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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Phase Noise Characterization of an Ultra-Stable Industrial Clock Laser System via Cross-Correlation Analysis

Dewni Pathegama, Florian Schäfer, Filippo Bregolin

Published Jun 23, 2025• 0 citations
Publisher

Abstract

As more and more quantum applications are brought outside of laboratories, the demand for compact, robust laser systems with high frequency stability are increasing [1]–[3]. For many such applications, achieving fractional frequency stability below$2\times 10^{-15}$for averaging times between 0.1 and 100 seconds is critical. Here we present the characterization of a TOPTICA Photonics Clock Laser System (CLS), a transportable, rack-mounted ultra-stable laser system, and discuss its suitability for quantum applications. Further, we quantify the system performance with respect to the impact of environmental conditions such as seismic and acoustic vibrations, fiber induced phase noise and optical power fluctuations. We characterize our system over an offset frequency range from 1 mHz up to 10 MHz. We show that our system reaches a fractional frequency stability of$4\times 10^{-16}$at 1 s averaging time.

DOI
10.1109/cleo/europe-eqec65582.2025.11109434

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As more and more quantum applications are brought outside of laboratories, the demand for compact, robust laser systems with high frequency stability are increasing [1]–[3]. For many such applications, achieving fractional frequency stability below2×10−152\times 10^{-15}2×10−15for averaging times between 0.1 and 100 seconds is critical. Here we present the characterization of a TOPTICA Photonics Clock Laser System (CLS), a transportable, rack-mounted ultra-stable laser system, and discuss its suitability for quantum applications. Further, we quantify the system performance with respect to the impact of environmental conditions such as seismic and acoustic vibrations, fiber induced phase noise and optical power fluctuations. We characterize our system over an offset frequency range from 1 mHz up to 10 MHz. We show that our system reaches a fractional frequency stability of4×10−164\times 10^{-16}4×10−16at 1 s averaging time.
Optical clocks require an ultra-stable laser to probe and precisely measure the frequency of the narrow-linewidth clock transition. We introduce a portable ultraviolet (UV) laser system for use in an aluminum quantum logic clock, demonstrating a fractional frequency instability of approximately mod σy=2×10−16\mathrm{mod}\,σ_\mathrm{y} = 2 \times 10^{-16}modσy​=2×10−16. The system is based on an ultra-stable cavity with crystalline AlGaAs/GaAs mirror coatings, alongside with a frequency quadrupling system employing two single-pass second harmonic generation (SHG) stages. Its acceleration sensitivity, measured in all three axes, does not exceed 4(2)×10−124(2) \times 10^{-12}4(2)×10−12/(ms−2^{-2}−2) and is among the lowest recorded for transportable systems to date. Additionally, partial cancellation between photo-thermal noise and photo-birefringence noise is used to effectively mitigate noise induced by intra-cavity optical power fluctuation at lower Fourier frequencies.
Neutral atom optical standards require the highest levels of laser precision to operate near the limit set by quantum fluctuations. We develop state-of-the-art ultra-stable laser systems to achieve a factor of 10 enhancement in clock measurement precision and additionally demonstrate optical linewidths below 50 mHz. The most stable of these lasers reaches its thermal noise floor of 1 × 10-16 fractional frequency instability, allowing the attainment of near quantum-noise-limited clock operation with single-clock instabilities of 3×10-16 at 1 s. We utilize this high level of spectral resolution to operate a 87Sr optical lattice clock in a regime in which quantum collisions play a dominant role in the dynamics, enabling the study of quantum many-body physics. With a fractional level of precision of near 1 × 10-16 at 1 s, we clearly resolve the signatures of many-body interactions. We find that the complicated interplay between the p wave-dominated elastic and inelastic interaction processes between lattice-trapped atoms leads to severe lineshape distortion, shifts, and loss of Ramsey fringe contrast. We additionally explore the theoretical prediction that these many-body interactions will modify the quantum fluctuations of the system and we find that in certain parameter regimes the quantum noise distribution exhibits a quadrature dependence. We further present technological advancements that will permit ultra-stable lasers to operate with reduced thermal noise, leading to a potential gain of an additional factor of 10 in stability. This indicates that laser fractional frequency instabilities of 1 × 10-17 are within experimental reach, as is a fully-quantum-limited regime of optical clock operation.
