arXiv subjects

John Jeffers

Publications and source records attributed to John Jeffers.

At least 19 recordsLinked to original sources

arXiv2025

Decoy-state quantum key distribution over 227 km with a frequency-converted telecom single-photon source

We implement a decoy-state quantum key distribution scheme using a telecom C-band single-emitter source. The decoy states are created by varying the optical excitation of the quantum emitter to modulate the photon number distribution. We provide an analysis of our scheme based on existing security proofs, allowing the calculation of secret key rates including finite key effects. This enables us to demonstrate, with a realistic single-photon source, positive secret key rates using our scheme over 227 km of optical fiber, equivalent to a loss tolerance one order of magnitude greater than non-decoy schemes. This work broadens the scope of single-photon sources in future quantum networks by enabling long-distance QKD with realistic levels of single-photon purity.

quant-ph↗

arXiv2025

Spoofing resilience for simple-detection quantum illumination LIDAR

Object detection and range finding using a weak light source is vulnerable to jamming and spoofing attacks by an intruder. Quantum illumination with nonsimultaneous, phase-insensitive coincidence measurements can provide jamming resilience compared to identical measurements for classical illumination. We extend an experimentally-feasible object detection and range finding quantum illumination-based protocol to include spoofing resilience. This approach allows the system to be characterised by its experimental parameters and quantum states, rather than just its detector data. Therefore we can scope the parameter-space which provides some spoofing resilience without relying upon the prohibitive method of acquiring detector data for all combinations of the experimental parameters. We demonstrate that in certain regimes the intruder has an optimal relative detection basis angle to minimise the induced error. We also show that there are spoofing-vulnerable regimes where excessive background noise prevents any induced error, while it is still possible to perform object detection, i.e. our detectors have not been fully blinded. The sensing protocol which we describe can allow for the recognition of intrusion and the possible detection of our trustworthy return signal. Our results reinforce that quantum illumination is advantageous for spoofing resilience compared to a classical illumination-based protocol.

quant-ph↗

arXiv2024

Comparison of non-decoy single-photon source and decoy weak coherent pulse in quantum key distribution

Advancements in practical single-photon sources (SPS) exhibiting high brightness and low $g^{(2)}(0)$ have garnered significant interest for their application in quantum key distribution (QKD). To assess their QKD performance, it is essential to compare them with the widely employed weak coherent pulses (WCPs) in the decoy state method. In this work, we analyze the non-decoy efficient BB84 protocol for an SPS, partially characterising its photon statistics by its $g^{(2)}(0)$ and mean photon number. We compare it to the 2-decoy efficient BB84 with WCPs within the finite-key analysis framework while optimizing the parameters of both protocols. Our findings indicate that the non-decoy SPS with a mean photon number of $\langle n \rangle = 0.5$ and $g^{(2)}(0) = 3.6\%$ can enhance the secure key generation over the 2-decoy WCP for block sizes under $4.66 \cdot 10^9$ sent signals ($29$ seconds of acquisition time) at a channel loss of $10$ dB ($52.5$ km of optical fibre). Additionally, we demonstrate an increase in the maximum tolerable channel loss for SPSs with mean photon number $\langle n \rangle \geq 0.0142$ at block sizes below $10^8$ sent signals ($0.62$ seconds of acquisition time). These results suggest that SPSs hold potential for key rate enhancement in short-range QKD networks, though further research is required to evaluate their key generation capabilities when integrated into the decoy method.

quant-ph↗

arXiv2023

Using random coherent states to mimic quantum illumination

Quantum illumination uses quantum correlations to enhance the detection of an object in the presence of background noise. This advantage has been shown to exist even if one uses non-optimal direct measurements on the two correlated modes. Here we present a protocol that mimics the behaviour of quantum illumination, but does not use correlated or entangled modes. Instead, the protocol uses coherent (or phase-randomized coherent) pulses with randomly chosen intensities. The intensities are drawn from a distribution such that the average state looks thermal. Under appropriate conditions, the mimic protocol can perform similarly to quantum illumination schemes that use direct measurements. This holds even for a reflectance as low as $10^{-7}$. We also present an analytic condition which allows one to determine the sets of parameters in which each protocol works best.

quant-ph↗

arXiv2023

Object detection and rangefinding with quantum states using simple detection

In a noisy environment with weak single levels, quantum illumination can outperform classical illumination in determining the presence and range of a target object even in the limit of sub-optimal measurements based on non-simultaneous, phase-insensitive coincidence counts. Motivated by realistic experimental protocols, we present a theoretical framework for analysing coincident multi-shot data with simple detectors. This approach allows for the often-overlooked non-coincidence data to be included, as well as providing a calibration-free threshold for inferring the presence and range of an object, enabling a fair comparison between different detection regimes. Our results quantify the advantage of quantum over classical illumination when performing target discrimination in a noisy thermal environment, including estimating the number of shots required to detect a target with a given confidence level.

