Quantum entanglement has been generated and verified in cold-atom experiments
and used to make atom-interferometric measurements below the shot-noise limit.
However, current state-of-the-art cold-atom devices exploit separable (i.e.
unentangled) atomic states. This perspective piece asks the question: can
entanglement usefully improve cold-atom sensors, in the sense that it gives new
sensing capabilities unachievable with current state-of-the-art devices? We
briefly review the state-of-the-art in precision cold-atom sensing, focusing on
clocks and inertial sensors, identifying the potential benefits entanglement
could bring to these devices, and the challenges that need to be overcome to
realize these benefits. We survey demonstrated methods of generating
metrologically-useful entanglement in cold-atom systems, note their relative
strengths and weaknesses, and assess their prospects for near-to-medium term
quantum-enhanced cold-atom sensing.
Quantum entanglement has been generated and verified in cold-atom experiments
and used to make atom-interferometric measurements below the shot-noise limit.
However, current state-of-the-art cold-atom devices exploit separable (i.e.
unentangled) atomic states. This perspective piece asks the question: can
entanglement usefully improve cold-atom sensors, in the sense that it gives new
sensing capabilities unachievable with current state-of-the-art devices? We
briefly review the state-of-the-art in precision cold-atom sensing, focusing on
clocks and inertial sensors, identifying the potential benefits entanglement
could bring to these devices, and the challenges that need to be overcome to
realize these benefits. We survey demonstrated methods of generating
metrologically-useful entanglement in cold-atom systems, note their relative
strengths and weaknesses, and assess their prospects for near-to-medium term
quantum-enhanced cold-atom sensing.
Quantum entanglement has been generated and verified in cold-atom experiments and used to make atom-interferometric measurements below the shot-noise limit. However, current state-of-the-art cold-atom devices exploit separable (i.e., unentangled) atomic states. This perspective piece asks the question: can entanglement usefully improve cold-atom sensors, in the sense that it gives new sensing capabilities unachievable with current state-of-the-art devices? We briefly review the state-of-the-art in precision cold-atom sensing, focusing on clocks and inertial sensors, identifying the potential benefits entanglement could bring to these devices, and the challenges that need to be overcome to realize these benefits. We survey demonstrated methods of generating metrologically useful entanglement in cold-atom systems, note their relative strengths and weaknesses, and assess their prospects for near-to-medium term quantum-enhanced cold-atom sensing.
Recent developments in atomic physics have enabled the experimental generation of many-body entangled states to boost the performance of quantum sensors beyond the Standard Quantum Limit (SQL). This limit is imposed by the inherent projection noise of a quantum measurement. In this Perspective article, we describe the commonly used experimental methods to create many-body entangled states to operate quantum sensors beyond the SQL. In particular, we focus on the potential of applying quantum entanglement to state-of-the-art optical atomic clocks. In addition, we present recently developed time-reversal protocols that make use of complex states with high quantum Fisher information without requiring sub-SQL measurement resolution. We discuss the prospects for reaching near-Heisenberg limited quantum metrology based on such protocols.
Abstract Atom interferometry is one of the most promising technologies for high precision measurements. It has the potential to revolutionise many different sectors, such as navigation and positioning, resource exploration, geophysical studies, and fundamental physics. After decades of research in the field of cold atoms, the technology has reached a stage where commercialisation of cold atom interferometers has become possible. This article describes recent developments, challenges, and prospects for quantum sensors for inertial sensing based on cold atom interferometry techniques.
Entangled quantum states can enhance measurement precision. But quantum mechanics harbors other possibilities for enhancing precision, including some that have nothing to do with entanglement. This review surveys various strategies with unentangled probes by which measurements have been improved. Among the approaches considered are those that rely on particle statistics and correlations in highly mixed states. Often nonentangled states are more robust, and these approaches are feasible in current experiments: here the current states of research are shown in cold atoms, nonlinear optics, and nanomechanical oscillators.
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.
