Free-space quantum key distribution links in an urban environment have demanding operating needs such as functioning in daylight and under atmospheric turbulence, which can dramatically impact their performance. Both effects are usually mitigated with a careful design of the field of view of the receiver. However, a trade-off is often required, since a narrow field of view improves background noise rejection but is linked to an increase in turbulence-related losses. We present a high-speed automatic tracking system to overcome these limitations. Both a reduction in the field of view to decrease the background noise and a mitigation of the losses caused by atmospheric turbulence are addressed. Two different designs are presented and discussed, along with technical considerations for the experimental implementation. Finally, preliminary experimental results of beam wander correction are used to estimate the potential improvement of both the quantum bit error rate and secret key rate of a free-space quantum key distribution system.
This paper presents recent progress in the development of a scanning time-of-flight imaging system employing
time-correlated single-photon counting (TCSPC) designed for the acquisition of depth information at kilometre
ranges. The device is capable of acquiring information on non-cooperative target surfaces at eye-safe average
optical power levels in the near-IR regime (<1 mW at 842 nm illumination wavelength). Target illumination
is periodic or non-periodic at typical repetition frequencies in the MHz domain, utilising a sub-ns pulse-width
laser diode. The system output is steered over the optical field of interest, and return photons from the target
are routed towards a single-photon detector. Measurements are performed with a silicon single-photon avalanche
diode (SPAD). Effective optical spatial and spectral filtering techniques permit operation in bright daylight
Results in the form of depth images from a variety of targets, taken under various environmental conditions,
are presented. Achieved improvements of this first-generation system are discussed in terms of parametric
enhancement of quantities such as spatial and spectral filtering, internal optical attenuation and beam size.
We detail progress in the design process both based on theoretical assumptions and actual measurements at
distances between few 100's of metres and several km. The trade-offs between acquisition time, maximum range
and excitation laser power levels are discussed and projections made for this and future depth imaging systems.
State-of-the-art TCSPC hardware solutions facilitate the rapid transfer and storage of large quantities
of raw data. This renders possible real-time analysis with speed-optimised algorithms such as fast Fourier
transform-supported cross-correlation methods, as well as gathering additional information about the scene in
post-processing steps, based on approaches such as reversible-jump Markov-chain Monte Carlo (RJMCMC).
This algorithm dynamically adapts the number of degrees of freedom of a range measurement, resulting in
multi-surface resolution and the possible identification of targets obscured by objects such as foliage.
Proc. SPIE. 6900, Quantum Sensing and Nanophotonic Devices V
KEYWORDS: Mirrors, Photon counting, 3D acquisition, Detection and tracking algorithms, Sensors, Semiconductor lasers, Data processing, Signal processing, Single photon detectors, Pulsed laser operation
Time-correlated single-photon counting techniques using individual optimized detectors have been applied to
time-of-flight ranging and depth imaging. This paper describes recent progress in photon-counting systems
performing surface mapping of non-cooperative targets. This includes systems designed for short ranges of the
order of 1-50 meters, and longer ranges of up to ten kilometers. The technique has also been applied to distributed
surfaces. We describe the measurement approach, techniques used for scanning, as well as the signal analysis
methodology and algorithm selection.
The technique is fundamentally flexible: the trade-off between the integrated number of counts (or acquisition
time) against range repeatability or depth resolution allows its application in a number of diverse fields. The
inherent time gating of the technique, allied to the spatial filtering provided by small active area single-photon
detectors, can lead to operation under high ambient light conditions even with low average optical power pulsed
We have demonstrated three-dimensional imaging of meter-dimensioned objects where reverse engineering
methods using cooperative targets cannot be routinely employed: e.g. delicate objects, or objects with more than
one reflective surface. Using more advanced signal processing algorithms, we have been able to improve the system
performance significantly, as measured by the depth resolution at short and long ranges. Furthermore, the
application of these methodologies has allowed us to characterize the positions and amplitudes of multiple returns.
Hence, the approach can be used for characterization of distributed non-cooperative targets at kilometer ranges,
even in environments where low-light level and and/or eye-safe operation is necessary.
The technique has also been applied in conjunction with a rapid scanning approach, to acquire three-dimensional
information of a target scene with frame times of approximately 1 second.
In this paper we demonstrate the application of multi-user quantum key distribution (QKD) to typical broadband fibrebased
passive optical access links to metropolitan area networks. We propose a technique to utilize the currently unused
850nm waveband in standard telecommunications fiber for QKD in two network architectures. Net bit rates of up to
100's of kilobits<sup>-1</sup> were achieved for each receiver, depending on the network topology. The transmission distances
between sender and receivers were compatible with the typical span of optical access links ( ≤ 10km).
In recent years quantum information research has lead to the discovery of a number of remarkable new paradigms for information processing and communication. These developments include quantum cryptography schemes that offer unconditionally secure information transport guaranteed by quantum-mechanical laws. Such potentially disruptive security technologies could be of high strategic and economic value in the future. Two major issues confronting researchers in this field are the transmission range (typically <100km) and the key exchange rate, which can be as low as a few bits per second at long optical fiber distances. This paper describes further research of an approach to significantly enhance the key exchange rate in an optical fiber system at distances in the range of 1-20km. We will present results on a number of application scenarios, including point-to-point links and multi-user networks.
Quantum key distribution systems have been developed, which use standard telecommunications optical fiber, and which are capable of operating at clock rates of up to 2GHz. They implement a polarization-encoded version of the B92 protocol and employ vertical-cavity surface-emitting lasers with emission wavelengths of 850 nm as weak coherent light sources, as well as silicon single-photon avalanche diodes as the single photon detectors. The point-to-point quantum key distribution system exhibited a quantum bit error rate of 1.4%, and an estimated net bit rate greater than 100,000 bits-1 for a 4.2 km transmission range.