Few-photon imaging at 1550 nm using a low-timing-jitter superconducting nanowire single-photon detector

Nanowires: Exponential speedup in quantum computing

This review paper examines the crucial role of nanowires in the field of quantum computing, highlighting their importance as versatile platforms for qubits and vital building blocks for creating fault-tolerant and scalable quantum information processing systems. Researchers are studying many categories of nanowires, including semiconductor, superconducting, and topological nanowires, to explore their distinct quantum features that play a role in creating various qubit designs. The paper explores the interdisciplinary character of quantum computing, combining the fields of quantum physics and materials science. This text highlights the significance of quantum gate operations in manipulating qubits for computation, thus creating the toolbox of quantum algorithms. The paper emphasizes the key research areas in quantum technology, such as entanglement engineering, quantum error correction, and a wide range of applications spanning from encryption to climate change modeling. The research highlights the importance of tackling difficulties related to decoding mitigation, error correction, and hardware scalability to fully utilize the transformative potential of quantum computing in scientific, technical, and computational fields.

Long range 3D imaging through atmospheric obscurants using array-based single-photon LiDAR

Single-photon light detection and ranging (LiDAR) has emerged as a strong candidate technology for active imaging applications. In particular, the single-photon sensitivity and picosecond timing resolution permits high-precision three-dimensional (3D) imaging capability through atmospheric obscurants including fog, haze and smoke. Here we demonstrate an array-based single-photon LiDAR system, which is capable of performing 3D imaging in atmospheric obscurant over long ranges. By adopting the optical optimization of system and the photon-efficient imaging algorithm, we acquire depth and intensity images through dense fog equivalent to 2.74 attenuation lengths at distances of 13.4 km and 20.0 km. Furthermore, we demonstrate real-time 3D imaging for moving targets at 20 frames per second in mist weather conditions over 10.5 km. The results indicate great potential for practical applications of vehicle navigation and target recognition in challenging weather.

Advancement on target ranging and tracking by single-point photon counting lidar

Laser tracking with a cooperative target has been widely used in many fields and becomes increasingly important while the non-cooperative target tracking is still a challenge. In this article, a pure laser scanning, ranging and tracking system based on a single-point single photon detector (SP-SPD) is proposed, which can achieve a non-cooperative target real-time tracking without any other passive detection sensor. Through laboratory tracking experiment, we realized the real-time angular measurement, ranging and tracking of a small unmanned aerial vehicle (UAV) at a distance of about 38 m. The results show that the system and its tracking strategy have the ability to achieve a non-cooperative target real-time ranging and tracking in conditions of weak echo signals (a few tenths of a photoelectron), which means that the pure lidar tracking of the non-cooperative target in far distance become reality. It has important guiding significance and application value for a non-cooperative long-distance target ranging and tracking in the airspace.

Frequency-modulated continuous-wave 3D imaging with high photon efficiency

Frequency-modulated continuous-wave (FMCW) light detection and ranging (LIDAR), which offers high depth resolution and immunity to environmental disturbances, has emerged as a strong candidate technology for active imaging applications. In general, hundreds of photons per pixel are required for accurate three-dimensional (3D) imaging. When it comes to the low-flux regime, however, depth estimation has limited robustness. To cope with this, we propose and demonstrate a photon-efficient approach for FMCW LIDAR. We first construct a FMCW LIDAR setup based on single-photon detectors where only a weak local oscillator is needed for the coherent detection. Further, to realize photon-efficient imaging, our approach borrows the data from neighboring pixels to enhance depth estimates, and employs a total-variation seminorm to smooth out the noise on the recovered depth map. Both simulation and experiment results show that our approach can produce high-quality 3D images from ∼10 signal photons per pixel, increasing the photon efficiency by 10-fold over the traditional processing method. The high photon efficiency will be valuable for low-power and rapid FMCW applications.

Single photon imaging with multi-scale time resolution

To avoid echo photons to be submerged in noise in rough terrain or dynamic applications, a single photon imaging mechanism with multi-scale time resolution is proposed in this paper. Combining with adaptively thresholding technique, multiple histograms with different time resolutions are produced to cluster the echo photons into a time bin and then separate them from the noise. With microsecond-scale resolution, uncertainty in the position of an object can be reduced from several kilometers to 300 meters, and therefore the computational overheads are saved by only investigating depths with picosecond-scale resolution where an object is present. Reconstructed results of the two near surfaces show that the depth accuracy is less than 0.15 m in the conditions of 8 echo photons and 1 Mcps background count rate, even though the pulse width of laser source reaches 3.5 ns (equivalent to an uncertainty of 0.525 m). In addition, the echo can be distinguished from the noise clearly when the background count rate varies from 200 kcps to 1 Mcps. The proposed method is suitable for implementation in digital signal processor (DSP) due to low data volumes and computational overheads.

