This PDF file contains the front matter associated with SPIE Proceedings Volume 10659, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

This paper reports on the state of the art of the Quanta Image Sensor (QIS) being developed by Dartmouth. The QIS is a photon-counting image sensor. Experimental 1Mpixel devices have been implemented in a modified backside-illuminated stacked CMOS image sensor process. Without the use of avalanche multiplicative gain, the sensors have achieved room temperature average read noise of 0.22e- rms (analog readout) permitting photon counting, and over 1000fps readout at under 20mW total power dissipation including pads (single-bit digital readout).

The human eye contains millions of rod photoreceptor cells, and each one is a single-photon detector. Whether people can actually see a single photon|which requires the rod signal to propagate through the rest of the noisy visual system and be perceived in the brain|has been the subject of research for nearly 100 years. Early experiments hinted that people could see just a few photons, but classical light sources are poor tools for answering these questions. Single-photon sources have opened up a new area of vision research, providing the best evidence yet that humans can indeed see single photons, and could even be used to test quantum effects through the visual system. We discuss our program to study the lower limits of human vision with a heralded single-photon source based on spontaneous parametric downconversion, and present two proposed experiments to explore quantum effects through the visual system: testing the perception of superposition states, and using a human observer as a detector in a Bell test.

This paper will describe recent developments in the state-of-the art for InP/InGaAs Geiger Mode focal plane arrays developed at MIT Lincoln Laboratory. Fabrication details of highly-dense arrays on a 25-micron pitch (256 x 256) will be presented, along with techniques developed to suppress crosstalk in neighboring pixels. These dense arrays are hybrized to highly efficient read-out circuits capable of simultaneous photon-counting imaging and photon time extraction for multiple user-defined regions of interest. Matching 256 x 256 microlens arrays are attached to the hybrized APD array/ROIC. Performance data and applications of the focal plane arrays will be discussed

Time-resolved imaging by means of Single Photon Avalanche Diodes (SPADs) has been subject to a widespread interest in recent years, especially since technological breakthroughs have opened the way to the development of multichannel Time Correlated Single Photon Counting (TCSPC) acquisition systems. Nevertheless, a main drawback of TCSPC has to be taken into account: it is an intrinsically slow technique because it requires the collection of a statistically-significant number of events to build a histogram that accurately reconstructs the time-domain waveform. As a result, the acquisition time needed for imaging can be relatively long. Two main solutions can be adopted to push the speed of a TCSPC measurement: the increment of the acquisition rate of the single channel and the exploitation of a high number of channels operating in parallel. The actual implementation of these solutions requires complex highperformance electronics designed on purpose. In this paper we report and discuss fully-integrated solutions for the development of a high-throughput and high-performance TCSPC acquisition system.

In recent years we assisted to a marked trend toward the development of large Single Photon Avalanche Diode arrays for Time-Correlated Single Photon Counting (TCSPC) measurements. Indeed, multichannel systems feature a higher counting capability compared to single-channel systems, thus permitting a speedup of TCSPC experiments. Above all, the exploitation of CMOS technology has paved the way to the integration of both detectors and electronics on the same chip, leading to systems with thousands of independent channels, but resulting in a trade-off with the performance. Even worse, the integration of a high number of channels has not led to a proportional increment of the measurement speed, thus limiting the advantages of a multichannel approach. In order to break the trade-off that currently affects TCSPC imagers, we propose a router-based architecture, which allows us to choose the best-suited technology for the design of different parts of the system, that is detectors, sensing electronics and time-measurement circuits. The system is based on a limited number of high-performance converters shared with a much larger detector array. During each excitation cycle, a smart routing logic selects a subset of the detectors carrying a valid signal and connects them to the external converters. Here we present a new routing algorithm exploiting digital gates distributed in a tree structure within a large 32x32 array. Our solution has a double advantage: it permits to maximize the measurement speed and to minimize the number of interconnections crossing the system, which is a major issue in dense multichannel arrays.

