Quantum 'Cat' States Stop Bouncing Back as Chinese Team Adds More Qubits
Fuzhou University physicists watched a microwave superposition recover its coherence with one qubit nearby, then lose it for good when the environment grew to eight.
A team at Fuzhou University in China has watched a quantum superposition lose its ability to recover, and has pinned down how a larger environment makes the loss stick.
The study, led by Ri-Hua Zheng and published in Physical Review Letters, is titled "Observation of the Transition from Reversible to Irreversible Decoherence of Mesoscopic Quantum Superpositions." Phys.org reported the work on October 23.
The researchers prepared a microwave field in what physicists call a cat state, a superposition of an empty state and a state containing light, named after Schrödinger's thought experiment. They stored it in a superconducting resonator. They then built an artificial environment out of superconducting qubits and increased the number of qubits from one to eight to see how the field's coherence behaved.
With a single qubit, the result was a partial reprieve. Interference between the two parts of the cat state faded as the qubit picked up information that distinguished them. About 38 nanoseconds in, the qubit nearly finished a full oscillation, the distinguishing information was effectively erased, and the interference returned. The authors wrote that "the quantum coherence is nearly revived."
With eight qubits, it never came back. A measure of distinguishability, which runs from 0 when nothing tells the two components apart to 1 when they are unambiguously distinct, climbed toward 1 after about 15 nanoseconds and stayed near that level through 80 nanoseconds. Maps of the field's quantum state showed the interference fading and not returning between 0 and 30 nanoseconds.
The explanation is a matter of coordination. Different qubits trade energy with the field at different rates. To erase the record the environment holds, all of them have to swing back together to states that can no longer tell the two components apart. Each added qubit makes that collective return less likely, so the revival that rescues coherence in a small environment effectively disappears in a larger one.
The team was careful about what it showed. The results demonstrate that revival was suppressed during the observation window, not that decoherence is permanent, and the experiment did not offer a way to defeat it.
Even so, the setup gives physicists a controlled place to study a problem that shadows every quantum technology. Cat states are useful for quantum computing and precision measurement, but they are fragile, and understanding exactly how the surrounding world destroys them is a prerequisite to protecting them. Seeing the move from reversible to irreversible loss in a lab, rather than in theory alone, is a step in that direction.