Single photon imaging based on a photon driven sparse sampling

Single photon three-dimensional (3D) imager can capture 3D profile details and see through obscuring objects with high sensitivity, making it promising in sensing and imaging applications. The key capabilities of such 3D imager lie on its depth resolution and multi-return discrimination. For conventional pulsed single photon lidar, these capabilities are limited by transmitter bandwidth and receiver bandwidth simultaneously. A single photon imager is proposed and experimentally demonstrated to implement time-resolved and multi-return imaging. Time-to-frequency conversion is performed to achieve millimetric depth resolution. Experimental results show that the depth resolution is better than 4.5 mm, even though time jitter of the SPAD reaches 1 ns and time resolution of the TCSPC module reaches 10 ns. Furthermore, photon driven sparse sampling mechanism allows us to discriminate multiple near surfaces, no longer limited by the receiver bandwidth. The simplicity of the system hardware enables low-cost and compact 3D imaging.

Implementation of field two-way quantum synchronization of distant clocks across a 7 km deployed fiber link

The two-way quantum clock synchronization has been shown to provide femtosecond-level synchronization capability and security against symmetric delay attacks, thus becoming a prospective method to compare and synchronize distant clocks with enhanced precision and safety. In this letter, a field test of two-way quantum synchronization between a H-maser and a Rb clock linked by a 7 km-long deployed fiber is implemented by using time-energy entangled photon-pair sources. Limited by the intrinsic frequency stability of the Rb clock, the achieved time stability at 30 s is measured as 32 ps. By applying a fiber-optic microwave frequency transfer technology to build frequency syntonization between the separated clocks, the limit set by the intrinsic frequency stability of the Rb clock is overcome. A significantly improved time stability of 1.9 ps at 30 s is achieved, which is mainly restrained by the low number of acquired photon pairs due to the low sampling rate of the utilized coincidence measurement system. Such implementation demonstrates the high practicability of the two-way quantum clock synchronization method for promoting field applications.

Single-Pixel Photon-Counting Imaging Based on Dual-Comb Interferometry

We propose and experimentally demonstrate single-pixel photon counting imaging based on dual-comb interferometry at 1550 nm. Different from traditional dual-comb imaging, this approach enables imaging at the photon-counting regime by using single-photon detectors combined with a time-correlated single-photon counter to record the returning photons. The illumination power is as low as 14 pW, corresponding to 2.2 × 10^-3 photons/pulse. The lateral resolution is about 50 μm. This technique paves the way for applying dual-comb in remote sensing and imaging.

Polarization resolving and imaging with a single-photon sensitive superconducting nanowire array

Superconducting nanowire single-photon detectors (SNSPDs) have attracted remarkable interest for visible and near-infrared single-photon detection due to their outstanding performance. However, conventional SNSPDs are generally used as binary photon-counting detectors. Another important characteristic of light, i.e., polarization, which can provide additional information of the object, has not been resolved using the standalone SNSPD. In this work, we present a first prototype of the polarimeter based on a four-pixel superconducting nanowire array, capable of resolving the polarization state of linearly-polarized light at the single-photon level. The detector array design is based on a division of focal plane configuration in which the orientation of each nanowire division (pixel) is offset by 45°. Each single nanowire pixel operates as a combination of a photon detector and almost linear polarization filter, with an average polarization extinction ratio of ∼10. The total system detection efficiency of the array is ∼1% at a total dark count rate of 680 cps, with a timing jitter of 126 ps, when the detector array is free-space coupled and illuminated with 1550-nm photons. The mean errors of the measured angle of polarization and degree of linear polarization were about -3° and 0.12, respectively. Furthermore, we successfully demonstrated polarization imaging at low-light level using the proposed detector. Our results pave the way for the development of a single-photon sensitive, fast, and large-scale integrated polarization polarimeter or imager. Such detector may find promising application in photon-starved polarization resolving and imaging with high spatial and temporal resolution.

