Session Z10: Quantum Enhanced Metrology

Quantum enhanced sensing by echoing spin-nematic squeezing in atomic Bose-Einstein condensate

Enhanced precision beyond standard quantum limit (SQL) can be provided by quantum entanglement. However, due to the difficulty of preparing, maintaining, manipulating and detecting entanglement, observing large enhancement is still a challenge. Here, we present interaction-based readout interferometry protocols based on echoing spin-nematic squeezing to achieve record high enhancement factors in atomic Bose-Einstein condensate. The echo is realized by a state-flip of the spin-nematic squeezed vacuum, which serves as the probe state and is refocused back to the vicinity of the initial coherent state while carrying out near noiseless amplification of a signal encoded. We observe a sensitivity of 15.6±0.5 decibels (dB) for a small-angle Rabi rotation beyond the three-mode SQL of 26400 atoms as well as 16.6±1.3 dB for phase sensing in a Ramsey interferometer. The absolute phase sensitivity for the latter extrapolates to 103 pT/√Hz at a probe volume of 18 μm³ for near-resonant microwave field sensing. Our work highlights the power of spin-nematic squeezing echo for quantum metrology. It could potentially find practical applications in atomic magnetometer, atomic clock, and atomic momentum interferometer.   

Spin squeezing in ensembles of neutral atoms via Rydberg dressing

Engineering quantum entanglement provides a powerful route towards enhanced precision measurement of time, electromagnetic fields, and acceleration. While past experiments have focused on leveraging all-to-all interactions to generate metrologically useful entanglement, many promising platforms for metrology more naturally offer short-ranged interactions which drop off with distance. One such platform, neutral atoms interacting via Rydberg dressing, offers coherent optical control over local interactions, and has been theoretically proposed to generate metrologically useful entanglement in the form of squeezed spin states [1]. In this talk, we present the creation of spin-squeezed ensembles of neutral atoms via coherent Rydberg dressing, achieving a factor of up to 0.77(4) reduction in phase variance below the standard quantum limit. Additionally, we realize metrological gain across five spatially separated ensembles in parallel. This work demonstrates the potential of local interactions not only to enhance the precision of atomic tweezer clocks via Rydberg interactions, but also to offer simultaneous enhancement of both sensitivity and spatial resolution for quantum sensing.

[1] L.I.R. Gil, et. al. Spin Squeezing in a Rydberg Lattice Clock. Phys. Rev. Lett. 112, 103601 (2014).   

Spin squeezing in a programmable optical clock with Rydberg interactions

In recent years, ever-improving optical lattice clocks have been complemented by a novel platform: Atom arrays assembled by individual optical tweezers. Together with Rydberg interactions, the control at the single-particle level makes atom arrays an ideal platform for studying how many-body interactions can be harnessed for quantum-enhanced measurements of time.

Here, we report on realizing spin-squeezing in a programmable strontium optical clock using Rydberg interactions. We assemble near defect-free arrays of up to 140 atoms in an optical lattice potential utilizing dynamic optical tweezers. To entangle the atoms and produce spin-squeezed states, we employ a Rydberg-dressing protocol where a laser off-resonantly couples the clock state to a high-lying Rydberg state. We characterize the improved sensitivity of spin-squeezed states in a Ramsey interferometer by comparing two or more sub-ensembles of the atom array. This directly reveals enhanced fractional frequency stability below the standard quantum limit at a fixed averaging time. Our work paves the way for utilizing the programmability of atom arrays in more complex protocols for quantum-enhanced metrology, such as non-Gaussian states and variational optimization.   

