[Show abstract][Hide abstract] ABSTRACT: Multispectral light detection and ranging (LiDAR) has the potential to recover structural and physiological data from arboreal samples and, by extension, from forest canopies when deployed on aerial or space platforms. In this paper, we describe the design and evaluation of a prototype multispectral LiDAR system and demonstrate the measurement of leaf and bark area and abundance profiles using a series of experiments on tree samples “viewed from above” by tilting living conifers such that the apex is directed on the viewing axis. As the complete recovery of all structural and physiological parameters is ill posed with a restricted set of four wavelengths, we used leaf and bark spectra measured in the laboratory to constrain parameter inversion by an extended reversible jump Markov chain Monte Carlo algorithm. However, we also show in a separate experiment how the multispectral LiDAR can recover directly a profile of Normalized Difference Vegetation Index (NDVI), which is verified against the laboratory spectral measurements. Our work shows the potential of multispectral LiDAR to recover both structural and physiological data and also highlights the fine spatial resolution that can be achieved with time-correlated single-photon counting.
IEEE Transactions on Geoscience and Remote Sensing 07/2014; 52(8). · 3.47 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Digital signatures are widely used to provide security for electronic communications, for example, in financial transactions and electronic mail. Currently used classical digital signature schemes, however, only offer security relying on unproven computational assumptions. In contrast, quantum digital signatures offer information-theoretic security based on laws of quantum mechanics. Here, security against forging relies on the impossibility of perfectly distinguishing between nonorthogonal quantum states. A serious drawback of previous quantum digital signature schemes is that they require long-term quantum memory, making them impractical at present. We present the first realization of a scheme that does not need quantum memory and which also uses only standard linear optical components and photodetectors. In our realization, the recipients measure the distributed quantum signature states using a new type of quantum measurement, quantum state elimination. This significantly advances quantum digital signatures as a quantum technology with potential for real applications.
[Show abstract][Hide abstract] ABSTRACT: Quantum optical amplification that beats the noise addition limit for
deterministic amplifiers has been realized experimentally using several
different nondeterministic protocols. These schemes either require
single-photon sources, or operate by noise addition and photon subtraction.
Here we present an experimental demonstration of a protocol that allows
nondeterministic amplification of known sets of coherent states with high gain
and high fidelity. The experimental system employs the two mature quantum
optical technologies of state comparison and photon subtraction and does not
rely on elaborate quantum resources such as single-photon sources. The use of
coherent states rather than single photons allows for an increased rate of
amplification and a less complex photon source. Furthermore it means that the
amplification is not restricted to low amplitude states. With respect to the
two key parameters, fidelity and amplified state production rate, we
demonstrate, without the use of quantum resources, significant improvements
over previous experimental implementations.
[Show abstract][Hide abstract] ABSTRACT: The response of cells to changes in their physico-chemical micro-environment is essential to their survival. For example, bacterial magnetotaxis uses the Earth's magnetic field together with chemical sensing to help microorganisms move towards favoured habitats. The studies of such complex responses are lacking a method that permits the simultaneous mapping of the chemical environment and the response of the organisms, and the ability to generate a controlled physiological magnetic field. We have thus developed a multi-modal microscopy platform that fulfils these requirements. Using simultaneous fluorescence and high-speed imaging in conjunction with diffusion and aerotactic models, we characterized the magneto- aerotaxis of Magnetospirillum gryphiswaldense. We assessed the influence of the magnetic field (orientation; strength) on the formation and the dynamic of a micro-aerotactic band (size, dynamic, position). As previously described by models of magnetotaxis, the application of a magnetic field pointing towards the anoxic zone of an oxygen gradient results in an enhanced aerotaxis even down to Earth's magnetic field strength. We found that neither a ten-fold increase of the field strength nor a tilt of 45° resulted in a significant change of the aerotactic efficiency. However, when the field strength is zeroed or when the field angle is tilted to 90°, the magneto-aerotaxis efficiency is drastically reduced. The classical model of magneto-aerotaxis assumes a response proportional to the cosine of the angle difference between the directions of the oxygen gradient and that of the magnetic field. Our experimental evidence however shows that this behaviour is more complex than assumed in this model, thus opening up new avenues for research.
PLoS ONE 01/2014; 9(7):e101150. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Digital signatures are widely used to provide security for electronic
communications, for example in financial transactions and electronic mail.