We present the design and preliminary characterization of a 32 × 32 SPAD time-gated imager in a 0.16 μm BCD technology featuring an innovative pixel structure, composed of four single-photon detectors, independent event counters and a shared Time-to-Digital Converter (TDC). This approach allows our design to reach higher fill factor (9.6 % with a pixel pitch of 100 μm) than typical SPAD imagers with in-pixel TDC, as well as reducing the power dissipation of the chip itself. The imager targets primarily LIDAR applications, but can also be employed in scientific and biomedical applications, and also features a photon-coincidence operation mode intended for rejection of background light. The developed imager performs simultaneous photon-counting and photon-timing operation and implements a 12 bit, 75 ps LSB flash-type TDC, able to perform one conversion per each gate window and up to 62 gates per acquisition frame (100 kfps maximum frame rate). The TDC structure is finely tunable, allowing to compensate for process variations and mismatches. Different readout modes allow to fit the requirements of different applications, trading-off the amount of provided data (e.g., only counting, only timing, both timing and counting) and the achievable frame rate. The readout scheme allows for easy tiling into large imagers and avoids the readout speed bottleneck typically found in standard row select / column select approaches.

The Time Correlated Single Photon Counting (TCSPC) technique represents a key tool in many fields where picosecond precision is required to record fast and faint luminous signals. Unfortunately, TCSPC experiments involve a relatively long acquisition time to accumulate a statistically relevant number of events. Indeed, the maximum operating rate of a single acquisition channel is usually kept below 5% of the excitation frequency in order to guarantee an acceptable pileup distortion. In this work, we propose a novel technique to speed up TCSPC experiments by almost an order of magnitude, avoiding at the same time any distortion of the recorded curve. First of all, we demonstrate that zero-distortion operation is feasible if an extremely fast conversion electronics is used and the dead time of the detector is matched to the excitation period. In this scenario, the excitation power can be increased well above the pile-up limit, thus enabling unprecedented measurement speed. It is worth highlighting that our technique can be easily extended to a multichannel approach to further speed up the measurement. We also discuss that a practical use of our solution is already feasible exploiting recently-proposed time-measurement circuits with negligible dead time and a Single Photon Avalanche Diode coupled to a fast Active Quenching Circuit, featuring a short and finely tunable dead time. We provide a deep theoretical analysis of the technique to show its potential; in addition, the main issues related to nonidealities of a practical implementation have been widely investigated by means of numerical simulations.

We review recent progress on GaSb-based AlInAsSb avalanche photodetectors (APD), including preliminary room temperature Geiger mode single photon counting and gains >1000x. AlInAsSb APDs grown digitally [1] on GaSb have recently shown tunable cutoff wavelengths across the telecommunications band, and noise comparable to that of silicon, independent of alloy composition [2]. We review this progress and describe initial Geiger mode single photon counting success at room temperature. Geiger performance of an Al0.7In0.3As0.3Sb0.7 APD was characterized with a capacitance-balanced gated quenching circuit in which an avalanche event was quenched by the trailing edge of the gate pulse. The width of driving gate was 5 ns, and repetition rate was 1 MHz. The excess bias was varied from 0.3% to 1% of breakdown voltage. Both photon-induced breakdown probability and dark count probability increase with excess bias until photon-induced breakdown probability saturates at 55%, which is due to high dark count rate. The dark count probability per gate was as high as 60%, which is consistent with the 0.17-nA primary dark current. [1] Maddox, March, Bank, ACS Crystal Growth & Design, vol. 16, no. 7, pp. 3582–3586, June 2016. [2] Woodson, Ren, Maddox, Chen, Bank, Campbell, Appl. Phys. Lett., vol. 108, no. 8, pp. 081102, Feb. 2016.