Quantum technologies are nowadays emerging as enabling tools for practical applications, such as quantum sensing, quantum computing and quantum metrology. Lasers play a central role in many of these technological platforms, e.g. for atomic clocks, ion-based or neutral atom-based quantum computers or atom interferometers. Here we present a complete laser system to cool, trap and control strontium atoms in an optical lattice or in tweezer arrays. A sub-Hz linewidth master laser, locked to a high-finesse optical cavity provides the frequency reference for an ultra-low noise comb. The rack-mounted laser system consists of all cooling, repumping, and clock lasers stabilized to the optical frequency comb. Each of the involved laser frequencies can therefore be tuned and mapped in the frequency domain with a high degree of stability. The system is controlled via a software interface, allowing to operate the cold-atom-based physics package autonomously. The system is tailored for the operation of 88Sr or 87Sr optical lattice clocks, or for quantum computing applications, but other sub-Hz lasers could be obtained by phase locking additional clock laser frequencies to the ultra-stable comb, enabling convenient and accurate optical frequency ratio measurements. The laser system architecture and the relevant characterization measurements will be presented, proposing some user-cases such as quantum computing and atom interferometry on strontium atoms. This represents a technological leap for quantum optics, allowing to explore further applications of quantum sensors outside a traditional lab.
Abstract The increasing performance of optical lattice clocks has made them attractive for scientific applications in space and thus has pushed the development of their components including the interrogation lasers of the clock transitions towards being suitable for space, which amongst others requires making them more power efficient, radiation hardened, smaller, lighter as well as more mechanically stable. Here we present the development towards a space-compatible interrogation laser system for a strontium lattice clock constructed within the Space Optical Clock (SOC2) project where we have concentrated on mechanical rigidity and size. The laser reaches a fractional frequency instability of 7.9 × 10 −16 at 300 ms averaging time. The laser system uses a single extended cavity diode laser that gives enough power for interrogating the atoms, frequency comparison by a frequency comb and diagnostics. It includes fibre link stabilisation to the atomic package and to the comb. The optics module containing the laser has dimensions 60 × 45 × 8 cm 3 ; and the ultra-stable reference cavity used for frequency stabilisation with its vacuum system takes 30 × 30 × 30 cm 3 . The acceleration sensitivities in three orthogonal directions of the cavity are 3.6 × 10 −10 /g, 5.8 × 10 −10 /g and 3.1 × 10 −10 /g, where g ≈ 9.8 m/s 2 is the standard gravitational acceleration.
An optical clock network requires the establishment of an optical frequency transmission link between multiple optical clocks, utilizing narrow-linewidth lasers. Despite achieving link noise levels of ${10}^{\ensuremath{-}20}$, the final accuracy is limited by the phase noise of the clock laser. Correlation spectroscopy has been developed to transmit frequency information between two optical clocks directly, enabling optical clock comparison beyond the phase noise limit of clock lasers, and significantly enhancing the measurement accuracy or shortening the measurement time. In this article, two compact transportable 40Ca+{}^{40}\mathrm{Ca}{}^{+}40Ca+ clocks are employed to accomplish a correlation spectroscopy comparison, demonstrating a 10-cm level measurement accuracy of chronometric leveling using a mediocre clock laser with linewidth of 200 Hz. The relative frequency instability reaches $6.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}/\sqrt{\ensuremath{\tau}/s}$, which is about 20 times better than the result with Rabi spectroscopy using the same clock laser. This method circumvents the harsh requirements on the performance of the clock laser, so that an ordinary stable laser can also be employed in the construction of an optical clock network, which is essential for field applications, especially for chronometric leveling.
Highly frequency-stable lasers are ubiquitous tools for optical-frequency metrology, precision interferometry, and quantum information science. While making a universally applicable laser is unrealistic, spectral noise can be tailored for specific applications. Here we report a high-power 698-nm clock laser with a maximum output of 4W and minimized frequency noise up to a few kHz Fourier frequency, together with long-term instability of 3.5 × 10 − 17 at one to thousands of seconds. The laser-frequency noise is precisely characterized with atom-based spectral analysis that employs a pulse sequence designed to suppress sensitivity to intensity noise. This method provides universally applicable tunability of the spectral response and analysis of quantum sensors over a wide frequency range. With the optimized laser system characterized by this technique, we achieve an average single-qubit Clifford gate fidelity of up to F 1 2 = 0.999 64 ( 3 ) when simultaneously driving 3000 optical qubits with a homogeneous Rabi frequency ranging from 10 Hz to 1 kHz. This result represents the highest single optical-qubit-gate fidelity for a large number of atoms.