A research team led by Professor Lu Zhengtian and Researcher Xia Tian from the University of Science and Technology of China (USTC) has achieved a breakthrough in quantum metrology by realizing a Schrödinger-cat state with an unprecedented minute-scale lifetime. Using optically trapped cold atoms, this advance significantly enhances the sensitivity of quantum measurements and opens new doors for fundamental physics research. The study was published in Nature Photonics.
In quantum metrology, particle spins serve as highly sensitive probes for measuring magnetic fields, inertia, and a range of physical phenomena. These measurements are crucial not only for advancing technology but also for exploring potential deviations from the Standard Model of particle physics. Among the various quantum states utilized in such measurements, the high-spin Schrödinger-cat state stands out for its extraordinary properties. This state, which represents a quantum superposition of two oppositely directed and maximally separated spin states, offers enhanced sensitivity and robustness against noise.
The high-spin nature of Schrödinger-cat states amplifies precession frequency signals, making them particularly effective for detecting minute changes in physical quantities. Additionally, these states are less susceptible to certain types of environmental interference, suppressing noise and improving measurement accuracy. However, despite their theoretical advantages, the practical application of Schrödinger-cat states has long been hindered by a critical limitation: their coherence time, or how long the state can maintain its quantum properties, has typically been too short for meaningful experimentation.
The USTC research team overcame this hurdle through an innovative experimental approach. They began by trapping ytterbium-173 (¹⁷³Yb) atoms, which have a high spin quantum number of 5/2, in an optical lattice. This lattice, created using laser light, acts as a controllable environment for the atoms. By precisely manipulating laser pulses, the researchers induced nonlinear light shifts in the atoms’ ground states. This technique allowed them to prepare a Schrödinger-cat state composed of two spin projections, +5/2 and -5/2.
What sets this Schrödinger-cat state apart is its resilience to decoherence. The researchers found that the state resides within a decoherence-free subspace, meaning it is immune to common sources of noise, such as fluctuations in laser intensity and spatial variations within the optical lattice. These properties stem from the fact that the cat state experiences identical light shifts for both spin projections, effectively canceling out external disturbances.
Experimental measurements demonstrated that the coherence time of this Schrödinger-cat state exceeded 20 minutes, a groundbreaking achievement in the field of quantum physics. To test the state’s capabilities, the team employed Ramsey interferometry, a technique widely used in precision measurements. The results showed that the phase measurement sensitivity of the system approached the Heisenberg limit, the theoretical maximum sensitivity achievable in quantum mechanics.
The implications of this work are profound. The long-lived Schrödinger-cat state enhances the potential of atomic magnetometry, enabling highly sensitive detection of magnetic fields with applications ranging from medical imaging to navigation. It also represents a significant step forward for quantum computation, where long coherence times are essential for maintaining the fidelity of quantum information. Furthermore, the ability to achieve such precise measurements could help scientists explore physics beyond the Standard Model, including the detection of exotic particles or interactions.
By addressing the longstanding challenge of coherence time, this research not only advances quantum metrology but also sets the stage for new discoveries in fundamental physics. The innovative techniques developed by the USTC team are likely to inspire further investigations into quantum systems and their applications across various scientific and technological domains.