Naeimeh Behbood

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.

Spin cooling via incoherent feedback in an ensemble of cold 87Rb atoms

We report an experimental study of a new technique for spin cooling an ensemble of ultracold atoms via quantum non-demolition measurement and incoherent feedback. This technique has direct application in generating macroscopic singlet state.

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.