Ghost imaging (GI) can reconstruct the image of an object by measuring the correlation function of two beams, none of which carries the structure information of the object independently. This powerful technology makes it possible to obtain high-quality imaging of the object even in the presence of noise. Here, we introduce the GI method into quantum device evaluation in the time domain. We realized a proof-of-principle experiment to evaluate the temporal detection efficiency of a gated-mode single-photon avalanche detector (SPAD). The experimental results show that high-quality evaluation of temporal characteristics of the SPAD can be realized by the method of temporal GI (TGI). Our work indicates that the TGI method is an effective tool to monitor the temporal characteristics of quantum devices in real time and will bring a new perspective to the security evaluation of quantum communication.
Ghost imaging (GI) can reconstruct the image of an object by measuring the correlation function of two beams, none of which carries the structure information of the object independently. This powerful technology makes it possible to obtain high-quality imaging of the object even in the presence of noise. Here, we introduce the GI method into quantum device evaluation in the time domain. We realized a proof-of-principle experiment to evaluate the temporal detection efficiency of a gated-mode single-photon avalanche detector (SPAD). The experimental results show that high-quality evaluation of temporal characteristics of the SPAD can be realized by the method of temporal GI (TGI). Our work indicates that the TGI method is an effective tool to monitor the temporal characteristics of quantum devices in real time and will bring a new perspective to the security evaluation of quantum communication.
Quantum Ghost Imaging (QGI) is a scheme which uses the temporal correlation of entangled photon pairs (signal and idler) to identify detections belonging to actual entangled pairs and separate them from uncorrelated detections. By also exploiting the spatial correlation of the photon pair, this identification allows to extract image information of the idler photons by imaging the signal photons. This can even be done for non-degenerate photon pairs, allowing imaging at wavelengths without sophisticated cameras in this regime. Recent advances in detector technology, especially single photon avalanche diodes (SPADs), allow the design for new cameras capable of single photon detection with both high spatial and temporal resolution. These new detectors are especially useful for application in QGI, allowing to detect each photon individually and identify the correlated pairs after the measurement. It further allows to analyse the time-of-flight of photons and thus enables 3D imaging, the first realization of which will be presented here.
Quantum Ghost Imaging (QGI) is a scheme using entangled pairs of photons (signal and idler) in order to perform imaging with both single photons and with only a single-element detector in the spectrum of interest. It utilises the temporal coincidence of the photons to identify associated pairs, while their spatial correlation allows to obtain image information from of the idler photon from the measurement of the signal photon. It is especially useful when using nondegenerate photon pairs, allowing to keep the signal photon in the silicon detection window, while the interacting wavelength can be freely chosen.However, current schemes are limited, as they rely on time-gating and heralding. Recent advances in single photon avalanche diodes (SPADs) allow the design of new single photon cameras, which can be outfitted with dedicated in-pixel timing circuitry. This allows to register single photons in both time and space. These detectors allowed us to design a new scheme for QGI, in which the coincidence is evaluated after the measurement. It also allows us to perform depth-resolved 3D imaging based on the time-of-flight of photons, first results of which are presented here.
Abstract Quantum ghost imaging is a method that utilizes the correlated detection of two photons to generate an image. One photon is detected by an imaging sensor and the other by a single-element bucket detector. The selection of the imaging sensor and its capabilities relative to the bucket detector impact the quality of the ghost images. This work examines a SPAD array and a photocathode detector as imaging sensors for quantum ghost imaging. We discuss how to achieve optimal images using these two technologies. We also demonstrate that these devices are able to generate ghost images at 1Hz frame rates, expanding the technique to biologically relevant time scales. Graphical Abstract
Low-light-level imaging in the infrared regime could find applications in areas ranging from technological to biological. Still, its development is limited by the inefficiency of silicon-based detectors in this regime. Quantum Ghost Imaging (QGI) can address this limitation by allowing the separation of illumination and imaging [1]. This is accomplished by generating non-degenerate correlated photon pairs, whose infrared photon probes the object and is detected by an InGaAs bucket detector, while the spatial distribution of the visible one is collected by a single-photon sensitive silicon camera. Traditional QGI setups evaluate photon correlations through “heralding”, by triggering the camera upon idler detection and thus require an image-preserving delay-line, which limits their use for remote sensing [1]. In asynchronous QGI (aQGI) cameras are replaced with SPAD arrays [2] able to record photon timestamp with picoseconds precision. By identifying pairs through timestamps comparison, the mentioned limitations are resolved [3], enabling applications with reflective or diffusive objects where the timestamp delay depends on the object's position. In this work, we propose and demonstrate an experimental QGI setup for remote sensing in reflection configuration (Fig. 1). Collinear non-degenerate pairs of single-photons at 550 nm and 1550 nm are generated by pumping a 2 mm-long type-0 ppKTP crystal with a 405 nm laser. To enable imaging of objects at varying distances, we implemented a momentum-correlation-based setup. Infrared photons interacting with the object were collected by commercially available InGaAs SPAD detectors, while the visible photons were directly captured using a prototype silicon SPAD array developed by Politecnico di Milano [4]. It is based on a 32×32 SPAD, with in-pixel TDCs, and on-chip microlenses are employed to reach a fill factor of 78%. The time resolution is adjustable down to 250 ps, with 1 μs Full Scale Range (FSR). The measurement duty cycle (i.e. the ration of the TDC FSR and the frame time) can be increased with a multi-gate acquisition scheme. To validate the efficiency of our setup, we tested it using a proof-of-principle object, as shown in Fig. 1b. The reconstructed images, captured with the object placed at 30 cm intervals in an acquisition time of only 30 s, are shown in Fig. 1c. In summary, the shown results confirm the potential of the proposed setup for remote sensing applications, particularly for reflective or diffusive objects. Moreover, the proposed approach could offers the possibility of significantly reduce acquisition times compared to standard QGI setups.
This work experimentally demonstrates that the imaging quality of quantum ghost imaging (GI) with entangled photons can be significantly improved by properly handling the errors caused by the imperfection of optical devices. We also consider compressive GI to reduce the number of measurements and thereby the data acquisition time. The image reconstruction is formulated as a sparse total least square problem which is solved with an iterative algorithm. Our experiments show that, compared with existing methods, the new method can achieve a significant performance gain in terms of mean square error and peak signal⁻noise ratio.
Quantum key distribution (QKD) can generate secure key bits between remote users with quantum mechanics. However, the gap between the theoretical model and practical realizations gives eavesdroppers opportunities to intercept the secret key. The most insidious attacks, known as quantum hacking, are the ones with no significant discrepancy of the measurement results using side-channel loopholes of QKD systems. Depicting full-time-scale characteristics of the quantum signals, the quantum channel, and the QKD system can provide legitimate users extra capabilities to defeat malicious attacks. Here, we propose the method exploring a temporal ghost-imaging (TGI) scheme to perceive quantum hacking with temporal fingerprints and experimentally verify its validity. The scheme presents a common approach to promote QKD's practical security from a unique perspective of signals and systems.