Preparing and stabilizing quantum states through engineered dissipation

A quantum computer can solve certain problems exponentially faster than a classical computer. However, current realizations of its building blocks, qubits, are plagued by errors induced by uncontrolled interactions between quantum bits (or qubits) and the environment. Such interactions introduce dissipation, which destroys quantum entanglement, a necessary resource in quantum algorithms, and drives a quantum processor toward trivial, classical states. This decoherence process introduces errors, corrupting the utility of the quantum processor. As such, environmental dissipation is the main limiting factor for the current-generation quantum processors. Paradoxically, while uncontrolled dissipation is a bane of current quantum processors, engineered dissipation — created with a carefully tailored environment — may in fact aid a quantum processor by steering it into a desired entangled state.

In our latest work, “Stable Quantum-Correlated Many Body States via Engineered Dissipation”, published in Science, we explore this counterintuitive effect of engineered dissipation. Not only does this method offer a reliable way to prepare a quantum state, but it also naturally stabilizes the state so that it survives much longer than it would without the dissipation. These experiments lay the groundwork for improved methods to prepare strongly correlated states (which may be found in e.g. magnets) in a quantum processor, which could help physicists learn about exotic quantum materials, such as high-temperature superconductors, in the future.

In the past, some groups have prepared entangled states of up to eight atoms with engineered dissipation. But more intricate, many-body entangled states have been out of reach due to environmental decoherence caused by other, undesirable dissipation channels. With our Sycamore processor, we have developed fast reset capabilities that makes the desired engineered dissipation dominant over unwanted dissipation channels.

When we apply techniques previously developed to mitigate errors, we found that we could dissipatively prepare ground states with fidelities of 90% for an 18-qubit system. This value matches the highest achieved fidelities at comparable system sizes using unitary circuits. We also demonstrated theoretically, in the same work, that the dissipative state preparation protocol scales more favorably vs. system size in terms of preserving state fidelity. These comparisons highlight the promise of the dissipative approach. The cooling algorithm is expected to be even more advantageous for more intricate correlated states, because once the coupling to the tailored environment is engineered, then you can just “set it and forget it.” The system naturally evolves to the desired state and stays there as long as the dissipation is happening.

Finally, we demonstrated that engineered dissipation has applications to non-equilibrium quantum phenomena beyond state preparation. Here we use a 1D system of 26 qubits to emulate a Heisenberg spin chain, a paradigmatic correlated system. We drive a current through the chain by periodically resetting the spins of two auxiliary qubits at either end. We first studied transferred magnetization, whose time dependence is observed to vary strongly depending on the underlying parameters of the model, in agreement with established theory (see below). At the so-called “isotropic point,” where exact solutions of the model are unknown, the team discovered a previously unknown phenomenon. At this point, we observed that the spin current has a distinct power-law dependence with time, which indicates a form of previously unknown, anomalous diffusion for this spin model. This result highlights the increasing functionality of the quantum processor as a tool for fundamental physics discovery, where the introduction of engineered dissipation unveils quantum phenomena that have never been observed or theoretically realized.

Conclusion

This work shows that engineered dissipation strongly enhances the functionality of quantum processors, allowing for a conceptually new approach to preparing and probing quantum-correlated states. Engineered dissipation will aid in the preparation of strongly correlated states, which give rise to new quantum phases of matter that are found in materials such as magnets. Quantum materials are considered among the most promising applications of quantum processors, yet most other protocols for correlated state preparation are severely limited by unwanted decoherence. We believe the methods demonstrated here will also enable the preparation of intricate many-body states such as quantum spin liquids, which may aid in the synthesis of high temperature superconductors or new materials for data-storage.