A linear mode photon counting HgCdTe avalanche photodiode (APD) focal plane array (FPA) detector was developed for space lidar applications. An integrated detector cooler assembly (IDCA) was manufactured using a miniature Stirling cooler. The HgCdTe APD demonstrated a greater than 60% photon detection efficiency from 0.9 to 4.3 μm wavelength and a dark count rate less than 250,000/s. The IDCA cooled the FPA to 110K from ambient room temperature at a total electrical power of 7 W. The IDCA has passed environmental tests, including vibration, thermal cycling and thermal vacuum tests.

CMOS SPADs are nowadays an established imaging technology for applications requiring single-photon sensitivity in a compact form-factor (e.g. three-dimensional LIDAR imaging and fluorescence lifetime FLIM microscopy). However, we aimed at further enhance overall SPAD performances, by exploiting smart power technologies, such as the BCD (Bipolar-CMOS-DMOS) one. We achieved the present state-of-the-art SPADs fabricated in the 0.16 μm BCD technology by STMicroelectronics, attaining >60% photon detection efficiency at 500 nm, dark count rate density < 0.2 cps/μm², and less than 30 ps FWHM timing jitter.

Superconducting single photon detectors consist of a narrow and thin wire of superconductor and can detect single photons with high efficiency and high speed. Simultaneous optimization of the detection efficiency and detector response time is difficult because the response time is set by the dimensions of the wire (cross section and length) while the detection efficiency is determined by both the internal detection mechanism and the dimensions of the wire. Wider and shorter wires are easier to fabricate, are more robust and lead to shorter detector reset time, but are generally less efficient. Experiments employing detector tomography provide important insights into the photon detection mechanism and indicate a detection mechanism where the edges of the wire are more efficient. This leads to a position dependent local detection efficiency that can be explained in the context of a photon-assisted-vortex-entry model and predicts an optimum wire width ~70 nm. A 50 nm wide silicon nanowire deposited on top of a 150 nm wide NbN nanowire directs light at 1550 nm wavelength to the edges and improves both the total absorption efficiency and the internal detection efficiency of the wire. The total absorption efficiency can be enhanced by 30% while the internal detection efficiency is increased by 70%. Assuming that the wire covers a similar area the detector response time is reduced 4-fold compared to the standard design using a 70 nm wide wire.

Superconducting nanowire single photon detectors (SNSPD) offer excellent performance for infrared single photon detection, combining high efficiency, low timing jitter, low dark count rates and high photon counting rates. Promising application areas for SNSPDs include quantum key distribution, space-to-ground communications and single photon remote sensing [1]. SNSPDs are typically made with ultrathin niobium nitride (NbN) films with thickness 4 nm and a superconducting transition temperature above 9 K. NbN offers high performance in the near infrared but their sensitivity drops at wavelengths beyond 2 um. There is growing interest in potential photon counting applications in the mid infrared domain (for example remote sensing of greenhouse gases in the atmosphere [2]). One way to overcome the wavelength limit in NbN SNSPDs is to use films with a lower superconducting energy gap [3]. Here we report on the study of SNSPDs fabricated with thin films of titanium nitride (TiN). We compare TiN films deposited by atomic layer deposition (ALD) and by magnetron sputtering. The TiN films range in thickness from 5 to 60 nm, with superconducting transition temperatures from ~1 K to 3.5 K. We have analyzed the films via transmission electron microscopy and variable angle spectroscopic ellipsometry. We characterize TiN SNSPDs performance from near to mid-infrared at wavelengths (1-4 um) with fast optical parametric oscillator (OPO) source. We compare the performance of TiN SNSPDs to devices based on other lower gap materials: MoSi, NbTiN, WSi. [1] Natarajan et al Superconductor Science and Technology 25 063001 (2012) [2] Abshire et al Laser Applications to Chemical, Security and Environmental Analysis, (Optical Society of America, 2008) paper LMA4 [3] Verma et al Applied Physics Letters 105 022602 (2014)

