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European Frequency and Time Forum

Towards a quantum-enhanced atomic clock on a chip

Tommaso Mazzoni, Meng-Zi Huang, Théo Laudat, Carlos L. Garrido Alzar, Jakob Reichel

Published Apr 1, 2018• 0 citations
Publisher

Abstract

The Dick effect and quantum projection noise (QPN) are among the most stringent limiting factors for state-of-the-art atomic frequency standards, including both primary and secondary standards, as well as compact clocks. A possible solution to the Dick effect relies on Quantum Non-Destructive (QND) detection, which allows multiple interrogations with weak measurements on the same atomic ensemble. Furthermore, spin-squeezed states, which can also be prepared by such QND detection, have the potential to enable clock measurements beyond the standard quantum limit, by employing quantum correlations. Our experimental platform is a Trapped-Atom Clock on a Chip (TACC) that uses ultracold Rb atoms, having demonstrated metrological stability. In the second generation of this experiment, we target quantum enhancement by using QND detection and spin-squeezed ultra-cold atoms generated by cavity QED interactions. To reach this goal, we have integrated two fiber Fabry-Perot cavities on the clock chip, realizing a platform for cavity QED in the strong coupling and weak coupling regimes. With this new device we aim to explore spin squeezing at a metrologically relevant level of stability. Here, we report on the current status and perspectives of this project.

