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Laser & Photonics Review

High‐Precision Laser Spectrum Analyzer via Digital Decoherence (Laser Photonics Rev. 20(1)/2026)

Zhongwang Pang, Chunyi Li, Hongfei Dai, Wenlin Li, Dongqi Song, Fei Meng, Yige Lin, Bo Wang

Published Jan 1, 2026• 1 citation
Publisher

Abstract

Digital Decoherence for Sub-Hertz Laser Analyzer In Research Article e01773, Bo Wang and co-workers propose the concept of digital decoherence. This allows the decoherence process to be realized through a digital delay. Based on this method, a laser spectrum analyzer is achieved for accurate frequency-noise power spectral density measurements of sub-Hertz linewidth lasers. This advance benefits quantum metrology, especially optical clocks.

DOI
10.1002/lpor.70753

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Digital Decoherence for Sub-Hertz Laser Analyzer In Research Article e01773, Bo Wang and co-workers propose the concept of digital decoherence. This allows the decoherence process to be realized through a digital delay. Based on this method, a laser spectrum analyzer is achieved for accurate frequency-noise power spectral density measurements of sub-Hertz linewidth lasers. This advance benefits quantum metrology, especially optical clocks.
Decoherence induced by laser frequency noise is one of the most important obstacles in quantum information processing. In order to suppress this decoherence, the noise power spectral density needs to be accurately characterized. In particular, the noise spectrum measurement based on the coherence characteristics of qubits would be a meaningful and still challenging method. Here, we theoretically analyze and experimentally obtain the spectrum of laser frequency noise based on the continuous dynamical decoupling technique. We first estimate the mixture-noise (including laser and magnetic noises) spectrum up to $(2\ensuremath{\pi})530$ kHz by monitoring the transverse relaxation from an initial state +X+X+X, followed by a gradient descent data process protocol. Then the contribution from the laser noise is extracted by encoding the qubits on different Zeeman sublevels. We also investigate two sufficiently strong noise components by making an analogy between these noises and driving lasers whose linewidth is assumed to be negligible. This method is verified experimentally and finally helps to characterize the noise.
Interactions between atoms and lasers provide the potential for unprecedented control of quantum states. Fulfilling this potential requires detailed knowledge of frequency noise in optical oscillators with state-of-the-art stability. We demonstrate a technique that precisely measures the noise spectrum of an ultrastable laser using optical lattice-trapped 87Sr atoms as a quantum projection noise-limited reference. We determine the laser noise spectrum from near dc to 100 Hz via the measured fluctuations in atomic excitation, guided by a simple and robust theory model. The noise spectrum yields a 26(4) mHz linewidth at a central frequency of 429 THz, corresponding to an optical quality factor of 1.6×10(16). This approach improves upon optical heterodyne beats between two similar laser systems by providing information unique to a single laser and complements the traditionally used Allan deviation which evaluates laser performance at relatively long time scales. We use this technique to verify the reduction of resonant noise in our ultrastable laser via feedback from an optical heterodyne beat. Finally, we show that knowledge of our laser's spectrum allows us to accurately predict the laser-limited stability for optical atomic clocks.
Highest-resolution laser spectroscopy has generally been limited to single trapped ion systems because of the rapid decoherence that plagues neutral atom ensembles. Precision spectroscopy of ultracold neutral atoms confined in a trapping potential now shows superior optical coherence without any deleterious effects from motional degrees of freedom, revealing optical resonance linewidths at the hertz level with a good signal-to-noise ratio. The resonance quality factor of 2.4 x 10(14) is the highest ever recovered in any form of coherent spectroscopy. The spectral resolution permits direct observation of the breaking of nuclear spin degeneracy for the 1S0 and 3P0 optical clock states of 87Sr under a small magnetic bias field. This optical approach for excitation of nuclear spin states allows an accurate measurement of the differential Landé g factor between 1S0 and 3P0. The optical atomic coherence demonstrated for collective excitation of a large number of atoms will have a strong impact on quantum measurement and precision frequency metrology.
Optical atomic clocks based on laser-cooled trapped ions and atoms have advanced rapidly over the past decade. With fractional frequency uncertainties now surpassing 10 −18 , they are some of the most precise measurement tools ever built. Yet researchers are still pursuing new avenues of research to explore the fundamental limits to their stability, accuracy, and reproducibility. In this mini-review, we provide a survey of the current state of the art by describing the fundamental principles and techniques that underpin this progress, the architectures used to realize optical clocks, and the supporting laser technologies that are essential to their operation. We also examine the progress that has been made toward a redefinition of the second in the International System of Units and the inclusion of optical clocks into the global time and frequency metrology infrastructure. Finally, we discuss emerging applications of optical clocks and look at the prospects for making their precision more readily accessible to end users.
The frequency noise of an ordinary diode laser, often a nuisance in spectroscopy, can be used to measure atomic coherence. When atoms are prepared in a coherent superposition of two ground states, each random shift in laser frequency changes how much light they absorb, so the transmitted light carries a record of that coherence. Correlating the intensity fluctuations of two fields, each linking one ground state to a shared excited state, reads this record out, a technique known as noise correlation spectroscopy. The correlation signal is narrower than the transparency window the same fields create and switches sign as their frequency difference is tuned. Its width is set by the rate at which the atoms lose coherence and, over a wide range, does not grow with laser power. Over two decades the method has spread from warm vapors to cold atoms, to multi-field schemes, and to diamond and rare-earth-doped crystals. This review, to the author's knowledge the first, brings together its theory, experiments and applications, and examines its limits, the challenges that remain, and the opportunities that could make it a standard spectroscopic tool.