Superconducting nanowire single photon detectors (SNSPDs) have emerged as a leading choice for high performance single photon detectors due to their low timing jitter, high detection efficiency, and low dark count rates. SNSPDs have typically been biased using a passive quenching scheme in which the bias current of the device is shunted through a resistive load to allow for recovery after a detection event. To prevent latching, the shunting resistor must be approximately an order of magnitude smaller than the peak normal domain resistance of the SNSPD. Consequentially, the pulse amplitude (∝IBRL) and recovery time (∝LK/RL) are both negatively impacted. In this talk, we will describe a novel approach to the bias and read-out of SNSPDs based upon active quenching. We will present detailed design considerations for an active quenching architecture and will show that such an approach has the potential to improve count rates while increasing signal swings to the point where external amplification is no longer required. A silicon germanium (SiGe) active-reset chip design has been designed, implemented, and integrated with a NbTiN SNSPD. The procedure for the SiGe chip design will be described and simulation results will be presented. Finally, detailed measurement results of the complete system will be shown and compared to measurements of the same detector when biased and read-out using a standard passive quenching scheme. It will be shown that the active quenching configuration enables a considerable enhancement to the system performance.

Photon heterodyning affords the opportunity to single out photon detection events at a designated timing in the otherwise elusive transient interference fringes of single photons. Albeit essentially a time-domain analogue of Young's two-slit interference experiment at a single-photon level, it must be borne out by experiment using true single-photon streams. More importantly, it presents a technical challenge even without a tangible screen unlike space domain. Here, we demonstrate single-photon self-heterodyne beats that are obtained by letting the nonclassical light field associated with a photon interfere with itself. This is made possible by splitting, folding and mapping point-by-point the otherwise only poorly correlated time-series data onto a virtual capture frame. Photon heterodyning is of significance in view of conceivable applications. In fact, it marks a great step forward in benefiting such applications that handle interfering light fields in the time domain. An unmatched immunity to dc noise at an extremely low light level opens up the avenue for single-photon interferometry and interferometric biometrics of a highly light-sensitive target. Super-resolution and quantum optical coherence tomography, which are based on the nonclassical nature of photons, are those which should take full advantage of the photon heterodyning technology. On the other hand, the potential of photon heterodyning is most likely to be manifest in the field of quantum information processing by implementing frequency-encoded photon states. Some of these examples will be discussed in more detail.

As optical quantum information processing protocols and experiments become increasingly more complex, integrated optics provide a small and robust alternative to traditional bulk optics. Specifically, waveguide technology allows for the creation of bright single-photon sources based on the fact that photon pairs can be created at any location along the waveguide. For our goals, we are working on the characterization of a highly nondegenerate Spontaneous Parametric Down-Conversion (SPDC) waveguide source on a periodically poled KTP (PPKTP) crystal. Our current waveguide source uses type-II phase-matching to create collinear signal and idler photons at 1550 nm and 810 nm, respectively, with the promise of generating simultaneous time-bin and polarization entanglement in future iterations. Our intended source application is for use in quantum key distribution and superdense teleportation protocols between a space platform and collection telescopes on Earth.

Coherent light traversing disordered media usually attains a random field both in amplitude and phase with independent Gaussian statistics and results in thermal light (associated with Bose-Einstein photon-number statistics) upon ensemble averaging. This is expected according to the central limit theorem, which dictates the addition of a large number of independent random variables leads to a normal (Gaussian) distribution. Here, we show that certain network topologies that light travels within preclude the central limit theorem and result in non-Gaussian statistics. We realize such networks in the form of evanescently-coupled waveguide arrays (photonic lattices) and obtain the photon statistics at the output by time gating and averaging over multiple realizations of disordered photonic lattices. The effect of lattice topology, however, only exists when the photonic lattice is endowed with chiral-symmetric eigenmode pairs a disorder-immune symmetry where the eigenmodes appear in pairs with oppositely signed eigenvalues and the coherent input field satisfies certain conditions. We specifically examine one-dimensional arrays of randomly coupled identical waveguides (off-diagonal disorder) arranged on linear and ring topologies. The emerging field exhibits super-thermal statistics (associated with modified Bose-Einstein photon-number statistics) only for ring lattices with even parity and linear lattices (independent of its parity), whereas input coherent fields traversing ring lattices with odd parity attain sub-thermal statistics. By controlling the relative phase of a coherent input field exciting two neighboring lattice sites, we also demonstrate a deterministic tuning of photon-number statistics, namely photon bunching, while maintaining the mean photon number fixed.

