image: Figure | Schematics of quantum correlation-enhanced DCS (QC-DCS). a, DCS with intensity-correlated combs. b, Dual-comb results. c, Mid-infrared experimental data and simulation results. d, Broadband molecular spectra. In panel a, the signal and local combs mix on a photodetector (PD), generating dual-comb signals, while a separate idler comb is detected on a second PD to capture intensity noise correlated with the local comb. By subtracting the photocurrents, partial cancellation of shot noise in the dual-comb signals is achieved, lowering the noise floor in the spectrum derived from a fast Fourier transform (FFT) of the time-domain (t) signal. A low-pass filter (LPF) further suppresses background signals at the combs' repetition frequencies. The FFT yields dual-comb spectra with resolved comb lines. In this measurement, the signal comb power is only 1 nW, while the local comb power is 80 µW. The squeezed RF spectrum (orange) exhibits a reduced noise floor between 1 and 3.5 MHz compared to a standard quantum noise limit (SNL) dual-comb spectrum (blue). For the spectral results in panels b, c and d, a 10-cm-long gas cell filled with pure CH₄ at 2000 Pa (temperature: 275 K) was used.
Credit: Wan, Z., Chen, Y., Zhang, X. et al.
Dual-comb spectroscopy (DCS) has rapidly become a cornerstone of precision molecular detection, offering unparalleled resolution, broad spectral coverage, and fast acquisition speed. From fundamental research to real-world applications—such as atmospheric monitoring, trace gas detection, and even medical diagnostics—DCS has proven to be a versatile and powerful tool. Yet, despite its many strengths, DCS faces a fundamental barrier: quantum shot noise, which limits its sensitivity when probing extremely weak signals.
This limitation arises because dual-comb systems distribute optical energy across millions of frequency modes. While this feature enables high-resolution multiplexed detection, it also means that the power per mode is extremely low, resulting in quantum-limited noise that masks faint molecular signatures. Overcoming this shot-noise barrier has remained one of the key challenges in advancing the next generation of spectroscopic techniques.
In a new paper published in Light: Science & Applications, a team of scientists, led by Professors Heping Zeng and Ming Yan from State Key Laboratory of Precision Spectroscopy, and Hainan Institute, East China Normal University, China, and co-workers has demonstrated a breakthrough solution: quantum correlation-enhanced dual-comb spectroscopy (QC-DCS). This technique integrates intensity-difference squeezing into a dual-comb interferometric system, enabling the detection of molecular signals that would otherwise be obscured by quantum noise.
The concept of squeezing—reducing quantum noise in one observable at the expense of increased uncertainty in a conjugate variable—is a well-established technique in quantum optics. In the QC-DCS system, the team generated bright, intensity-correlated twin combs via seeded four-wave mixing in a highly nonlinear fiber. These twin combs are quantum-mechanically linked such that their intensity difference noise is below the standard quantum limit.
This quantum enhancement allowed the researchers to suppress shot noise while maintaining the DCS signal strength, effectively increasing the signal-to-noise ratio beyond the classical limit. In their proof-of-concept demonstration, the team recorded high-resolution (7.5 pm) molecular absorption spectra in the 3 μm region—an important window for detecting organic and atmospheric molecules. They achieved a 2 dB improvement in signal-to-noise ratio beyond the shot-noise limit, corresponding to a 2.6× increase in measurement speed.
Importantly, the researchers emphasize that their technique is not merely a laboratory curiosity but is compatible with a wide range of spectroscopic platforms. Since the squeezed combs are shielded from direct interaction with the sample, the method can be applied in scenarios involving strong absorption, high optical loss, or complex optical environments. This makes QC-DCS an attractive solution for field-deployable systems such as open-path sensors, resonance-enhanced cavities, multi-pass cells, or hollow-core fibers.
“Our method establishes a new paradigm for quantum-enhanced spectroscopy,” the authors note. “By preserving quantum correlations throughout the detection chain, we not only mitigate the fundamental noise limits of DCS but also enable seamless integration with other sensitivity-enhancing technologies.”
The implications of this work are far-reaching. “By reducing noise without sacrificing bandwidth or resolution, QC-DCS opens new frontiers in ultrasensitive molecular detection, breath-based disease diagnostics, combustion and plasma analysis, and even quantum metrology and light detection and ranging (LiDAR),” the scientists forecast. “As dual-comb systems continue to mature and miniaturize, integrating quantum techniques like squeezing could become a critical component in pushing the frontiers of precision spectroscopy.”
Journal
Light Science & Applications
Article Title
Quantum correlation-enhanced dual-comb spectroscopy