State Preparation on Quantum Computers via Quantum Steering

Quantum computers present a compelling platform for the study of open quantum systems, namely, the nonunitary dynamics of a system. Here, we investigate and report digital simulations of Markovian nonunitary dynamics that converge to a unique steady state. The steady state is programmed as a desired target state, yielding semblance to a quantum state preparation protocol. By delegating ancilla qubits and system qubits, the system state is driven to the target state by repeatedly performing the following steps: 1) executing a designated system–ancilla entangling circuit; 2) measuring the ancilla qubits; and 3) reinitializing ancilla qubits to known states through active reset. While the ancilla qubits are measured and reinitialized to known states, the system qubits undergo a nonunitary evolution and are steered from arbitrary initial states to desired target states. We show results of the method by preparing arbitrary qubit states and qutrit (three-level) states on contemporary quantum computers. We also demonstrate that the state convergence can be accelerated by utilizing the readouts of the ancilla qubits to guide the protocol in a nonblind manner. Our work serves as a nontrivial example that incorporates and characterizes essential operations, such as qubit reuse (qubit reset), entangling circuits, and measurement. These operations are not only vital for near-term noisy intermediate-scale quantum applications but are also crucial for realizing future error-correcting codes.