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Bulletin of the American Physical Society

Progress toward a spin squeezed optical atomic clock beyond the standard quantum limit

Boris Braverman, Akio Kawasaki, Vladan Vuletić

Published Jun 11, 2015• 0 citations
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Optical atomic clocks with unrivaled precision and accuracy have advanced the frontier of precision measurement science and opened new avenues for exploring fundamental physics. A fundamental limitation on clock precision is the standard quantum limit (SQL), which stems from the uncorrelated projection noise of each atom. State-of-the-art optical lattice clocks interrogate large ensembles to minimize the SQL, but density-dependent frequency shifts pose challenges to scaling the atom number. The SQL can be surpassed, however, by leveraging entanglement, though it remains an open problem to achieve quantum advantage from spin squeezing at state-of-the-art stability levels. Here, we demonstrate clock performance beyond the SQL, achieving a fractional frequency precision of 1.1×10^{-18} for a single spin-squeezed clock. With cavity-based quantum nondemolition measurements, we prepare two spin-squeezed ensembles of ∼30 000 strontium atoms confined in a two-dimensional optical lattice. A synchronous clock comparison with an interrogation time of 61 ms achieves a metrological improvement of 2.0(2) dB beyond the SQL, after correcting for state preparation and measurement errors. These results establish the most precise entanglement-enhanced clock to date and offer a powerful platform for exploring the interplay of gravity and quantum entanglement.

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Spin squeezing can improve atomic precision measurements beyond the standard quantum limit (SQL), and unitary spin squeezing is essential for improving atomic clocks. We report substantial and nearly unitary spin squeezing in ^{171}Yb, an optical lattice clock atom. The collective nuclear spin of ∼10^{3} atoms is squeezed by cavity feedback, using light detuned from the system's resonances to attain unitarity. The observed precision gain over the SQL is limited by state readout to 6.5(4) dB, while the generated states offer a gain of 12.9(6) dB, limited by the curvature of the Bloch sphere. Using a squeezed state within 30% of unitarity, we demonstrate an interferometer that improves the averaging time over the SQL by a factor of 3.7(2). In the future, the squeezing can be simply transferred onto the optical-clock transition of ^{171}Yb.

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Optical atomic clocks utilizing strontium are the gold standard of timekeeping. Generating entanglement by spin-squeezing has the potential to continue pushing the limits of clock stability below the fundamental performance limit for unentangled ensembles, the standard quantum limit (SQL). One way to do so is by using an optical cavity to transduce interactions between the atoms and to improve the cavity readout, one needs to minimize the noise of the stabilized cavity length. Noise in a control loop can arise from many different sources, including mechanical resonances of the piezo (PZT) used to control the cavity length, so addressing this noise can improve clock stability and maximize the capabilities of quantum sensing. For this project, I measured the transfer function of the piezo-cavity system, inverted this transfer function, and extracted the poles and zeros. I then programmed this tailored digital filter on a Red Pitaya using an open-source Python package, PyRPL. Challenges still remain with the device implementation into the control loop, but afterwards this would ultimately contribute to improved clock precision and attain better spin-squeezing generated by the cavity.

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Optical atomic clocks with unrivaled precision and accuracy have advanced the frontier of precision measurement science and opened new avenues for exploring fundamental physics. A fundamental limitation on clock precision is the standard quantum limit (SQL), which stems from the uncorrelated projection noise of each atom. State-of-the-art optical lattice clocks interrogate large ensembles to minimize the SQL, but density-dependent frequency shifts pose challenges to scaling the atom number. The SQL can be surpassed, however, by leveraging entanglement, though it remains an open problem to achieve quantum advantage from spin squeezing at state-of-the-art stability levels. Here, we demonstrate clock performance beyond the SQL, achieving a fractional frequency precision of 1.1×10^{-18} for a single spin-squeezed clock. With cavity-based quantum nondemolition measurements, we prepare two spin-squeezed ensembles of ∼30 000 strontium atoms confined in a two-dimensional optical lattice. A synchronous clock comparison with an interrogation time of 61 ms achieves a metrological improvement of 2.0(2) dB beyond the SQL, after correcting for state preparation and measurement errors. These results establish the most precise entanglement-enhanced clock to date and offer a powerful platform for exploring the interplay of gravity and quantum entanglement.
Spin squeezing can improve atomic precision measurements beyond the standard quantum limit (SQL), and unitary spin squeezing is essential for improving atomic clocks. We report substantial and nearly unitary spin squeezing in ^{171}Yb, an optical lattice clock atom. The collective nuclear spin of ∼10^{3} atoms is squeezed by cavity feedback, using light detuned from the system's resonances to attain unitarity. The observed precision gain over the SQL is limited by state readout to 6.5(4) dB, while the generated states offer a gain of 12.9(6) dB, limited by the curvature of the Bloch sphere. Using a squeezed state within 30% of unitarity, we demonstrate an interferometer that improves the averaging time over the SQL by a factor of 3.7(2). In the future, the squeezing can be simply transferred onto the optical-clock transition of ^{171}Yb.
Optical atomic clocks utilizing strontium are the gold standard of timekeeping. Generating entanglement by spin-squeezing has the potential to continue pushing the limits of clock stability below the fundamental performance limit for unentangled ensembles, the standard quantum limit (SQL). One way to do so is by using an optical cavity to transduce interactions between the atoms and to improve the cavity readout, one needs to minimize the noise of the stabilized cavity length. Noise in a control loop can arise from many different sources, including mechanical resonances of the piezo (PZT) used to control the cavity length, so addressing this noise can improve clock stability and maximize the capabilities of quantum sensing. For this project, I measured the transfer function of the piezo-cavity system, inverted this transfer function, and extracted the poles and zeros. I then programmed this tailored digital filter on a Red Pitaya using an open-source Python package, PyRPL. Challenges still remain with the device implementation into the control loop, but afterwards this would ultimately contribute to improved clock precision and attain better spin-squeezing generated by the cavity.