Detection signal to noise ratio lies at the heart of any remote sensing task. Matched filters have long been deployed as the optimal devices for radio and microwave receiving. However, their implements in the optics domain are challenging due to a much higher bandwidth and the intensity-only measurement by incoherent detectors. For applications involving high noise and low signal, their utilities are overly restricted by a fundamental trade-off between the signal detection efficiency and noise rejection. In this talk, I will describe a nascent approach, quantum parametric mode sorting, to addressing these practical and fundamental challenges. It is realized via quantum frequency conversion at the phase matching edge, where the signal distillation, picosecond timing tagging, and wavelength transduction are accomplished during a single pass through a nonlinear waveguide or crystal. I will report our experimental progress on using it a variety of single photon sensing and imaging tasks.
Detection signal to noise ratio lies at the heart of any remote sensing task. Matched filters have long been deployed as the optimal devices for radio and microwave receiving. However, their implements in the optics domain are challenging due to a much higher bandwidth and the intensity-only measurement by incoherent detectors. For applications involving high noise and low signal, their utilities are overly restricted by a fundamental trade-off between the signal detection efficiency and noise rejection. In this talk, I will describe a nascent approach, quantum parametric mode sorting, to addressing these practical and fundamental challenges. It is realized via quantum frequency conversion at the phase matching edge, where the signal distillation, picosecond timing tagging, and wavelength transduction are accomplished during a single pass through a nonlinear waveguide or crystal. I will report our experimental progress on using it a variety of single photon sensing and imaging tasks.
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.
Quantum parametric mode sorting has been shown to enable photon counting with precise time gating and exceptional noise rejection that significantly exceeds what is possible with linear filters. While previous experimental demonstrations were in a collinear optical configuration, its response to off-axis scattering must be understood to apply it more broadly in remote sensing missions. To evaluate this prospect, we use a laboratory testbed to examine its performance for detecting photons at small angles, along both forward and backward directions, after passing through strongly scattering media. Our results find no measurable degradation in detecting noncollinear photons along both directions. This finding indicates that the key intra-pulse coherence essential to quantum parametric mode sorting is maintained at a small scattering angle, permitting its applications on a moving platform.
Active imagers capable of reconstructing 3-dimensional (3D) scenes in the presence of strong background noise are highly desirable for many sensing and imaging applications. A key to this capability is the time-resolving photon detection that distinguishes true signal photons from the noise. To this end, quantum parametric mode sorting (QPMS) can achieve signal to noise exceeding by far what is possible with typical linear optics filters, with outstanding performance in isolating temporally and spectrally overlapping noise. Here, we report a QPMS-based 3D imager with exceptional detection sensitivity and noise tolerance. With only 0.0006 detected signal photons per pulse, we reliably reconstruct the 3D profile of an obscured scene, despite 34-fold spectral-temporally overlapping noise photons, within the 6 ps detection window (amounting to 113,000 times noise per 20 ns detection period). Our results highlight a viable approach to suppress background noise and measurement errors of single photon imager operation in high-noise environments.
Time-frequency (TF) filtering of analog signals has played a crucial role in the development of radio-frequency communications and is currently being recognized as an essential capability for communications, both classical and quantum, in the optical frequency domain. How best to design optical time-frequency (TF) filters to pass a targeted temporal mode (TM), and to reject background (noise) photons in the TF detection window? The solution for 'coherent' TF filtering is known-the quantum pulse gate-whereas the conventional, more common method is implemented by a sequence of incoherent spectral filtering and temporal gating operations. To compare these two methods, we derive a general formalism for two-stage incoherent time-frequency filtering, finding expressions for signal pulse transmission efficiency, and for the ability to discriminate TMs, which allows the blocking of unwanted background light. We derive the tradeoff between efficiency and TM discrimination ability, and find a remarkably concise relation between these two quantities and the time-bandwidth product of the combined filters. We apply the formalism to two examples-rectangular filters or Gaussian filters-both of which have known orthogonal-function decompositions. The formalism can be applied to any state of light occupying the input temporal mode, e.g., 'classical' coherent-state signals or pulsed single-photon states of light. In contrast to the radio-frequency domain, where coherent detection is standard and one can use coherent matched filtering to reject noise, in the optical domain direct detection is optimal in a number of scenarios where the signal flux is extremely small. Our analysis shows how the insertion loss and SNR change when one uses incoherent optical filters to reject background noise, followed by direct detection, e.g. photon counting. We point out implications in classical and quantum optical communications. As an example, we study quantum key distribution, wherein strong rejection of background noise is necessary to maintain a high quality of entanglement, while high signal transmission is needed to ensure a useful key generation rate.
Spatial-mode-selective frequency conversion is potentially useful for both classical and quantum communication applications. By a judicious choice of the quasi-phase-matching period in a Kai(2) multimode waveguide, such conversion can be achieved with high efficiency (close to 100%) and with low crosstalk (< -20 dB). For space-division multiplexing application with classical signals, where each spatial mode represents a separate signal channel, the selective conversion of a spatial mode without disrupting other signal modes can be used for reconfigurable spatial-mode de-multiplexing. This classical de-multiplexing capability can be also extended to the quantum regime, where the quantum state of the signal is preserved during frequency conversion, owing to the unitary nature of the sum-frequency generation (SFG) process. Building upon our previous experimental demonstration of the classical spatial-mode-selective frequency up-conversion in a two-mode PPLN waveguide, here we report the extension of this work into the single-photon-level regime. The signal (1540 nm) in either a single mode (TM00 or TM01) or a superimposition mode (TM00+TM01, TM00+iTM01) of the waveguide is selectively up-converted into TM01 SFG mode, by interacting with an appropriate pump mode (1560 nm). An accurate measurement of the single-photon-level SFG signals requires thorough filtering of the unwanted photons contributed by the second harmonic of the pump, residual pump noise extending to the signal band, and the Raman noise generated in the waveguide. We have investigated these unwanted photon sources and suppressed them by a combination of thin-film-interference and volume-Bragg-grating filters. Resulting single-photon-counting measurements show >70% internal conversion efficiency, better than -12dB crosstalk, and >100 ratio of the signal to background photon counts for all selected modes and mode superpositions.
High resolution three dimensional (3D) optical imaging in the turbid underwater scenarios over extended length remains an outstanding challenge, primarily impeded by the absorption and scattering in turbid water, which result in substantial signal attenuation over short propagation distances. Overcoming water absorption by using optimum illumination wavelengths (480- 600 nm) of the visible spectrum, however, still requires one to address the strong scattering effects. To address the above challenge, we introduce a novel 3D imaging modality based on quantum parametric mode sorting (QPMS). It is a nascent quantum measurement technique that utilizes mode-selective quantum frequency conversion (QFC) in a χ2 nonlinear waveguide to up convert signal photons in a single spatiotemporal mode efficiently. Undesirable photons in other modes, even if they spectrally and temporally overlap with the signal, are converted with much less efficiency. This unique feature, combining with picosecond time gated detection as defined by the pump pulse width, can isolate signal photon backscattered by the target from multiscattered photons by the obstacle. It thus enables imaging through a strongly scattering medium, where the background photons are orders of magnitude stronger. With QPMS, we demonstrate 3D imaging of a target occluded by strongly scattering turbid media with optical depth < 9 (<18 round trip), while needing only 105 detected photons/pulse to achieve sub-millimeters resolution. This makes our single photon sensitive 3D imager suitable for imaging and remote sensing applications in photon-starved natural water environments where it's high sensitivity and excellent temporal resolution can be exploited to its full extent.