Currently used classical digital signature schemes, however, only offer
security relying on unproven computational assumptions. In contrast, quantum
digital signatures (QDS) offer information-theoretic security based on laws of
quantum mechanics (e.g. Gottesman and Chuang 2001). Here, security against
forging relies on the impossibility of perfectly distinguishing between
non-orthogonal quantum states. A serious drawback of previous QDS schemes is
however that they require long-term quantum memory, making them unfeasible in
practice. We present the first realisation of a scheme (Dunjko et al 2013) that
does not need quantum memory, and which also uses only standard linear optical
components and photodetectors. To achieve this, the recipients measure the
distributed quantum signature states using a new type of quantum measurement,
quantum state elimination (e.g. Barnett 2009, Bandyopadhyay et al 2013). This
significantly advances QDS as a quantum technology with potential for real
[Show abstract][Hide abstract] ABSTRACT: As society becomes more reliant on electronic communication and
transactions, ensuring the security of these interactions becomes more
important. Digital signatures are a widely used form of cryptography
which allows parties to certify the origins of their communications,
meaning that one party, a sender, can send information to other parties
in such a way that messages cannot be forged. In addition, messages are
transferrable, meaning that a recipient who accepts a message as genuine
can be sure that if it is forwarded to another recipient, it will again
be accepted as genuine. The classical digital signature schemes
currently employed typically rely on computational complexity for
security. Quantum digital signatures offer the potential for increased
security. In our system, quantum signature states are passed through a
network of polarization maintaining fiber interferometers (a multiport)
to ensure that recipients will not disagree on the validity of a
message. These signatures are encoded in the phase of photonic coherent
states and the choice of photon number, signature length and number of
possible phase states affects the level of security possible by this
approach. We will give a brief introduction into quantum digital
signatures and present results from our experimental demonstration
[Show abstract][Hide abstract] ABSTRACT: We have used an InGaAs/InP single-photon avalanche diode detector module in conjunction with a time-of-flight depth imager operating at a wavelength of 1550 nm, to acquire centimeter resolution depth images of low signature objects at stand-off distances of up to one kilometer. The scenes of interest were scanned by the transceiver system using pulsed laser illumination with an average optical power of less than 600 µW and per-pixel acquisition times of between 0.5 ms and 20 ms. The fiber-pigtailed InGaAs/InP detector was Peltier-cooled and operated at a temperature of 230 K. This detector was used in electrically gated mode with a single-photon detection efficiency of about 26% at a dark count rate of 16 kilocounts per second. The system's overall instrumental temporal response was 144 ps full width at half maximum. Measurements made in daylight on a number of target types at ranges of 325 m, 910 m, and 4.5 km are presented, along with an analysis of the depth resolution achieved.
[Show abstract][Hide abstract] ABSTRACT: Intensity correlation measurements form the basis of many experiments based on spontaneous parametric down-conversion. In the most common situation, two single-photon avalanche diodes and coincidence electronics are used in the detection of the photon pairs, and the coincidence count distributions are measured by making use of some scanning procedure. Here we analyze the measurement of intensity correlations using multielement detector arrays. By considering the detector parameters such as the detection and noise probabilities, we found that the mean number of detected photons that maximizes the visibility of the two-photon correlations is approximately equal to the mean number of noise events in the detector array. We provide expressions predicting the strength of the measured intensity correlations as a function of the detector parameters and on the mean number of detected photons. We experimentally test our predictions by measuring far-field intensity correlations of spontaneous parametric down-conversion with an electron multiplying charge-coupled device camera, finding excellent agreement with the theoretical analysis.
Physical Review A 07/2013; 88(1):013816. · 3.04 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This paper highlights a significant advance in time-of-flight depth imaging: by using a scanning transceiver which incorporated a free-running, low noise superconducting nanowire single-photon detector, we were able to obtain centimeter resolution depth images of low-signature objects in daylight at stand-off distances of the order of one kilometer at the relatively eye-safe wavelength of 1560 nm. The detector used had an efficiency of 18% at 1 kHz dark count rate, and the overall system jitter was ~100 ps. The depth images were acquired by illuminating the scene with an optical output power level of less than 250 µW average, and using per-pixel dwell times in the millisecond regime.
[Show abstract][Hide abstract] ABSTRACT: Direct monitoring of singlet oxygen (<sup>1</sup>O<sub>2</sub>) luminescence is a particularly challenging infrared photodetection problem. <sup>1</sup>O<sub>2</sub>, an excited state of the oxygen molecule, is a crucial intermediate in many biological processes. We employ a low noise superconducting nanowire single-photon detector to record <sup>1</sup>O<sub>2</sub> luminescence at 1270 nm wavelength from a model photosensitizer (Rose Bengal) in solution. Narrow band spectral filtering and chemical quenching is used to verify the <sup>1</sup>O<sub>2</sub> signal, and lifetime evolution with the addition of protein is studied. Furthermore, we demonstrate the detection of <sup>1</sup>O<sub>2</sub> luminescence through a single optical fiber, a marked advance for dose monitoring in clinical treatments such as photodynamic therapy.
