USTC keeps 10,000 atoms coherent
- USTC physicists reported a minute-scale Schrödinger-cat state in trapped ytterbium-173 atoms, turning a notoriously fragile quantum superposition into something that survives for practical measurement times. - The headline number is 1.4 × 10^3 seconds of coherence — about 23 minutes — protected inside a decoherence-free subspace against lattice light-shift noise. - That matters because long-lived cat states can push atomic sensing toward the Heisenberg limit, not just the usual standard quantum limit.
Quantum metrology is about using weird quantum states to measure things better than ordinary physics lets you do with independent particles. The problem is that the useful states are also the delicate ones. They usually die fast. What changed here is that a team at the University of Science and Technology of China kept a Schrödinger-cat state in trapped ytterbium-173 atoms coherent for about 1,400 seconds — roughly 23 minutes — and then used it for magnetic-field sensing. (nature.com) ### What is the “cat” here? It is not a cat made of 10,000 atoms sitting in two places at once. This experiment used the internal spin state of each ytterbium-173 atom. The cat state is a superposition of the two furthest-apart nuclear-spin projections, m = +5/2 and m = −5/2. Basically, the atom is put into two maximally distinct spin orientations at the same time. (nature.com) ### Why is that(nature.com)n a magnetic field than an ordinary, more classical-looking state would. That extra phase sensitivity is the whole metrology play. In ideal conditions, a cat state can approach the Heisenberg limit — the best scaling quantum mechanics allows — instead of the standard quantum limit that governs uncorrelated measurements. (nature.com)ore specifically, anything that nudges one branch of the superposition differently from the other branch will wash out the phase relation between them. Optical lattices are great for trapping atoms, but the trapping light can shift energy levels unevenly across the sample. For a cat state, that kind of inhomogeneity is poison. (nature.com) ### So w(nature.com)ecoherence-free subspace. That sounds fancy, but the idea is simple — encode the quantum information in a way that makes the dominant noise hit both parts equally, so the damaging difference mostly cancels out. Here, the target noise was inhomogeneous light shifts from the optical lattice, and that protection is the main reason the coherence stretched to 1.4(1) × 10^3 seconds. (nature.com) ### Why use ytterbium-173? Ytterbium is already a favorite atom in precision measurement because it has very stable internal transitions and plays nicely with optical trapping. The isotope used here, ytterbium-173, has nuclear spin 5/2, which gives the researchers a six-level spin system to work with. That larger spin is what makes the “furthest-apart” cat-state construction possible in the first place. (nature.com) use it? They used it. The paper shows magnetic-field measurement with Ramsey interferometry, which is the standard way to let a quantum phase accumulate and then read it out. The point was not only “look, it lives a long time,” but “look, it stays useful long enough to do sensing with near-fundamental precision scaling.” (nature.com) ### Is this a(nature.com)um-control and quantum-sensing breakthrough. But the overlap is real — long-lived protected states are exactly what quantum computing and quantum memories need. The catch is that this was a very tailored setup aimed at one kind of encoded state and one dominant noise source, not a general-purpose fault-tolerant processor. (nature.com)ts made a cat state.” People have done that before. The big deal is that this one lasted minutes, not moments, while staying useful for precision measurement. That pushes quantum sensing closer to the regime where exotic states are not just lab demos, but working tools. (nature.com)