Quantum Fisher information and spin squeezing

Quantum metrological sensors allow us to reach the fundamentally limited precision of measurements, the Heisenberg limit (HL), which scales as the number of atoms N in contrast to the standard quantum limit (SQL) scaling N^1/2 . A common strategy to achieve scaling beyond the SQL is the generation of spin squeezed states. These states are characterized by non-classical correlations that reduce the variance of one measurement quadrature in the collective state while increasing the variance of the quadrature orthogonal to the measurement. In this work we analyze spin squeezing generation by considering dynamics of a cold atomic ensemble subject to a nonlinear Hamiltonian (one-axis twisting). The analysis is done using Dicke state and Wigner distribution function representations. Quantum Fisher information of various entangled states is calculated and analyzed in parallel, to determine what kind of nonclassical correlations between atoms are responsible for the metrological gain in Ramsey-type measurement schemes.   

Controlling trapped-ion motional modes for precision measurement

Motional modes of trapped ions have been shown to be a useful tool for quantum sensing, making use of time reversal protocols. This application requires the ability to prepare well-defined motional states with high fidelity. Many of these states can be generated from motional ground states without the use of laser fields. We report our progress towards generation of one-mode and two-mode squeezed states using parametric excitation. These operations help to create motional state interferometers and can be used to achieve Heisenberg-limited phase sensitivities. We present a preliminary implementation of an SU(1,1) interferometer using two motional modes of a ⁴⁰Ca⁺ ion in a Paul trap, and compare the performance to the more traditional SU(2) Mach-Zender interferometer. We also report on preliminary results for using single-mode squeezed states and coherent states as the interferometer inputs to further increase phase sensitivity. To characterize the input and output motional states of the interferometers, the ions’ motion is coupled to internal ‘spin’ states, which are distinguishable through spin-dependent fluorescence. The calculation of the Fisher information from experimental data can be used to quantify the phase sensitivity that we can achieve in our setup.   

Coherent Control and Precision Measurement of Nuclear Spin States in Laser-Trapped Yb Atoms

Quantum non-demolition (QND) measurement enhances the detection efficiency and measurement fidelity, and is highly desired for its applications in precision measurements and quantum information processing. We develop a QND scheme for the spin state of laser-trapped atoms, allowing the fractions in each spin state to be probed repeatedly. On ¹⁷¹Yb (I=1/2) atoms held in an optical dipole trap, a spin-selective cycling transition is produced by introducing an ancillary laser beam. We measure the phase of spin precession of the ¹⁷¹Yb atoms in a 20-mG bias magnetic field. The QND approach reduces the measurement noise by ~19 dB, to an inferred level of 2.4 dB below the atomic quantum projection noise. This method is used in the measurement of the electric dipole moment of ¹⁷¹Yb. We also apply this QND approach in a Ramsey experiment on ¹⁷³Yb (I=5/2), a multilevel quantum system.   

Optimizing one-axis twists for realistic variational Bayesian quantum metrology

Variational Bayesian quantum metrology is a promising avenue toward quantum advantage in sensing which optimizes both the state preparation (or encoding) and measurement (or decoding) procedures and takes prior information into account. For the sake of practical advantage, it is important to understand how effective various parametrized protocols are as well as how robust they are to the effects of complex noise, such as spatially correlated noise. First, we propose a new family of parametrized encoding and decoding protocols called arbitrary-axis twist ansatzes, and show that it can lead to a substantial reduction in the number of one-axis twists needed to achieve a target estimation error. Second, using a polynomial-size tensor network algorithm, we analyze practical variational metrology beyond the symmetric subspace of a collective spin, and find that quantum advantage persists for shallow-depth ansatzes under realistic noise level.   

Robust Estimators of Genuine Multiparticle Indistinguishability

The dynamics of noninteracting bosons has attracted interest due to the BosonSampling problem and its computational difficulty. A challenge in experimental systems implementing these dynamics is verifying that the output distributions are close to the desired ones. Motivated by a cold atom optical lattice experiment, we formalize the notion of genuine multiparticle indistinguishability, making use of tools from representation theory. We then construct novel witnesses of it, aiming for small variance and robustness to small miscalibrations of the unitary that governs the dynamics. We also discuss the possibility of using the representation theoretic framework to infer the linear optical unitary, by preparing many initial states.