Physicists successfully maintained atoms in a state of superposition for 23 minutes.

Metrology is the science that deals with measurement issues, their accuracy, and methods of implementation. In quantum metrology, particle spins serve as tools for measuring magnetic fields, inertia, and other physical phenomena. Furthermore, they open up the possibility of exploring new physics beyond the Standard Model.

Chinese physicists have focused on studying the "Schrödinger's cat" state — a superposition representing the simultaneous existence of two oppositely directed and maximally separated spin states within a single atom. This atomic state has unique advantages for conducting precise measurements in quantum systems.

The main challenge in applying "Schrödinger's cat" states in experiments remains maintaining a sufficient duration of coherence — a condition where the spin superposition and clear separation between their phases are preserved.

To overcome this issue, a research team led by Professor Lu Zhengtian (Lu Zhengtian) and researcher Xia Tian (Xia Tian) from the University of Science and Technology of China (USTC) utilized approximately 10,000 atoms of the isotope ytterbium-173 (¹⁷³Yb), which possesses a spin of (5/2).

The scientists cooled the atoms to nearly absolute zero and stabilized their position using a laser. Under these conditions, the quantum states of the atoms can be controlled with high precision, and the physicists placed each atom in a superposition state with two very distant spins: +5/2 and -5/2.

The spin superposition state, "Schrödinger's cat," exhibits increased sensitivity to magnetic fields. Additionally, it is resilient to noise associated with random changes in laser intensity and spatial imperfections of the lattice in which the ytterbium-173 isotopes are placed.

Typically, superposition remains stable for milliseconds, but in this experiment, the physicists managed to maintain the "Schrödinger's cat" state for over 20 minutes. To confirm the coherence time, they checked the sensitivity to phase shifts. A phase shift is related to the rotation of the spin vector of the atoms in superposition. It leads to a change in the relative phase between states, which can be measured using a technique called Ramsey interferometry.

The results of this new research confirmed that phase measurements approach the Heisenberg limit. This is the theoretically maximum possible accuracy of phase measurement in quantum systems, constrained by the fundamental laws of quantum mechanics. Such accuracy validates the effectiveness of the employed technique and the stability of the "Schrödinger's cat" state.

The long-lived superposition state of ytterbium-173 atoms opens new prospects for atomic magnetometry, quantum computing, and investigations into physics beyond the Standard Model. The findings of this study broaden the possibilities for metrology and technologies related to quantum computing and sensors.

The scientists' article was published in the journal Nature Photonics.