[Show abstract][Hide abstract] ABSTRACT: The design, modeling, fabrication, and characterization of single-photon avalanche diode detectors with an epitaxial Ge absorption region grown directly on Si are presented. At 100 K, a single-photon detection efficiency of 4% at 1310 nm wavelength was measured with a dark count rate of ~ 6 megacounts/s, resulting in the lowest reported noise-equivalent power for a Ge-on-Si single-photon avalanche diode detector (1×10-14 WHz-1/2). The first report of 1550 nm wavelength detection efficiency measurements with such a device is presented. A jitter of 300 ps was measured, and preliminary tests on after-pulsing showed only a small increase (a factor of 2) in the normalized dark count rate when the gating frequency was increased from 1 kHz to 1 MHz. These initial results suggest that optimized devices integrated on Si substrates could potentially provide performance comparable to or better than that of many commercially available discrete technologies.
IEEE Transactions on Electron Devices 01/2013; 60(11):3807-3813. · 2.06 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Ge-on-Si single-photon detectors are fabricated and characterized at 1310 and 1550 nm. 4 % single-photon detection efficiency is observed at 1310 nm demonstrating the lowest reported noise equivalent power for Ge-on-Si single-photon detectors (1×10-14 WHz-1/2).
Group IV Photonics (GFP), 2013 IEEE 10th International Conference on; 01/2013
[Show abstract][Hide abstract] ABSTRACT: Digital signatures are frequently used in data transfer to prevent impersonation, repudiation and message tampering. Currently used classical digital signature schemes rely on public key encryption techniques, where the complexity of so-called 'one-way' mathematical functions is used to provide security over sufficiently long timescales. No mathematical proofs are known for the long-term security of such techniques. Quantum digital signatures offer a means of sending a message, which cannot be forged or repudiated, with security verified by information-theoretical limits and quantum mechanics. Here we demonstrate an experimental system, which distributes quantum signatures from one sender to two receivers and enables message sending ensured against forging and repudiation. Additionally, we analyse the security of the system in some typical scenarios. Our system is based on the interference of phase-encoded coherent states of light and our implementation utilizes polarization-maintaining optical fibre and photons with a wavelength of 850 nm.
[Show abstract][Hide abstract] ABSTRACT: Using an electron multiplying CCD camera we observe both image plane
(position) and far field (momentum) correlations between photon pairs
produced from spontaneous parametric down-conversion when using a 201 x
201 bi-dimensional array of pixels and a flux of around 0.02
photons/pixel. After background subtraction we characterize the strength
of signal and idler correlations in both transverse dimensions by
applying entanglement and EPR criteria, showing good agreement with the
theoretical predictions. The application of such devices in quantum
optics could have a wide range, including quantum computation with
spatial degrees of freedom of single photons.
[Show abstract][Hide abstract] ABSTRACT: We present a general purpose theoretical model of single-photon
detectors in quantum key distribution systems and apply it to an
autonomous gigahertz clocked phase basis set system operating at a
wavelength of 850 nm over a standard telecommunications fiber quantum
channel. The system has been demonstrated using a variety of different
singlephoton detectors, including thick and thin junction silicon
single-photon avalanche photodiodes and the first implementation of a
resonant cavity thin junction silicon single-photon avalanche diode. We
show, by means of the theoretical model, how improvements to certain
detector parameters can optimize key exchange rates.
[Show abstract][Hide abstract] ABSTRACT: The light produced by parametric down-conversion shows strong spatial entanglement that leads to violations of EPR criteria for separability. Historically, such studies have been performed by scanning a single-element, single-photon detector across a detection plane. Here we show that modern electron-multiplying charge-coupled device cameras can measure correlations in both position and momentum across a multi-pixel field of view. This capability allows us to observe entanglement of around 2,500 spatial states and demonstrate Einstein-Podolsky-Rosen type correlations by more than two orders of magnitude. More generally, our work shows that cameras can lead to important new capabilities in quantum optics and quantum information science.
[Show abstract][Hide abstract] ABSTRACT: We have built and tested the first experimental demonstration of a quantum digital signature test-bed system. We will present a case for quantum digital signatures, overview of the protocol, description of the system and results.
[Show abstract][Hide abstract] ABSTRACT: We have developed a new approach to measuring the spatial position of a
single photon. Using fibers of different length, all connected to a
single detector allows us to use the high timing precision of single
photon avalanche diodes (SPAD) to spatially locate the photon. We have
built two 8-element detector arrays to measure the full-field quantum
correlations in position, momentum and intermediate bases for photon
pairs produced in parametric down conversion. The strength of the
position-momentum correlations is found to be an order of magnitude
below the classical limit.