quant-ph↗

arXiv2023

Demonstration of quantum-enhanced rangefinding robust against classical jamming

In this paper we demonstrate operation of a quantum-enhanced lidar based on a continuously pumped photon pair source combined with simple detection in regimes with over 5 orders of magnitude separation between signal and background levels and target reflectivity down to -52 dB. We characterise the performance of our detector using a log-likelihood analysis framework, and crucially demonstrate the robustness of our system to fast and slow classical jamming, introducing a new protocol to implement dynamic background tracking to eliminate the impact of slow background changes whilst maintaining immunity to high frequency fluctuations. Finally, we extend this system to the regime of rangefinding in the presence of classical jamming to locate a target with an 11 cm spatial resolution limited only by the detector jitter. These results demonstrate the advantage of exploiting quantum correlations for lidar applications, providing a clear route to implementation of this system in real-world scenarios.

quant-ph↗

arXiv2022

Quantum illumination with multiplexed photodetection

The advantages of using quantum states of light for object detection are often highlighted in schemes that use simultaneous and optimal measurements. Here, we describe a theoretical but experimentally realizable quantum illumination scheme based on non-simultaneous and non-optimal measurements which can maintain this advantage. In particular, we examine the multi-click heralded two mode squeezed vacuum state as a probe signal in a quantum illumination process. The increase in conditioned signal intensity associated with multi-click heralding is greater than that from a single detector-heralded signal. Our results show, for lossy external conditions, the presence of the target object can be revealed earlier using multi-click measurements. We demonstrate this through sequential shot measurements based on Monte-Carlo simulation.

quant-ph↗

arXiv2022

Single-emitter quantum key distribution over 175 km of fiber with optimised finite key rates

Quantum key distribution with solid-state single-photon emitters is gaining traction due to their rapidly improving performance and compatibility with future quantum network architectures. In this work, we perform fibre-based quantum key distribution with a quantum dot frequency-converted to telecom wavelength, achieving count rates of 1.6 MHz with $g^{\left(2\right)}\left(0\right) = 3.6 \%$. We demonstrate positive key rates up to 175 km in the asymptotic regime. We then show that the community standard analysis for non-decoy state QKD drastically overestimates the acquisition time required to generate secure finite keys. Our improved analysis using the multiplicative Chernoff bound reduces the required number of received signals by a factor of $10^8$ over existing work, with the finite key rate approaching the asymptotic limit at all achievable distances for acquisition times of one hour. Over a practical distance of 100 km we achieve a finite key rate of 13 kbps after one minute of integration time. This result represents major progress towards the feasibility of long-distance single-emitter QKD networks.

quant-ph↗

arXiv2022

Measurement disturbance tradeoffs in three-qubit unsupervised quantum classification

We consider measurement disturbance tradeoffs in quantum machine learning protocols which seek to learn about quantum data. We study the simplest example of a binary classification task, in the unsupervised regime. Specifically, we investigate how a classification of two qubits, that can each be in one of two unknown states, affects our ability to perform a subsequent classification on three qubits when a third is added. Surprisingly, we find a range of strategies in which a non-trivial first classification does not affect the success rate of the second classification. There is, however, a non-trivial measurement disturbance tradeoff between the success rate of the first and second classifications, and we fully characterise this tradeoff analytically.

quant-ph↗

arXiv2021

A practical compact source of heralded single photons for simple detection LIDAR

Optical quantum technologies such as quantum sensing, quantum cryptography and quantum computation all utilize properties of non-classical light, such as precise photon-number and entangled photon-pair states, to surpass technologies based on the classical light. A common route for obtaining heralded single photons is spontaneous four-wave mixing in optical fibers, allowing for a well-defined spatial mode, for high efficiency integration into optical fiber networks. These fibers are typically pumped using large, commercial, pulsed lasers requiring high-power (~10 W) pump lasers and are limited to ~MHz repetition rate. Here we propose a cost-efficient, compact and mobile alternative. Photon pairs at 660 nm and 960 nm will be created using four-wave mixing in commercial birefringent optical fiber, pumped using transform limited picosecond pulses with GHz repetition rates derived from a 785 nm CW laser diode using cavity-enhanced optical frequency comb generation. The pulses are predicted to have average power of 275 mW, a peak power of >40 W, and predicted photon yield of >2000 pairs detected per second. This design will be later utilized to implement a quantum illumination scheme based on a coincidence count between idler and signal photons - instead of joint measurement between signal and idler. This will allow for quantum advantage over classic LIDAR without the requirement for maintaining an interferometric stability in free space.

