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Register a new userImpact of Non-Unitary Spin Squeezing on Atomic Clock Performance
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Spin squeezing is a form of entanglement that can improve the stability of quantum sensors operating with multiple particles, by inducing inter-particle correlations that redistribute the quantum projection noise. Previous analyses of potential metrological gain when using spin squeezing were performed on theoretically ideal states, without incorporating experimental imperfections or inherent limitations which result in non-unitary quantum state evolution. Here, we show that potential gains in clock stability are substantially reduced when the spin squeezing is non-unitary, and derive analytic formulas for the clock performance as a function of squeezing, excess spin noise, and interferometer contrast. Our results highlight the importance of creating and employing nearly pure entangled states for improving atomic clocks.
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State-of-the-art atomic clocks are based on the precise detection of the energy difference between two atomic levels, measured as a quantum phase accumulated in a given time interval. Optical-lattice clocks (OLCs) now operate at or near the standard quantum limit (SQL) that arises from the quantum noise associated with discrete measurement outcomes. While performance beyond the SQL has been achieved in microwave clocks and other atomic sensors by engineering quantum correlations (entanglement) between the atoms, the generation of entanglement on an optical-clock transition and operation of such a clock beyond the SQL represent major goals in quantum metrology that have never been demonstrated. Here we report creation of a many-atom entangled state on an optical transition, and demonstrate an OLC with an Allan deviation below the SQL. We report a metrological gain of $4.4^{+0.6}_{-0.4}$ dB over the SQL using an ensemble consisting of a few hundred 171Yb atoms, allowing us to reach a given stability $2.8{pm}0.3$ times faster than the same clock operated at the SQL. Our results should be readily applicable to other systems, thus enabling further advances in timekeeping precision and accuracy. Entanglement-enhanced OLCs will have many scientific and technological applications, including precision tests of the fundamental laws of physics, geodesy, or gravitational wave detection.
Arrays of atoms trapped in optical tweezers combine features of programmable analog quantum simulators with atomic quantum sensors. Here we propose variational quantum algorithms, tailored for tweezer arrays as programmable quantum sensors, capable of generating entangled states on-demand for precision metrology. The scheme is designed to generate metrological enhancement by optimizing it in a feedback loop on the quantum device itself, thus preparing the best entangled states given the available quantum resources. We apply our ideas to generate spin-squeezed states on Sr atom tweezer arrays, where finite-range interactions are generated through Rydberg dressing. The complexity of experimental variational optimization of our quantum circuits is expected to scale favorably with system size. We numerically show our approach to be robust to noise, and surpassing known protocols.
In this article we present a concrete proposal for spin squeezing the ultracold ground state polar paramagnetic molecule OH, a system currently under fine control in the laboratory. In contrast to existing work, we consider a single, non-interacting molecule with angular momentum greater than $1/2$. Starting from an experimentally relevant effective Hamiltonian, we identify a parameter regime where different combinations of static electric and magnetic fields can be used to realize the single-axis twisting Hamiltonian of Kitagawa and Ueda [M. Kitagawa and M. Ueda, Phys. Rev. A 47, 5138 (1993)], the uniform field Hamiltonian proposed by Law et al. [C. K. Law, H. T Ng and P. T. Leung, Phys. Rev. A 63, 055601 (2001)], and a model of field propagation in a Kerr medium considered by Agarwal and Puri [G. S. Agarwal and R. R. Puri, Phys. Rev. A 39, 2969 (1989)]. To support our conclusions, we provide analytical expressions as well as numerical calculations, including optimization of field strengths and accounting for the effects of field misalignment. Our results have consequences for applications such as precision spectroscopy, techniques such as magnetometry, and stereochemical effects such as the orientation-to-alignment transition.
We have studied relative-intensity fluctuations for a variable set of orthogonal elliptic polarization components of a linearly polarized laser beam traversing a resonant $^{87}$Rb vapor cell. Significant polarization squeezing at the threshold level (-3dB) required for the implementation of several continuous variables quantum protocols was observed. The extreme simplicity of the setup, based on standard polarization components, makes it particularly convenient for quantum information applications.
In a non-reciprocal optical amplifier, gain depends on whether the light propagates forwards or backwards through the device. Typically, one requires either the magneto-optical effect, a temporal modulation, or an optical nonlinearity to break reciprocity. By contrast, here, we demonstrate non-reciprocal amplification of fibre-guided light using Raman gain provided by spin-polarized atoms that are coupled to the nanofibre waist of a tapered fibre section. The non-reciprocal response originates from the propagation direction-dependent local polarization of the nanofibre-guided mode in conjunction with polarization-dependent atom-light coupling. We show that this novel mechanism does not require an external magnetic field and that it allows us to fully control the direction of amplification via the atomic spin state. Our results may simplify the construction of complex optical networks. Moreover, suitable solid-state based quantum emitters provided, our scheme could be readily implemented in photonic integrated circuits.
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