arXiv subjects

Alexander Ling

Publications and source records attributed to Alexander Ling.

At least 55 records · Page 3Linked to original sources

arXiv2021

A Prototype of a Compact Rubidium-Based Optical Frequency Reference for Operation on Nanosatellites

Space-borne optical frequency references based on spectroscopy of atomic vapors may serve as an integral part of compact optical atomic clocks, which can advance global navigation systems, or can be utilized for earth observation missions as part of laser systems for cold atom gradiometers. Nanosatellites offer low launch-costs, multiple deployment opportunities and short payload development cycles, enabling rapid maturation of optical frequency references and underlying key technologies in space. Towards an in-orbit demonstration on such a platform, we have developed a CubeSat-compatible prototype of an optical frequency reference based on the D2-transition in rubidium. A frequency instability of 1.7e-12 at 1 s averaging time is achieved. The optical module occupies a volume of 35 cm^3, weighs 73 g and consumes 780 mW of power.

physics.atom-ph↗

arXiv2021

Realizing quantum nodes in space for cost-effective, global quantum communication: in-orbit results and next steps

Quantum sources and receivers operating on-board satellites are an essential building block for global quantumnetworks. SpooQy-1 is a satellite developed at the Centre for Quantum Technologies, which has successfully demonstrated the operation of an entangled photon pair source on a resource-constrained CubeSat platform. This miniaturized and ruggedized photon pair source is being upgraded to be capable of space-to-ground quantum keydistribution and long-range entanglement distribution. In this paper, we share results from SpooQy-1, discuss their relevance for the engineering challenges of a small satellite quantum node, and report on the development of the new light source.

quant-ph↗

arXiv2021

Advances in Space Quantum Communications

Concerted efforts are underway to establish an infrastructure for a global quantum internet to realise a spectrum of quantum technologies. This will enable more precise sensors, secure communications, and faster data processing. Quantum communications are a front-runner with quantum networks already implemented in several metropolitan areas. A number of recent proposals have modelled the use of space segments to overcome range limitations of purely terrestrial networks. Rapid progress in the design of quantum devices have enabled their deployment in space for in-orbit demonstrations. We review developments in this emerging area of space-based quantum technologies and provide a roadmap of key milestones towards a complete, global quantum networked landscape. Small satellites hold increasing promise to provide a cost effective coverage required to realised the quantum internet. We review the state of art in small satellite missions and collate the most current in-field demonstrations of quantum cryptography. We summarise important challenges in space quantum technologies that must be overcome and recent efforts to mitigate their effects. A perspective on future developments that would improve the performance of space quantum communications is included. We conclude with a discussion on fundamental physics experiments that could take advantage of a global, space-based quantum network.

quant-ph↗

arXiv2020

Temperature insensitive type II quasi-phasematched spontaneous parametric downconversion

The temperature dependence of the refractive indices of potassium titanyl phosphate (KTP) are shown to enable quasi-phasematched type II spontaneous parametric downconversion (SPDC) with low temperature sensitivity. Calculations show the effect to be maximised for emission of photons at around 1165nm, as well as producing potentially useful regions for wavelengths throughout the telecommunications bands. We demonstrate the effect experimentally, observing temperature-insensitive degenerate emission at 1326nm, within the telecommunications O band. This result has practical applications in the development of entangled photon sources for resource-constrained environments, and we demonstrate a simple polarization entangled source as a proof of concept.

quant-ph↗

arXiv2020

Stable Polarization Entanglement based Quantum Key Distribution over Metropolitan Fibre Network

We demonstrate a quantum key distribution implementation over deployed dark telecom fibers with polarisation-entangled photons generated at the O-band. One of the photons in the pairs are propagated through 10km of deployed fiber while the others are detected locally. Polarisation drifts experienced by the photons propagating through the fibers are compensated with liquid crystal variable retarders. This ensures continuous and stable QKD operation with an average QBER of 6.4% and a final key rate of 109 bits/s.

