Two-Photon Signals Cut Noise for Clearer Sensing
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Scientists are continually seeking methods to enhance the sensitivity of precision measurements, and a new study by Romain Dalidet, Sébastien Tanzilli, and Audrey Dot, working with colleagues from Université Côte d’Azur, CNRS, Institut de physique de Nice (INPHYNI), France, and Thales Research and Technology (TRT), Thales Group, Palaiseau, France, details a significant advance in quantum-enhanced sensing. Their research demonstrates a direct improvement in sensing capabilities within the Fourier domain by leveraging two-photon interference to reduce spectral noise. This collaborative effort establishes Fourier-domain super-sensitivity as a practical resource for precision interferometry, offering a 3 dB improvement in signal-to-noise ratio and maintaining performance even when classical signals are obscured by background noise, potentially impacting fields reliant on highly accurate measurements. Quantum sensing now bypasses limitations imposed by conventional measurement technology. Exploiting the subtle correlations of light unlocks detection of previously obscured signals, promising more sensitive instruments for applications ranging from medical imaging to materials science.
Scientists have long sought methods to improve the sensitivity of interferometric sensors, devices used in diverse applications such as gravitational wave detection and medical imaging. These sensors identify weak signals as subtle changes within a complex spectral background, in effect looking for small peaks amidst noise in the frequency domain.
This research demonstrates a direct pathway to quantum-enhanced sensing by actively reducing the noise floor against which signals are detected, establishing a form of ‘quantum super-sensitivity’ directly applicable to existing interferometric technologies. Previous explorations of quantum enhancement largely focused on the time or phase domains, while this work concentrates on the Fourier domain, where practical sensors are routinely optimised using power spectral density measurements. Investigations reveal that quantum correlations effectively lower the associated noise, leading to a measurable improvement in the signal-to-noise ratio, than increasing the height of a signal peak. The core of this advancement lies in the use of entangled photons. By comparing interference patterns generated by single photons with those created by entangled photons within a fibre-based interferometer, researchers obtained a common-mode reference, allowing for an unambiguous assessment of quantum advantage. Spectral analysis showed that the amplitude of the signal peak remained constant regardless of whether single or entangled photons were used, but the noise floor demonstrably decreased with the introduction of quantum correlations. This enhancement persisted even in the sub-shot-noise regime, where classical signals are typically obscured by background noise, while the two-photon contribution remained visible, revealing spectral features that would otherwise be hidden. Once implemented, this technique offers a pathway to improve the performance of broadband, noise-limited sensors without substantial redesigns, and opens possibilities for developing entirely new sensing modalities that exploit spectral noise reduction. Future applications could include more sensitive acoustic probes or improved coherence tomography for medical diagnostics. Generation of time-correlated photon pairs via parametric down-conversion and Franson interference A continuous-wave telecom laser emitting at 1560.61nm initiates the experimental procedure. Polarization control and a polarizing beam splitter prepare the laser beam before it diverges into two paths. One path passes through an electro-optic modulator, altering the phase of light based on applied voltage, while the other undergoes amplification and enters two type-0 lithium niobate waveguides. Within these waveguides, second-harmonic generation followed by spontaneous parametric down-conversion generates time-correlated photon pairs. A dense wavelength-division multiplexer recombines these photon pairs with the original pump laser, directing the combined signal via an optical circulator into a folded Franson interferometer. This interferometer, configured in a Michelson arrangement, comprises a balanced beam splitter, a 20m fibre coil serving as a phase-sensitive transducer, and two Faraday mirrors ensuring polarization compensation. The entire assembly resides within a thermally stabilised anechoic chamber, minimising external disturbances. At the interferometer outputs, cascaded DWDM separates photons for coincidence measurements using superconducting nanowire single-photon detectors coupled to a time-to-digital converter. Simultaneously, a balanced beam splitter directs light through 500MHz-bandwidth Bragg filters, allowing for both classical photodiode detection and controlled single-photon measurements via a variable optical attenuator. This dual detection scheme enables a direct comparison of single- and two-photon interference signals. The electro-optic modulator dynamically adjusts the phase, locking the interferometer at the mid-fringe point for two-photon interference, as static phase adjustments are insufficient. The upper Bragg filter selects the carrier frequency, detected by a photodiode within a PID feedback loop, while the lower filter isolates the +1 modulation sideband, allowing independent control of the single-photon interference. This methodology ensures synchronous, common-mode acquisition of classical and quantum interference in the Fourier domain. Quantum correlations enhance signal-to-noise ratio via noise floor reduction in Fourier-domain interferometry Initial analysis of the interferometric outputs revealed that quantum correlations reduce the associated noise floor, yielding a 3 dB improvement in signal-to-noise ratio, than increasing the amplitude of the modulation peak. This enhancement was consistently observed even in the sub-shot-noise regime, where classical signals become indistinguishable from spectral background noise, while the two-photon contribution remained resolvable. These findings firmly establish Fourier-domain super-sensitivity as a practical resource for precision interferometric measurements. Spectral analysis demonstrated that the height of the spectral peak remains independent of the number of correlated photons used, while the research focused on the noise floor, which decreased as 1/N, where N represents the number of photons. Calculations showed the signal-to-noise ratio scales as SNR(N) = λA²m 8 N, indicating a direct N-fold improvement over single-photon measurements, where λ represents the mean emission rate and Am is the modulation amplitude. Numerical simulations using parameters of λ = 2 × 10⁶ photons/s and Am ≃ 6.3 × 10⁻² rad showed that all interference fringes maintained the same amplitude, while the absolute noise decreased with increasing N. The experimental implementation involved a fibre-based interferometer where both single-photon and two-photon interference signals were recorded simultaneously. By ensuring identical technical noise conditions, the study provided a rigorous benchmark for isolating the effect of quantum correlations. A continuous-wave telecom laser at λ = 1560.61nm was utilised, and the interferometer incorporated a 20m fibre-coil transducer to introduce a measurable phase shift. Precision sensing via Fourier domain interference cancellation The pursuit of signals buried within noise has become a defining challenge of modern sensing. Recent work demonstrates a method of enhancing precision measurements by exploiting the properties of light in the Fourier domain, achieving improvements in signal-to-noise ratio without increasing signal amplitude. This approach actively diminishes surrounding interference, a distinction that carries considerable weight. For years, scientists have sought ways to extract information from increasingly faint signals, but this isn’t merely a technical refinement; it represents a shift in how we approach the problem of detection. By focusing on noise reduction rather than signal boosting, researchers circumvent some of the fundamental constraints that plague traditional methods. Unlike earlier attempts at super-resolution, this technique operates effectively even when the classical signal is almost entirely obscured, opening possibilities for sensing in exceptionally noisy environments. Applications ranging from gravitational wave detection to biomedical imaging could benefit from this more subtle approach to signal recovery. Translating this laboratory success into practical devices will not be straightforward. While the demonstrated enhancement is clear, maintaining these advantages outside of a carefully controlled fibre optic setup presents a significant hurdle. The complexity of implementing Fourier-domain techniques may limit its widespread adoption, particularly in fields where simpler methods already exist. The technique relies on specific optical configurations, and scaling it to more complex systems or different wavelengths requires further investigation. However, the principles are broadly applicable to any interferometric measurement, and we can anticipate efforts to adapt this noise-reduction strategy to diverse sensing modalities. In distributed acoustic sensing, this could lead to more sensitive monitoring of underground structures or pipelines. The future of precision measurement may lie in listening more carefully to the quietest whispers. 👉 More information 🗞 Quantum-enhanced sensing via spectral noise reduction 🧠 ArXiv: https://arxiv.org/abs/2602.16350 Tags: