Entangled particles play a crucial role in quantum computation and simulation. Recently, researchers have started using entangled atoms to design precise atomic clocks. Atomic transitions are recognized as accurate references for constructing atomic clocks. In the context of atomic clocks, a laser beam with an electromagnetic field oscillating at a given frequency is precisely adjusted to synchronize with one of the atomic transitions. Each of these oscillations sets the precision of timekeeping and frequency standards. To maintain the stability of the electromagnetic oscillation period, it is necessary to track its frequency by probing the excitation probability of an isolated atom or ion undergoing a Ramsey measurement. Any changes in the frequency manifest as alterations in the measurement outcome of the Ramsey sequence, which in turn allows for the correction of the oscillation frequency. However, in these systems, the precision of these measurements is limited by the incoherent quantum projection noise, which restricts the accuracy and speed at which the laser frequency can be measured.
Entangling many particles reduces measurement noise as atoms begin to experience quantum correlations. In long trapped ion chains, where each atomic ion acts as a pseudo-spin, the spin-spin interaction, a key element for generating these quantum correlations, decays with distance (termed finite-range interactions). Until recently, spin systems with finite-range interactions were considered less favorable choices for creating precise atomic clocks. A theoretical team led by Ana Maria Rey and her co-worker Sean R Muledy from NIST, USA, developed a theoretical protocol that made it possible to change the applicability of such systems for atomic clock experiments . Our experimental team from the University of Innsbruck and the Institute for Quantum Optics and Quantum Information (IQOQI) Innsbruck implemented the proposed theoretical protocol and verified the theoretical predictions in a programmable quantum simulator with up to 51 ions. Our experimental results have recently been published in Nature (see Franke et al). These results are seen as a milestone towards utilizing long ion chains to create more accurate atomic clocks. In this manuscript, we present the experimental results with these ion chains, demonstrating spin squeezing of ξ² = - 4 dB, which is a metric for reducing measurement noise with respect to unentangled ions. We also demonstrate the generation of multi-headed Schrödinger cat states, which could potentially be useful for many-body physics simulations and metrological applications with finite-range interacting atomic systems.
Enhancing spin squeezing using soft-core interactions, Young et al., Physical Review Research 5, 1, L012033 (2023).
Quantum-enhanced sensing on optical transitions via finite-range interactions, Johannes Franke, Sean R. Muleady, Raphael Kaubruegger, Florian Kranzl, Rainer Blatt, Ana Maria Rey, Manoj K. Joshi, and Christian F. Roos. Nature 2023 DOI: 10.1038/s41586-023-06472-z.
Image credit: Steven Burrows and the Rey Group/JILA