Brice Dubost

Interaction-based quantum metrology showing scaling beyond the Heisenberg limit

Quantum metrology aims to use entanglement and other quantum resources to improve precision measurement. An interferometer using N independent particles to measure a parameter can achieve at best the standard quantum limit of sensitivity, δ ∝ N−1/2. However, using N entangled particles and exotic states, such an interferometer can in principle achieve the Heisenberg limit, δ ∝ N−1. Recent theoretical work has argued that interactions among particles may be a valuable resource for quantum metrology, allowing scaling beyond the Heisenberg limit. Specifically, a k-particle interaction will produce sensitivity δ ∝ N−k with appropriate entangled states and δ ∝ N−(k−1/2) even without entanglement. Here we demonstrate ‘super-Heisenberg’ scaling of δ ∝ N−3/2 in a nonlinear, non-destructive measurement of the magnetization of an atomic ensemble. We use fast optical nonlinearities to generate a pairwise photon–photon interaction (corresponding to k = 2) while preserving quantum-noise-limited performance. We observe super-Heisenberg scaling over two orders of magnitude in N, limited at large numbers by higher-order nonlinear effects, in good agreement with theory. For a measurement of limited duration, super-Heisenberg scaling allows the nonlinear measurement to overtake in sensitivity a comparable linear measurement with the same number of photons. In other situations, however, higher-order nonlinearities prevent this crossover from occurring, reflecting the subtle relationship between scaling and sensitivity in nonlinear systems. Our work shows that interparticle interactions can improve sensitivity in a quantum-limited measurement, and experimentally demonstrates a new resource for quantum metrology.

Generation of a macroscopic singlet state in an atomic ensemble

We report on an experiment for generating singlet states in a cold atomic ensemble. We use quantum non-demolition measurement and feedback control to produce a macroscopic spin state with total spin zero and reduced spin fluctuations.

Interaction-based Quantum Metrology Showing Scaling Beyond the Heisenberg Limit

Atom-mediated optical nonlinearities, within an atom-light quantum interface, allow spin measurement with sensitivity scaling better than the Heisenberg limit. This demonstrates the use of interactions as a new resource for quantum metrology

Spin Squeezing of Large-Spin Ensembles via Quantum Non-demolition Measurement

We report the first demonstration of spin squeezing of a large-spin system via quantum non-demolition (QND) measurement. We observe 2 dB of metrological squeezing in an ensemble of ∼ 106 laser cooled 87Rb atoms in the F = 1 hyperfine ground state.

Generation of a Macroscopic Singlet State in an Atomic Ensemble

We report on an experiment underway for generating a singlet state in a cold atomic ensemble. The experiment is based on a recent proposal to generate these states by applying a quantum non-demolition (QND) measurement and feedback to an unpolarized ensemble [1]. Our criteria for generating the singlet state is the spin squeezing parameter equation where Fi are the components of the collective angular momentum, N is the number of atoms and f is the spin of a single particle. Any state with ξs < 1 is an entangled state [2]. Our procedure, described bellow, will lead to a highly entangled state with ξs ≪ 1 starting from a non-entangled state with ξ ∼ 1.

Interaction-based quantum metrology giving a scaling beyond the Heisenberg limit

Atom-mediated optical nonlinearities, generated within an atom-light quantum interface, allow spin measurement with sensitivity that scales better than the Heisenberg limit. This demonstrates interactions as a new resource for quantum metrology.

Ultra-sensitive Faraday rotation measurements from an atom-light quantum interface

Atomic ensembles have gained a lot of interest in the last decade for their prospective use as quantum memories, in precision measurements and other applications in quantum information processing. The atomic part of the interface presented here is an all-optically trapped atomic ensemble of 87Rb at a temperature of 25µK. The ensemble consists of 106 atoms and has an elongated shape with an aspect ratio of about 250:1. The resonant optical depth is around 50. The light part is an off-resonant polarized laser beam which dispersively probes the atomic state. The detection is a shot-noise limited polarization measurement of light pulses of a few µs and 3×105 photons. More details are given in [1].