Quantum-enhanced sensing via spectral noise reduction

Abstract

We report a direct demonstration of quantum-enhanced sensing in the Fourier domain by comparing single- and two-photon interference in a fiber-based interferometer under strictly identical noise conditions. The simultaneous acquisition of both signals provides a common-mode reference that enables a fair and unambiguous benchmark of quantum advantage. Spectral analysis of the interferometric outputs reveals that quantum correlations do not increase the amplitude of the modulation peak, but instead lower the associated noise floor, resulting in the expected 3 dB improvement in signal-to-noise ratio. This enhancement persists in the sub-shot-noise regime, where the classical signal becomes buried in the spectral background while the two-photon contribution remains resolvable. These observations establish Fourier-domain quantum super-sensitivity as an operational and broadly applicable resource for precision interferometric sensing.

Figures (4)

• Figure 1: (a) Mach-Zehnder-like folded Franson interferometer. The quantum probe, prepared in a N00N state, is coherently delocalized over the two arms, with the relative phase encoded in the upper arm. BS: beam splitter. (b) Numerical simulations of Eq. (\ref{['eq: probability N-coincidence']}) for $N=1,2$, and $4$ photons (blue, red, and green curves, respectively), using $\lambda = 2\times10^{6}$ photons/s, $f_{0}=1$ kHz, $f_{m}=20$ Hz, and $A_{m}\simeq6.3\times10^{-2}$ rad. All interference fringes have the same amplitude, while the absolute noise associated with Poissonian statistics decreases with increasing $N$. (c) Corresponding power spectral densities computed from the temporal signals in (b). The spectral peak has the same amplitude (about $30$ dB/Hz) for all $N$, whereas the noise floor scales inversely with $N$.
• Figure 2: Experimental setup for demonstrating quantum-enhanced sensing in the frequency domain. A phase-modulated laser field and photon pairs are injected into a folded Franson interferometer incorporating a 20 m fiber-coil transducer. The interferometer outputs are analyzed using single-photon detection and two-photon coincidence measurements. EOM: electro-optic modulator; VOA: variable optical attenuator; BS: beam splitter; SNSPD: superconducting nanowire single-photon detector; TDC: time-to-digital converter; DWDM: wavelength demultiplexer with rejection (REJ), passband (PASS), and common (COM) ports.
• Figure 3: Power spectral densities (PSDs) of single- and two-photon interference at 440 Hz. (a) Example at 88 % sound volume showing identical spectral peaks but different noise floors. (b) Mean PSD values versus sound volume. Lower curves: noise floors separated by 3 dB. Upper curves: peak amplitudes at 440 Hz, identical for both probes. The errors bars are calculated from the standard deviation of the 100 computed PSD per point. The green trace shows the measured SNR difference, consistent with the predicted 3 dB gain.
• Figure 4: Two-photon performance in the low-signal regime. (a) PSD at 22 % sound volume, where the single-photon peak is lost in the noise. (b) Averaged results demonstrating sub-shot-noise sensitivity with two-photon probes, confirming quantum super-sensitivity in the Fourier domain. The errors bars are calculated from the standard deviation of the 1000 computed PSD per point. SN: shot-noise.