Google Quantum AI Achieves Stable Quantum States, Pioneers New Spin Transport Regime

Google Quantum AI and collaborators have made strides in quantum computing by using engineered dissipation to prepare many-body quantum states. This method, which involves coupling a quantum system to a dissipative reservoir, offers a scalable alternative to unitary evolution for preparing entangled many-body states on noisy quantum processors. The team used 49 superconducting qubits to prepare low-energy states of the transverse-field Ising model. The results could be useful for quantum simulation of high-temperature superconductivity or quantum magnetism. The work also opens up possibilities for investigating nonequilibrium quantum phenomena.

Quantum State Preparation through Engineered Dissipation

The field of quantum simulation and computation is constantly seeking scalable algorithms for the preparation of correlated states, such as the ground state of interacting Hamiltonians. Traditional methods include adiabatic unitary evolution from an initial Hamiltonian to a desired one, and variational quantum algorithms. However, these methods have inherent limitations. Adiabatic state preparation is fundamentally challenging across quantum phase transitions where the many-body energy gaps close. Variational quantum algorithms, on the other hand, involve large optimization overheads and are challenged by the so-called barren plateaus. Furthermore, the lifetimes of states prepared through unitary evolution are limited by the coherence times of physical qubits, hindering their use as a basis for noise-biased qubits or topological quantum computation.

An alternative and potentially more robust approach to quantum state preparation is through engineered dissipation. In such schemes, the quantum system is coupled to a dissipative reservoir that is repeatedly entangled with the system and projected to a chosen state. Over time, the system is steered toward a steady state of interest by the reservoir. This process, known as dissipative cooling, involves the use of auxiliary qubits, each with an energy splitting close to the energy of low-lying excitations of a many-body quantum system. The entangling operation transfers excitations from the system into the auxiliaries, which are then removed via controlled dissipation, thereby cooling the system toward its ground state.

Experimental Evidence of Entanglement and Long-Range Quantum Correlations

Past experimental works have demonstrated the dissipative preparation of two-qubit Bell pairs of trapped ions and superconducting qubits, as well as an 8-qubit Mott insulator state in an analog quantum simulator. However, the dissipative preparation of many-body quantum states has remained experimentally challenging due to increased environmental decoherence which threatens to overwhelm the impact of the auxiliaries.

In this study, the researchers report on the preparation of many-body quantum states via dissipative cooling on a superconducting transmon quantum processor. They provide experimental evidence of entanglement and long-range quantum correlations in the steady state, and demonstrate a favorable scaling of dissipative state preparation over system sizes when compared to unitary evolution algorithms.

Exploring Non-Equilibrium Physics through Engineered Dissipation

Furthermore, the researchers extend the use of engineered dissipation beyond cooling and explore the non-equilibrium physics arising from coupling a many-body quantum system to two different reservoirs. This work is enabled by two technical advances: continuously tunable quantum gates with simultaneously operated two-qubit gate fidelities reaching 99.7% in 1D and 99.6% in 2D, and a fast reset mechanism.

The Future of Quantum State Preparation

The results of this study establish engineered dissipation as a scalable alternative to unitary evolution for preparing entangled many-body states on noisy quantum processors and an essential tool for investigating nonequilibrium quantum phenomena. The researchers’ work opens up new possibilities for preparing many-body quantum states and exploring non-equilibrium physics in quantum systems. It also raises important questions about the practical importance of engineered dissipation to current quantum hardware, and the potential for further advances in this area.