Abstract
Selective detection of signal over noise is essential to measurement and signal processing. Time-frequency filtering has been the standard approach for the optimal detection of non-stationary signals. However, there is a fundamental tradeoff between the signal detection efficiency and the amount of undesirable noise detected simultaneously, which restricts its uses under weak signal yet strong noise conditions. Here, we demonstrate quantum parametric mode sorting based on nonlinear optics at the edge of phase matching to improve the tradeoff. By tailoring the nonlinear process in a commercial lithium-niobate waveguide through optical arbitrary waveform generation, we demonstrate highly selective detection of picosecond signals overlapping temporally and spectrally but in orthogonal time-frequency modes as well as against broadband noise, with performance well exceeding the theoretical limit of the optimized time-frequency filtering. We also verify that our device does not introduce any significant quantum noise to the detected signal and demonstrate faithful detection of pico-second single photons. Together, these results point to unexplored opportunities in measurement and signal processing under challenging conditions, such as photon-starving quantum applications.
Highlights
To illustrate this, we consider a typical TF filter consisting of a temporal shutter with a Gaussian profile followed by a Gaussian-shape bandpass filter, as shown in Fig. 1(a), whose full-width half-maximum (FWHM) are T and B, respectively
While previous research has found its promises in quantum information applications using high-dimensional photons, here we demonstrate for the first time its potential for overcoming the SNR boundary with the TF filtering where the noise is randomly distributed in widespread, unknown modes
The present quantum parametric mode sorting” (QPMS) is realized through nonlinear mixing of a pump wave, a signal wave, and their sum frequency (SF) in a χ(2) waveguide, whose dynamics is described by a set of coupled Heisenberg equations of motion[6, 19]
Summary
We consider a typical TF filter consisting of a temporal shutter with a Gaussian profile followed by a Gaussian-shape bandpass filter, as shown in Fig. 1(a), whose full-width half-maximum (FWHM) are T and B, respectively. An example is the quantum key distribution in free space, where the receiving of single signal photons can be hampered by strong atmospheric scattering and background photons spanning the same spectrum and arrival time[7,8,9]. We demonstrate an approach based on nonlinear wave mixing for improving the tradeoff inherent with the TF filters, thereby enabling faithful measurement and processing of weak signals even in the presence of strong interfering noise (e.g., those overlapping in both frequency and time). We consider the parametric frequency conversion in nonlinear optical media that preserves the signals’ quantum states[13,14,15,16], including their entanglement with other parties We call it “quantum parametric mode sorting” (QPMS). Thanks to coherent nonlinear effects, a significant advantage can be established over the TF filtering approach
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