The Extremely Low-Resource Optical Identifier (ELROI) beacon is a concept for a milliwatt optical "license plate" that can provide unique ID numbers for everything that goes into space. Using photon counting to enable extreme background rejection in real time, the ID number can be uniquely identified from the ground in a few minutes, even if the ground station detects only a few photons per second. The ELROI concept has been validated in long-range ground tests, and orbital prototypes are scheduled for launch in 2018 and beyond. We discuss the design and signal characteristics of these prototypes, including a PC-104 form factor unit which was integrated into a CubeSat and is currently scheduled to launch in May 2018, and basic requirements on ground stations for observing them. We encourage others to consider observing our test flights.

A scanning depth imaging system is used for the investigation of three-dimensional image reconstruction and classification of targets in underwater environments. The system uses the Time-Correlated Single-Photon Counting (TCSPC) technique to measure single-photon time-of-flight. In this paper, we use both single and multiple wavelengths to interrogate underwater targets. This presentation will show laboratory measurements on several target scenarios, including targets in clutter. We demonstrate high resolution depth and intensity image reconstruction in highly scattering underwater scenarios, and show image reconstruction at up to nine attenuation lengths between transceiver and target. The system comprised a scanning transceiver unit, fiber coupled to a silicon single-photon avalanche diode (Si SPAD) and a supercontinuum laser system operating at the repetition rate of 19.5 MHz. An acousto-optic tunable filter (AOTF) is used to select an individual operational wavelength in the range 500 nm to 725 nm. The measurements used a range of system configurations, including both single wavelength and multiple wavelength measurements. Generally, the measurements used sub-milliwatt average optical power levels. Bespoke algorithms were developed to identify man-made objects hidden by marine vegetation in the scanned scene. Advanced statistical image processing methods were used to improve target discrimination and to reconstruct the target under different conditions, including reduced number of wavelengths and number of pixels, and reduced acquisition time. Particular attention will be given to the photon starved regime, which will be typical of data acquired at long distances in open ocean waters or in highly scattering environments.

We present a scanning depth imaging system which was used to investigate free-space imaging of targets through a variety of highly obscuring media. The system utilized the time-correlated single-photon counting (TCSPC) technique to obtain photon time-of-flight information. This was used to reconstruct high-resolution three-dimensional depth and intensity profiles of targets at stand-off distances of up to 24 meters in scenarios with very poor visibility. The results demonstrate the benefits of short-wave infrared (SWIR) wavelengths compared to visible band sensors for imaging through several forms of obscurants.

The system was configured for operation at a wavelength of 1550 nm and measurements were performed using a 26 meter long fog tunnel facility which was filled with obscurants of several different types and densities. The system was comprised of a custom-built scanning transceiver unit, fiber-coupled to a Peltier cooled InGaAs/InP single-photon avalanche diode (SPAD) detector. A picosecond pulsed laser was used to provide a fiber-coupled illumination wavelength of 1550 nm at an approximate average optical power level of just under 1.5 mW for all measurements.

Bespoke image processing algorithms were developed to reconstruct high resolution depth and intensity profiles of obscured targets in challenging environments with low visibilities. Such algorithms can allow for target reconstruction using low levels of optical power and shorter data acquisition times, thus enabling image acquisition in the sparse photon regime.