quant-ph↗

arXiv2020

Entanglement demonstration on board a nano-satellite

Global quantum networks for secure communication can be realised using large fleets of satellites distributing entangled photon-pairs between ground-based nodes. Because the cost of a satellite depends on its size, the smallest satellites will be most cost-effective. This paper describes a miniaturised, polarization entangled, photon-pair source operating on board a nano-satellite. The source violates Bell's inequality with a CHSH parameter of 2.6 $\pm$ 0.06. This source can be combined with optical link technologies to enable future quantum communication nano-satellite missions.

quant-ph↗

arXiv2020

Thermo-mechanical design for a miniaturized quantum light source on board the SpooQy-1 CubeSat

This paper presents the thermo-mechanical design of the quantum light source on board SpooQy-1, a 3U CubeSat that was deployed from the International Space Station on 17th June 2019. SpooQy-1 is a technology demonstrator for space-based quantum networks. The on-board light source generates and detects polarization-entangled photon pairs to validate its in-orbit performance. Entangled photons are generated using spontaneous parametric down-conversion (SPDC) necessitating stringent dimensional stability and temperature requirements. Under laboratory conditions these requirements are routinely met using off-the-shelf laboratory mounts and alignment mechanisms. However, when facing harsh environments such as the vibration during rocket launch or temperature changes due to fluctuating illumination conditions, custom thermo-mechanical solutions are required. In this work, the development and in-orbit demonstration of an isostatic payload mount is discussed. This mounting approach enables future space missions with quantum instruments on resource-constrained CubeSat platforms with limited thermal control capabilities.

physics.app-ph↗

arXiv2019

Broadband pumped polarization entangled photon-pair source in a linear beam displacement interferometer

We experimentally demonstrate a source of polarization entangled photon-pairs based on a single periodically-poled potassium titanyl phosphate (PPKTP) crystal pumped with a broadband, free running laser diode.The crystal is placed within a linear beam-displacement interferometer, and emits photon-pairs based ontype-0 spontaneous parametric downconversion (SPDC). We observe pair rates of 0.56 Mpairs/s/mW in a single spatial mode with a polarization visibility of 97.7% over a spectral range of 100 nm. This experiment demonstrates a pathway towards observing Gigacount rates of polarization entangled photon pairs by using high-power free-running laser diodes with fast multiplexed detectors.

quant-ph↗

arXiv2019

Experimental conversion of position correlation into polarization entanglement

This manuscript presents a method to convert position correlation of photon-pairs into polarization entanglement. This is achieved by individually manipulating the polarization state of photons generated in different parts of a non-linear medium and putting them in coherent superposition. This concept is experimentally demonstrated using photon-pairs produced by spontaneous parametric down conversion (SPDC). The method was used to implement a compact source producing an observed photon-pair rate of 120,000/s/mW with an entanglement fidelity of 0.99. This method can be extended to any photon-pair generation process with initial position correlation.

quant-ph↗

arXiv2019

Mapping a quantum walk by tuning the coupling coefficient

We present a method to map the evolution of photonic random walks that is compatible with nonclassical input light. Our approach leverages a newly developed flexible waveguide platform to tune the jumping rate between spatial modes, allowing the observation of a range of evolution times in a chip of fixed length. In a proof-of-principle demonstration we reconstruct the evolution of photons through a uniform array of coupled waveguides by monitoring the end-face alone. This approach enables direct observation of mode occupancy at arbitrary resolution, extending the utility of photonic random walks for quantum simulations and related applications.

quant-ph↗

arXiv2019

Experimental comparison of tomography and self-testing in certifying entanglement

We assess the quality of a source of allegedly pure two-qubit states using both standard tomography and methods inspired by device-independent self-testing. Even when the detection and locality loopholes are open, the latter methods can dispense with modelling of the system and the measurements. However, due to finite sample fluctuations, the estimated probability distribution usually does not satisfy the no-signaling conditions exactly. We implement data analysis that is robust against these fluctuations. We demonstrate a high ratio $f_s/f_t\approx 0.988$ between the fidelity estimated from self-testing and that estimated from full tomography, proving high performance of self-testing methods.