High-precision nonlocal temporal correlation identification of entangled photon pairs for quantum clock synchronization

High-precision nonlocal temporal correlation identification in entangled photon pairs is critical to measure the time offset between remote independent time scales for many quantum information applications. The first nonlocal correlation identification was reported in 2009, which extracts the time offset via the algorithm of iterative fast Fourier transformations and their inverse. The best identification resolution is restricted by the peak identification threshold of the algorithm, and thus the time offset calculation precision is limited. In this paper, an improvement for the identification is presented both in resolution and precision via a modified algorithm of direct cross correlation extraction. A flexible resolution down to 1 ps is realized, which is only dependent on the least significant bit resolution of the time-tagging device. The attainable precision is shown to be mainly determined by the inherent timing jitter of single photon detectors, the acquired pair rate, and acquisition time, and a sub-picosecond precision (0.72 ps) has been achieved at an acquisition time of 4.5 s. This high-precision nonlocal measurement realization provides a solid foundation for the field applications of entanglement-based quantum clock synchronization, ranging, and communications.

Contrast resolution of few-photon detectors

We investigate the minimum acquisition time, expressed as the number of image frames, and the minimum number of absorbed photons per pixel required to achieve a predefined contrast resolution in a monochromatic, pixelated image acquisition system at low light intensities (from well below one photon, to several hundred photons per pixel and frame). Primarily we compare systems based on the pixels of the photon-number-resolving (PNR) type of detectors and detectors that discriminate, in a binary fashion, between zero and non-zero photon numbers (so-called click detectors). We find that our model can seamlessly interpolate between the two. We also model detectors with intrinsic PNR capabilities and integrating detectors with a simple saturation model, derive the probability of errors in assigning the correct intensity (or ‘gray level’) and finally discuss how the estimated levels, which need to be based on threshold levels due to the stochastic nature of the detected photon number, should be assigned. Overall, we find that non-ideal PNR-detector-based systems offer advantages even over ideal click-detector-based systems when the incident mean photon number is sufficiently large, which is guaranteed to occur around ten photons per pixel and frame.

Scalable implementation of a superconducting nanowire single-photon detector array with a superconducting digital signal processor

A two-dimensional single-photon imaging system with high sensitivity and high time resolution is the ultimate camera and useful in a wide range of fields. A superconducting nanowire single-photon detector (SSPD or SNSPD) is one of the best candidates for realizing such an ultimate camera due to its high detection efficiency in a wide spectral range, low dark count rate without after-pulsing, and excellent time resolution. Here we propose a new readout scheme to realize a large-scale imaging array based on SSPD, where a row-column readout architecture is combined with a digital signal processor based on a single-flux-quantum (SFQ) circuit. A 16-pixel row-column readout SSPD array is fabricated and measured with an SFQ digital signal processor. We successfully acquired spatial information as encoded digital bit codes with the temporal information of the photon detection. The system timing jitter was measured as <80 ps for all 16 pixels even through the SFQ signal processor, indicating the potential for an imaging array with an extremely high time resolution.

Super-resolution single-photon imaging at 8.2 kilometers

Single-photon light detection and ranging (LiDAR), offering single-photon sensitivity and picosecond time resolution, has been widely adopted for active imaging applications. Long-range active imaging is a great challenge, because the spatial resolution degrades significantly with the imaging range due to the diffraction limit of the optics, and only weak echo signal photons can return but mixed with a strong background noise. Here we propose and demonstrate a photon-efficient LiDAR approach that can achieve sub-Rayleigh resolution imaging over long ranges. This approach exploits fine sub-pixel scanning and a deconvolution algorithm tailored to this long-range application. Using this approach, we experimentally demonstrated active three-dimensional (3D) single-photon imaging by recognizing different postures of a mannequin model at a stand-off distance of 8.2 km in both daylight and night. The observed spatial (transversal) resolution is ∼5.5 cm at 8.2 km, which is about twice of the system's resolution. This also beats the optical system's Rayleigh criterion. The results are valuable for geosciences and target recognition over long ranges.

Photon counting LIDAR at 2.3µm wavelength with superconducting nanowires

In this work, we show a proof-of-principle benchtop single-photon light detection and ranging (LIDAR) depth imager at 2.3µm, utilizing superconducting nanowire single-photon detectors (SNSPDs). We fabricate and fiber-couple SNSPDs to exhibit enhanced photon counting performance in the mid-infrared. We present characterization results using an optical parametric oscillator source and deploy these detectors in a scanning LIDAR setup at 2.3µm wavelength. This demonstrates the viability of these detectors for future free-space photon counting applications in the mid-infrared where atmospheric absorption and background solar flux are low.