physics.optics↗

arXiv2021

Theoretical comparison of quantum and classical illumination for simple detection-based LIDAR

Use of non-classical light in a quantum illumination scheme provides an advantage over classical illumination when used for LIDAR with a simple and realistic detection scheme based on Geiger-mode single photon detectors. Here we provide an analysis that accounts for the additional information gained when detectors do not fire that is typically neglected and show an improvement in performance of quantum illumination. Moreover, we provide a theoretical framework quantifying performance of both quantum and classical illumination for simple target detection, showing parameters for which a quantum advantage exists. Knowledge of the regimes that demonstrate a quantum advantage will inform where possible practical quantum LIDAR utilising non-classical light could be realised.

physics.optics↗

arXiv2021

Compressive Sampling Using a Pushframe Camera

The recently described pushframe imager, a parallelized single pixel camera capturing with a pushbroom-like motion, is intrinsically suited to both remote-sensing and compressive sampling. It optically applies a 2D mask to the imaged scene, before performing light integration along a single spatial axis, but previous work has not made use of the architecture's potential for taking measurements sparsely. In this paper we develop a strongly performing static binarized noiselet compressive sampling mask design, tailored to pushframe hardware, allowing both a single exposure per motion time-step, and retention of 2D correlations in the scene. Results from simulated and real-world captures are presented, with performance shown to be similar to that of immobile -- and hence inappropriate for satellite use -- whole-scene imagers. A particular feature of our sampling approach is that the degree of compression can be varied without altering the pattern, and we demonstrate the utility of this for efficiently storing and transmitting multi-spectral images.

eess.IV↗

arXiv2021

Quantum retrodiction: foundations and controversies

Prediction is the making of statements, usually probabilistic, about future events based on current information. Retrodiction is the making of statements about past events based on current information. We present the foundations of quantum retrodiction and highlight its intimate connection with the Bayesian interpretation of probability. The close link with Bayesian methods enables us to explore controversies and misunderstandings about retrodiction that have appeared in the literature. To be clear, quantum retrodiction is universally applicable and draws its validity directly from conventional predictive quantum theory coupled with Bayes' theorem.

quant-ph↗

arXiv2021

Gaussian State-Based Quantum Illumination with Simple Photodetection

Proofs of the quantum advantage available in imaging or detecting objects under quantum illumination can rely on optimal measurements without specifying what they are. We use the continuous-variable Gaussian quantum information formalism to show that quantum illumination is better for object detection compared with coherent states of the same mean photon number, even for simple direct photodetection. The advantage persists if signal energy and object reflectivity are low and background thermal noise is high. The advantage is even greater if we match signal beam detection probabilities rather than mean photon number. We perform all calculations with thermal states, even for non-Gaussian conditioned states with negative Wigner functions. We simulate repeated detection using a Monte Carlo process that clearly shows the advantages obtainable.

quant-ph↗

arXiv2021

Parity-Swap State Comparison Amplifier for Schrödinger Cat States

We propose a postselecting parity-swap amplifier for Schrödinger cat states that does not require the amplified state to be known a priori. The device is based on a previously-implemented state comparison amplifier for coherent states. It consumes only Gaussian resource states, which provides an advantage over some cat state amplifiers. It requires simple Geiger-mode photodetectors and works with high fidelity and approximately twofold gain.

quant-ph↗

arXiv2020

Compact Multi-Spectral Pushframe Camera for Nano-Satellites

In this paper we present an evolution of the single-pixel camera architecture, called 'pushframe', which addresses the limitations of pushbroom cameras in space-based applications. In particular, it is well-suited to observing fast moving scenes while retaining high spatial resolution and sensitivity. We show that the system is capable of producing colour images with good fidelity and scalable resolution performance. The principle of our design places no restriction on the spectral range to be captured, making it suitable for wide infrared imaging.

eess.IV↗

arXiv2017

Photonic Quantum Operations via the Quantum Carburettor Effect

The bosonic nature of light leads to counter-intuitive bunching effects. We describe an experimentally testable effect in which a single photon is induced through a highly reflecting beamsplitter by a large amplitude coherent state, with probability 1/e in the limit of large coherent state amplitude. We use this effect to construct a viable implementation of the bare raising operator on coherent states via conditional measurement, which succeeds with high probability and fidelity even in the high amplitude limit.

quant-ph↗

arXiv2016

Security against jamming and noise exclusion in imaging

We describe a protocol by which an imaging system could be protected against jamming by a malevolent party. Our protocol not only allows recognition of the jamming, but also allows for the recovery of the true image from the jammed one. We apply the method to jamming of quantum ghost imaging, for which the jamming detection probability is increased when the imaging light is entangled. The method can also be used to provide image recovery in general noisy environments.

quant-ph↗