Quantum devices work with entanglemend particles, whose properties stay linked in ways ordinary (non-quantum) physics cannot explain. Building useful quantum technology will require sharing entanglement between machines in different places. Today this is usually done by preparing entangled particles in one spot and then shipping them elsewhere. During the trip, outside disturbances degrade the fragile quantum state, a process called decoherence. This has been one of the biggest obstacles in the field.
Physicists at the University of Illinois Urbana-Champaign and the University of Chicago have now demonstrated a way to skip the risky transport step. They used dissipation - the unavoidable leak of energy from a quantum system into its surroundings - to create entanglement instead of destroying it. The methods and results of this study are published in Physical Review X.
A steady state that stays entangled
The experiment builds on an old theoretical idea called cascading, in which quantum objects continuously absorb and emit light. Some of that light leaks away, but if incoming light is added in just the right balance, the system settles into a steady state - a stable condition it holds on its own. With careful design, that resting state is one in which separated parts of the system are entangled. Because it is a steady state, the entanglement can in principle last indefinitely, and the particles never have to move; they only need a channel to communicate.
Earlier attempts at cascaded systems produced weak entanglement because of noise and imperfect hardware. The team's key advance is a method called synthetic squeezing, a way of tuning the experiment so that these real-world flaws no longer matter. They demonstrated it with two superconducting qubits - quantum bits made from circuits that carry electricity without resistance.
The groups are now extending the approach to systems with more qubits. They hope it will offer a sturdier way to network quantum computers, and plan to explore entanglement distillation, a technique that combines many weakly entangled qubits into a few strongly entangled ones.