Ultra-cold atoms are highly sensitive to both inertial effects and electromagnetic fields, making atom interferometers excellent candidates for a variety of precision sensing applications. Furthermore, highly tunable atomic interactions combined with well established experimental techniques for trapping, cooling and manipulating ensembles of ultra-cold atoms, make them well suited to quantum enhancement. For these reasons, ensembles of ultra-cold atoms are excellent candidates for quantum-enhanced measurement devices. Broadly, this thesis presents a number of theoretical results which share the aim of progressing the proof-of-principle regime of current experiments to genuinely useful, ultra-precise quantum enhanced sensors.In Chapter 4 we explore the challenge of devising a useful, quantum-enhanced inertial measurement - to date no such experiment has been performed. Specifically, we present a model of a spin-squeezed rotation sensor utilising the Sagnac effect in a spin-1 Bose-Einstein condensate in a ring trap. The two input states for the interferometer are seeded using Raman pulses with Laguerre-Gauss beams and are amplified by the bosonic enhancement of spin-exchange collisions, resulting in spin-squeezing and potential quantum enhancement in the interferometry. The ring geometry has an advantage over separated beam path atomic rotation sensors due to the uniform condensate density. We model the interferometer both analytically and numerically for realistic experimental parameters and find that significant quantum enhancement is possible. However, this enhancement is partially degraded when working in a regime with strong atomic interactions, indicating that perhaps interaction-based squeezing is not the best route to a quantum-enhanced inertial measurement.Continuing the theme of practical spin-squeezing with atomic interactions, in Chapter 5 we explore methods to combat detection noise, which is currently one of the most significant experimental limitations for quantum-enhanced atom interferometers. A useful quantum-enhanced interferometer requires nonclassical states with a high particle number and the (close to) optimal exploitation of the states' quantum correlations. Unfortunately, the single-particle detection resolution demanded by conventional protocols, such as spin squeezing via one-axis twisting, places severe limits on the particle number. Additionally, the challenge of finding optimal measurements (that saturate the quantum Cram´er-Rao bound) for an arbitrary nonclassical state limits most metrological protocols to only moderate levels of quantum enhancement. “Interaction-based readout” protocols have been shown to allow optimal interferometry or to provide robustness against detection noise at the expense of optimality. We prove that one has great flexibility in constructing an optimal protocol, thereby allowing it to also be robust to detection noise. This requires the full probability distribution of outcomes in an optimal measurement basis, which is typically easily-accessible and can be determined from specific criteria we provide. Additionally, we quantify the robustness of several classes of interaction-based readouts under realistic experimental constraints. We determine that optimal and robust quantum metrology is achievable in current spin-squeezing experiments.Macroscopic superposition states (known as spin-cat states in atomic ensembles) achieve the ultimate quantum precision limit. In Chapters 6 and 7 we turn our attention to studying decoherence that arises in creating and manipulating these states. In its simplest form, decoherence occurs when a quantum state is entangled with a second state, but the results of measurements made on the second state are not accessible. As the second state has effectively ``measured the first, in Chapter 6 we argue that the quantum Fisher information is the relevant metric for predicting and quantifying this kind of decoherence. In particular we apply this analysis to studying decoherence imparted on spin-cat states via atom-light interactions, which are an essential process in atom interferometry. We find that, compared to classical atomic states, spin-cat states are highly susceptible to this kind of decoherence. We investigate the required optical field occupation, such that this decoherence is negligible.Finally, in Chapter 7 we explore the deleterious effects of spatial and phase dynamics during the creation of spin-cat states via one-axis twisting in a two-component Bose-Einstein condensate. We simulate the breathing dynamics in 1 spatial dimension, employing the method of Sinatra and Castin, whereby the state is expanded into the Fock basis, with each basis state evolving within the Hartree-Fock approximation. This method is able to capture both multi-mode effects and quantum correlations, which allows us to calculate the quantum Fisher information. We find that so long as the time taken for the condensate to reach a spin-cat state is an integer multiple of the breathing period, it is sometimes possible to realise something extremely close to a spin-cat state, despite significant dynamics. These results indicate that large non-linearities, and the associated speed-up in entangling dynamics, are not fundamentally incompatible with the production of fragile, highly entangled states such as spin-cat states.