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arXiv:1904.07866 · Physical Review Research

Engineering spin squeezing in a 3D optical lattice with interacting spin-orbit-coupled fermions

P. He, M. A. Perlin, Sean R. Muleady, Robert J. Lewis-Swan, R. B. Hutson, J. Ye, A. M. Rey

Published Nov 5, 2019• 44 citations
arXivHTMLPDF

Abstract

The authors propose a protocol that uses both inter-atomic interactions and laser-induced momentum kicks, normally two undesirable decoherence mechanisms in current 3D optical lattice clocks, to engineer metrologically useful entangled states. Their proposal opens a path to push state-of-the-art quantum sensors at absolute limits of both precision and accuracy.

DOI
10.1103/physrevresearch.1.033075

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•Jan 1, 2016•0 citations
The authors propose a protocol that uses both inter-atomic interactions and laser-induced momentum kicks, normally two undesirable decoherence mechanisms in current 3D optical lattice clocks, to engineer metrologically useful entangled states. Their proposal opens a path to push state-of-the-art quantum sensors at absolute limits of both precision and accuracy.
We describe a theoretical scheme for generating scalable spin squeezing with nearest-neighbor interactions between spin-1/2 particles in a three-dimensional (3D) lattice, which are naturally present in state-of-the-art 3D optical lattice clocks. We propose to use strong isotropic Heisenberg interactions within individual planes of the lattice, forcing the constituent spin-1/2s to behave as large collective spins. These large spins are then coupled with XXZXXZXXZ anisotropic interactions along a third direction of the lattice. This system can be realized via superexchange interactions in a 3D optical lattice subject to an external linear potential, such as gravity, and in the presence of spin-orbit coupling to generate spin anisotropic interactions. We show there is a wide range of parameters in this setting where the spin squeezing improves with increasing system size even in the presence of holes.
We propose a protocol for generating generalized Greenberger-Horne-Zeilinger (GHZ) states using ultracold fermions in 3D optical lattices or optical tweezer arrays. The protocol uses the interplay between laser driving, on site interactions and external trapping confinement to enforce energetic spin- and position-dependent constraints on the atomic motion. These constraints allow us to transform a local superposition into a GHZ state through a stepwise protocol that flips one site at a time. The protocol requires no site-resolved drives or spin-dependent potentials, exhibits robustness to slow global laser phase drift, and naturally makes use of the harmonic trap that would normally cause difficulties for entanglement-generating protocols in optical lattices. We also discuss an improved protocol that can compensate for holes in the loadout at the cost of increased generation time. The state can immediately be used for quantum-enhanced metrology in 3D optical lattice clocks, opening a window to push the sensitivity of state-of-the-art sensors beyond the standard quantum limit.
Engineering a Hamiltonian system with tunable interactions provides opportunities to optimize performance for quantum sensing and explore emerging phenomena of many-body systems. An optical lattice clock based on partially delocalized Wannier-Stark states in a gravity-tilted shallow lattice supports superior quantum coherence and adjustable interactions via spin-orbit coupling, thus presenting a powerful spin model realization. The relative strength of the on-site and off-site interactions can be tuned to achieve a zero density shift at a "magic" lattice depth. This mechanism, together with a large number of atoms, enables the demonstration of the most stable atomic clock while minimizing a key systematic uncertainty related to atomic density. Interactions can also be maximized by driving off-site Wannier-Stark transitions, realizing a ferromagnetic to paramagnetic dynamical phase transition.
Building scalable quantum systems that demonstrate genuine performance enhancement based on entanglement is a major scientific goal for fields including computing, networking, simulations, and metrology. The tremendous challenge arises from the fragility of entanglement in increasingly larger sized quantum systems. Optical atomic clocks utilizing a large number of atoms have pushed the frontier of measurement science, building on precise engineering of quantum states and control of atomic interactions. However, today's state-of-the-art optical atomic clocks are limited by the quantum projection noise (QPN) defined by many uncorrelated atoms. Pioneering work on producing spin squeezed states of atoms has shown a path towards integrating entanglement into the best performing clocks. However, to directly demonstrate advantage of quantum entanglement in a working clock we must prevent backaction effects that degrade quantum coherence and introduce uncontrolled perturbations, as well as minimize the influence of technical noise arising from the interrogating clock laser. Here we present a new optical clock platform integrated with collective strong-coupling cavity QED for quantum non-demolition (QND) measurement. Optimizing the competition between spin measurement precision and loss of coherence, we measure a Wineland parameter of -1.8(7) dB for 1.9x104^44 atoms, thus verifying the presence of entanglement. Furthermore, a moving lattice allows the cavity to individually address two independent sub-ensembles, enabling us to spin squeeze two clock ensembles successively and compare their performance. This differential comparison between the two squeezed clocks directly verifies enhanced clock stability of 2.0(3) dB below QPN, and 0.6(3) dB above the standard quantum limit (SQL), at the measurement precision level of 10−17^{-17}−17, without subtracting any technical noise contributions.
Entanglement can improve the measurement precision of quantum sensors beyond the shot noise limit. Neutral atoms, the basis of some of the most precise and accurate optical clocks and interferometers, do not naturally exhibit the all-to-all interactions traditionally used to generate such entangled states. On the other hand, these systems exhibit exceedingly high degrees of experimental control over parameters such as temperature, spatial entropy, and itinerancy. In this work, we investigate spin squeezing in a highly coherent itinerant system of neutral atoms with magnetic dipole-dipole interactions. We achieve 7.1 dB of metrologically useful squeezing using finite-range spin-exchange interactions in an erbium quantum gas microscope, and we demonstrate that introducing atomic motion, realizing a dipolar t − J model, protects the spin sector coherence at low fillings, significantly improving the achievable spin squeezing in a 2D dipolar system. This work’s protocol can be implemented with most neutral atoms, opening the door to quantum-enhanced metrology in other itinerant dipolar systems, such as molecules or optical lattice clocks, and serves as a novel method for studying itinerant quantum magnetism with long-range interactions.
Advances in engineering quantum systems are expected to lead to a new generation of quantum technology with fundamentally new capabilities and no classical analogue. Specifically, in the near future, quantum entanglement may become useful for enhancing state-of-the-art atomic clocks and sensors. I have performed experiments using laser-cooled rubidium atoms trapped in a high finesse optical cavity to explore quantum and collective enhancements to precision measurements.\nIn this thesis, I will present a recent experiment to create record amounts of entanglement-enhancement, or spin squeezing, in a proof-of-principle atomic sensor using entanglement-generating collective measurements. We have demonstrated up to a factor of 60 in directly observed spin squeezing beyond the standard quantum limit for an unentangled quantum sensor and have demonstrated squeezing with real-time feedback to create deterministic entangled states. Second, I will present a new method that has generated over a factor of 10 in homogeneous entanglement that could be resolvable in free-space quantum sensors such as matter-wave interferometers and discuss a new method to reduce errors in manipulating collective spin states using reversible dephasing. These experiments and methods are directly applicable to some of the world's best optical lattice clocks such as those housed here at JILA and NIST.\nIn addition, I have studied and demonstrated a proof-of-principle superradiant laser that relies on collectively enhanced laser emission. These lasers have the potential to realize state-of-the-art frequency purity useful for optical atomic clocks and long baseline interferometry. I will discuss an experiment that demonstrates injection locking of a superradiant laser for the first time as well as explores the collective synchronization behaviors in the system. This study of synchronization informs research on current and future narrow linewidth superradiant lasers and may also provide a platform for future studies of quantum phase transitions in open quantum systems.