Ultra-broadband microfiber-coupled superconducting single-photon detector

Broadband photon detectors are a key enabling technology for various applications such as spectrometers, light detection and ranging. In this work, we report on an ultra-broadband single-photon detector based on a microfiber (MF)-coupled superconducting nanowires structure operating in the spectral range from visible to near-infrared light. The MF-coupled superconducting nanowire single-photon detector (SNSPD) is formed by placing an MF on top of superconducting niobium nitride (NbN) nanowires, allowing ultra-broadband photon detection due to their nearly lossless transmission/absorption and nearly unity internal efficiency for ultra-broad waveband. The simulation results indicate that with optimal device structure, the optical absorption with efficiency > 90% can be realized over a wavelength range of 350 nm to 2150 nm. The fabricated MF-coupled SNSPD shows unparalleled broadband system detection efficiencies (SDEs) of more than 50% from 630 nm to 1500 nm. The SDEs reach 66% at 785 nm and 45% at 1550 nm. These results pave the way for ultra-broadband weak light detection with quantum-limit sensitivity.

Photon-counting laser interferometer for absolute distance measurement on rough surface

We designed a dual-wavelength photon-counting laser interferometer for absolute distance measurement of noncooperative targets. The weak optical interference on the rough surface was measured by a single-photon detector. The range of nonambiguity of the dual-wavelength interferometer was less than 1.2 μm, as the maximum errors of L_g and L_r were 7.8 nm and 9.1 nm caused by the photon-counting measurement and the frequency shift of the two unlocked lasers. We integrated laser triangulation into the interferometer as a coarse measurement, thus increasing the range of nonambiguity to 6.5 mm. As a result, a measurement standard deviation of ∼18 nm was achieved within a range of 1.1 mm in the experiment.

Line-scan spectrum-encoded imaging by dual-comb interferometry

Herein, the method of spectrum-encoded dual-comb interferometry is introduced to measure a three-dimensional (3-D) profile with absolute distance information. By combining spectral encoding for wavelength-to-space mapping, dual-comb interferometry for decoding and optical reference for calibration, this system can obtain a 3-D profile of an object at a stand-off distance of 114 mm with a depth precision of 12 μm. With the help of the reference arm, the absolute distance, reflectivity distribution, and depth information are simultaneously measured at a 5 kHz line-scan rate with free-running carrier-envelope offset frequencies. To verify the concept, experiments are conducted with multiple objects, including a resolution test chart, a three-stair structure, and a designed "ECNU" letter chain. The results show a horizontal resolution of ∼22  μm and a measurement range of 1.93 mm.

Multi-beam single-photon-counting three-dimensional imaging lidar

Photon-counting laser ranging has attracted a lot of research interest for its application in the altimeter. In this letter, we report a large scale multi-beam photon-counting laser imaging system by using 100 laser beams in linear array as the light source. Taking advantage of a 100-channel low-noise high-efficiency single-photon detector, the three-dimensional image of remote targets could be constructed rapidly according to the time-of-flight measurement. This system provides a solution for a high-speed, high-resolution, low energy-consumption pushbroom airborne or spaceborne laser altimeter.

Improving the timing jitter of a superconducting nanowire single-photon detection system

Low timing jitter is a unique merit of superconducting nanowire single-photon detectors (SNSPDs) for time-correlated applications. Quantitative analysis was performed for the SNSPD system. Aided by an oscilloscope with an optimal signal amplitude, we were able to measure a full width at half-maximum system timing jitter as low as 14.2 ps for a high-switching-current SNSPD using a room-temperature low-noise amplifier. When using a time-correlated single-photon counting module, the system timing jitter was 17.3 ps. The detector's intrinsic timing jitter was estimated at ∼12.0  ps.

Ultrafast time measurements by time-correlated single photon counting coupled with superconducting single photon detector

Time resolution is one of the main characteristics of the single photon detectors besides quantum efficiency and dark count rate. We demonstrate here an ultrafast time-correlated single photon counting (TCSPC) setup consisting of a newly developed single photon counting board SPC-150NX and a superconducting NbN single photon detector with a sensitive area of 7 × 7 μm. The combination delivers a record instrument response function with a full width at half maximum of 17.8 ps and system quantum efficiency ∼15% at wavelength of 1560 nm. A calculation of the root mean square value of the timing jitter for channels with counts more than 1% of the peak value yielded about 7.6 ps. The setup has also good timing stability of the detector-TCSPC board.