 qubits at the critical point. In two dimensions, mutual information extending beyond nearest neighbors was found. By coupling the system to auxiliaries emulating reservoirs with different chemical potentials, transport in the quantum Heisenberg model was explored. The Science summary describes the dissipative approach as promising but experimentally challenging.

FAQ

Can this method prepare ground states?

Yes, dissipative cooling can prepare low-energy states. One study prepared low-energy states of the transverse-field Ising model by removing excitations. This effectively cools the system toward its correlated ground state. Long-range correlators of these states provided evidence of quantum correlations.

Is high fidelity achievable?

Reported values are high but specific to each study. The 2013 theory study found high fidelity for two-qubit entanglement. The 2023 experiment reported a ground-state fidelity of 0.86 for 18 qubits. A 2020 proposal aims for high-fidelity stabilization devoid of resonance resolution constraints.

Does this require specific hardware?

The 2020 paper says its protocol is readily accessible with state-of-the-art superconducting qubit technology. The 2013 scheme uses transmons, a resonator and microwave drives, and the 2023 experiment used a superconducting transmon processor.

Is this operationally deployed?

No. These are proposals, simulations and one laboratory experiment. None reports commercial use.

How does it handle decoherence?

The 2020 paper says reservoir engineering has improved robustness to decoherence, but that established methods rely on resolving resonances, so uncontrolled dissipation competes with stabilisation and limits fidelity. Its proposed parametric driving aims to avoid this.