Physicists at the Institute of Science and Technology Austria (ISTA) have developed a novel method for entangling quantum bits (qubits) that operates autonomously, a significant departure from current techniques requiring continuous active control and measurement. The breakthrough, detailed in Physical Review X, utilizes a "quantum bath" of correlated microwave photons to establish and maintain entanglement between qubits separated by distances. This approach confirms a theoretical prediction made over two decades ago and may offer a more straightforward path to connecting modules in future quantum computers.
Traditional methods for achieving distributed entanglement, which is crucial for scaling quantum processors and networks, often involve complex sequences of precisely timed control pulses and repeated measurements. In some cases, these methods rely on post-selection, where only successful entanglement events are kept, leading to inefficiencies. The ISTA team's experiment bypasses these complexities by employing the qubits' environment itself as the entangling mechanism.
The experiment involved two superconducting transmon qubits, each located 50 centimeters apart and without any direct interaction. These qubits were exposed to microwave photons generated by a shared source, a Josephson parametric converter. This source produced photons in a two-mode squeezed state, which carries continuous-variable entanglement. The challenge lay in transferring this continuous entanglement into the discrete states typically used by quantum computers. The quantum bath acts as a reservoir that continuously drives both qubits toward a stable, entangled steady state.
Professor Johannes Fink, a senior author on the study, explained that the quantum bath, which is essentially the qubits' environment, creates a new ground state through a continuous stream of correlated photons. This process stabilizes the entangled qubit state, making it consistently available for further quantum processing, even beyond the qubits' intrinsic operational lifetime. This contrasts with short-lived entanglement that must be utilized opportunistically.
The researchers confirmed the entanglement through quantum tomography, a technique that reconstructs the joint quantum state by combining measurements taken along different bases. The experiment utilized short microwave pulses, on the order of nanoseconds, to probe the underlying qubit states. The team's findings suggest that this autonomous entanglement method could be scaled to generate many entangled pairs from a single source, and it holds promise for hybrid quantum systems that integrate photons of different frequencies.
This work addresses a fundamental challenge in quantum information processing: the need for reliable and scalable entanglement. By enabling entanglement to be "always available," the quantum bath approach simplifies the requirements for future quantum computing architectures. The research also opens avenues for quantum-optics experiments and could contribute to the development of fault-tolerant quantum processors.
The theoretical foundation for this method dates back to work by Barbara Kraus and Ignacio Cirac in 2003, who proposed transforming continuous variable entanglement into its discrete counterpart by coupling qubits to a quantum bath. The ISTA team's experimental realization validates this concept, demonstrating that an interference effect between the bath and the qubits can lead to simultaneous excitation or de-excitation, resulting in complete entanglement without the need for synchronized laser pulses.
While the current experimental setup achieved a low degree of concurrence, a measure of entanglement, the researchers note limitations in waveguide couplings and losses. However, they believe that by generating squeezing over a broad range of microwave frequencies, the technology can be scaled for multi-mode, long-distance, and high-speed quantum networks.