DOI
10.1109/eftf.2018.8408997

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The Dick effect and quantum projection noise (QPN) are among the most stringent limiting factors for state-of-the-art atomic frequency standards, including both primary and secondary standards, as well as compact clocks. A possible solution to the Dick effect relies on Quantum Non-Destructive (QND) detection, which allows multiple interrogations with weak measurements on the same atomic ensemble. Furthermore, spin-squeezed states, which can also be prepared by such QND detection, have the potential to enable clock measurements beyond the standard quantum limit, by employing quantum correlations. Our experimental platform is a Trapped-Atom Clock on a Chip (TACC) that uses ultracold Rb atoms, having demonstrated metrological stability. In the second generation of this experiment, we target quantum enhancement by using QND detection and spin-squeezed ultra-cold atoms generated by cavity QED interactions. To reach this goal, we have integrated two fiber Fabry-Perot cavities on the clock chip, realizing a platform for cavity QED in the strong coupling and weak coupling regimes. With this new device we aim to explore spin squeezing at a metrologically relevant level of stability. Here, we report on the current status and perspectives of this project.
Atomic sensors are among the best devices for precision measurements of time, electric and magnetic fields, and inertial forces. However, all atomic sensors that utilise uncorrelated particles are ultimately limited by quantum projection noise, as is already the case for state-of-the-art atomic clocks. This so-called standard quantum limit (SQL) can be overcome by employing entanglement, a prime example being the spin-squeezed states. Spin squeezing can be produced in a quantum non-demolition (QND) measurement of the collective spin, particularly with cavity quantum electrodynamical (cQED) interactions. In this thesis, I present the second-generation trapped-atom clock on a chip (TACC) experiment, where we combine a metrology-grade compact clock with a miniature cQED platform to test quantum metrology protocols at a metrologically-relevant precision level. In a standard Ramsey spectroscopy, the stability of the apparatus is confirmed by a fractional frequency Allan deviation of 6E-13 at 1 s. We demonstrate spin squeezing by QND measurement, reaching 8(1) dB for 1.7E4 atoms, currently limited by decoherence due to technical noise. Cold collisions between atoms play an important role at this level of precision, leading to rich spin dynamics. Here we find that the interplay between cavity measurements and collisional spin dynamics manifests itself in a quantum amplification effect of the cavity measurement. A simple model is proposed, and is confirmed by initial measurements. New experiments in this direction may shed light on the surprising many-body physics in this sytem of interacting cold atoms.
Using the platform of a trapped-atom clock on a chip, we have generated spin-squeezed states and observed their time evolution on unprecedented timescales, showing that metrological spin squeezing is preserved for one second. We observe up to 8.1(9) dB of metrological squeezing in a cloud of 2times1042times 10^42times104 ultracold alkali atoms by quantum nondemolition (QND) measurement in a fiber Fabry-Perot microcavity. The experiment also reveals a surprising amplification effect in the final cavity measurement of the spin state. It results from a subtle interplay between the cavity coupling and the spin dynamics originating from cold collisions. Spin dynamics such as these are an important factor in real atomic clocks that previous proof-of-principle squeezing experiments had not been able to address. Our results open up encouraging perspectives for squeezing-enhanced atomic clocks in a metrologically relevant stability regime.
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.
Using the platform of a trapped-atom clock on a chip, we have generated spin-squeezed states with up to 8.1(9) dB of metrological squeezing in a cloud of 2×1042\times 10^42×104 ultracold alkali atoms by quantum nondemolition (QND) measurement in a fiber Fabry-Perot microcavity. Observing the time evolution of the squeezed state on unprecedented timescales of more than one second reveals a surprising measurement amplification effect in the final measurement of the spin state. It results from a subtle interplay between the spin dynamics of interacting indistinguishable particles and energy-dependent cavity coupling and leads to an increased cavity shift per spin, and thus to a higher signal per photon read out. Metrological spin squeezing is preserved for 1 s. Both results open up encouraging perspectives for squeezing-enhanced atomic clocks in a metrologically relevant stability regime.
Atom sensors are highly sensitive devices used in time and frequency standards, inertial sensing and precision measurements of electromagnetic fields. Nowadays, they are developed to the extent that they can be limited by their quantum nature, i.e., the standard quantum limit (SQL). This limit arises from the individual and uncorrelated behaviour of the used atoms. However, it has been demonstrated that one can overcome this limit via the generation of quantum correlations and entanglement between the atoms. Proof of principle entanglement generation can be accomplished via different protocols, but this has very seldom been done in metrology-grade devices. In this thesis, we use a cavity quantum electrodynamics (cQED) platform to create a type of quantum correlated state named spin squeezed. We use as a platform a trapped-atom clock on a chip (TACC) to generate these entangled states. This metrology-grade device allows us to study the dynamics due to spin interactions in the long time scale, on the order of a second. The stability of the apparatus is confirmed by a fractional frequency Allan deviation of 6×10−13 at 1 s, a performance beating commercially available compact atom clocks.
High-bandwidth, high-stability clocks offer unique chances to study fundamental physics beyond simple metrology. Today, the most stable clocks are optical lattice clocks and they can reach a fractional stability of 3.1 × 10^−18 at 1 s averaging time. Their applications are ranging from the redefinition of the SI time unit, the Second, to the possible detection of gravitational waves in space. The stability of optical lattice clocks is primarily limited by quantum projection noise and Dick noise. In this thesis, I present our effort in creating a platform for studying quantum enhanced metrology and its application towards optical lattice clocks. We demonstrated the laser cooling in an optical cavity with Raman-sideband cooling and achieved average vibration number of n = 0.2. We utilized cavity feedback squeezing to demonstrate unitary spin squeezing on magnetic sub-levels of 171Yb, achieving 6.5 dB quantum projection noise reduction over the standard quantum limit (SQL). With coherent optical state transfer by ultra stable laser pulses, we successfully transferred the spin squeezed state from magnetic sub-levels to the optical transition, and demonstrated quantum enhanced metrology in the optical lattice clock for the first time. We achieved 4.4 dB of metrological gain over the SQL, which corresponds to a reduction of the averaging time by a factor of 2.8 in clock operation. Furthermore, we studied quantum enhanced metrology with time-reversal-based protocols. We performed a signal amplification through time-reversed interaction protocol between the ground state magnetic sublevels of 171Yb, achieving the largest sensitivity improvement beyond SQL in any interferometers to date at 11.8dB. Besides that, we experimentally studied the connection between quantum information scrambling, by measuring out-of-time-order correlators (OTOCs), and quantum metrology, by measuring metrological gain. We found these values to agree with each other, and concluded that the time-reversal quantum metrology can saturate the attainable gain from the quantum information scrambling.