quant-ph↗

arXiv2019

Enhancing SPDC brightness using elliptical pump shapes

We report on the use of elliptical pump spatial modes to increase the observed brightness of spontaneous parametric downconversion in critically phase matched crystals. Simulations qualitatively predict this improvement which depends on the eccentricity and orientation of the pump ellipse. We experimentally confirm a factor of two improvement in brightness when compared to the traditional circular-symmetric pump spatial modes. These results support previous theoretical work that proposes the use of elliptical pump modes to enhance the performance of parametric processes in anisotropic materials.

physics.optics↗

arXiv2019

Satellite quantum communications when man-in-the-middle attacks are excluded

An application of quantum communications is the transmission of qubits to create shared symmetric encryption keys in a process called Quantum Key Distribution (QKD). Contrary to public-private key encryption, symmetric encryption is safe from (quantum) computing attacks, i.e. it provides forward security and is thus attractive for secure communications. In this paper we argue that for free-space quantum communications, especially with satellites, if one assumes that man-in-the-middle attacks can be detected by classical channel monitoring techniques, simplified quantum communications protocols and hardware systems can be implemented that offer improved key rates. We term these protocols photon key distribution (PKD) to differentiate them from the standard QKD protocols. We identify three types of photon sources and calculate asymptotic secret key rates for PKD protocols and compare them to their QKD counterparts. Results show that PKD protocols have roughly a factor of two higher rates as only one measurement basis is used and due to the relaxed security assumptions can establish keys at very high losses whereas in QKD the privacy amplification process becomes prohibitive.

quant-ph↗

arXiv2019

Satellite constellations for trusted node QKD networks

Quantum key distribution from satellites becomes particularly valuable when it can be used on a large network and on-demand to provide a symmetric encryption key to any two nodes. A constellation model is described which enables QKD-derived encryption keys to be established between any two ground stations with low latency. This is achieved through the use of low earth orbit, trusted-node QKD satellites which create a buffer of keys with the ground stations they pass over, and geostationary relay satellites to transfer secure combinations of the keys to the ground stations. Regional and global network models are considered and the use of inter-satellite QKD links for balancing keys is assessed.

quant-ph↗

arXiv2019

Elastomeric waveguide on-chip coupling of an encapsulated MoS$_2$ monolayer

We propose a robust photonic platform for encapsulation and addressing of optically active 2D- and nano-materials. Our implementation utilises a monolayer of MoS$_2$ transition metal dichalcogenide embedded in an elastomeric waveguide chip. The structure is manufactured from PDMS using soft-lithography and capable of sustaining a single mode of guided light. We prove that this setup facilitates addressing of the 2D material flake by pumping it with polarised laser light and gathering polarisation-resolved photoluminescence spectra with the extinction ratio of 31, which highlights the potential for selection-rule dependent measurements. Our results demonstrate improved handling of the material and experimental simplification compared to other techniques. Furthermore, inherent elasticity of the host provides an avenue for direct mechanical coupling to embedded materials.

physics.app-ph↗

arXiv2019

Characterizing nonlocal dispersion compensation in deployed telecommunications fiber

Propagation of broadband photon pairs over deployed telecommunication fibers is used to achieve nonlocal dispersion compensation without the deliberate introduction of negative dispersion. This is made possible by exploiting time-energy entanglement and the positive and negative dispersive properties of the fiber. We demonstrate preservation of photon timing correlations after transmission over two multi-segment 10km spans of deployed fiber and up to 80km of laboratory-based fiber.

quant-ph↗

arXiv2018

High fidelity field stop collection for polarization-entangled photon pair sources

We present an experimental demonstration of a bright and high fidelity polarization entangled photon pair source. The source is constructed using two critically phase matched $β$-Barium Borate crystals with parallel optical axes and photon pairs are collected after filtering with a circular field-stop. Near unity fidelities are obtained with detected pair rates exceeding \SI{100000}{pairs/\s/\mW} approaching the brightness of practical quasi-phase matched entangled photon sources. We find that the brightness scales linearly with the crystal length. We present models supporting the experimental data and propose strategies for further improvement. The source design is a promising candidate for emerging quantum applications outside of laboratory environments.

quant-ph↗

arXiv2018

Tunable interferometers on a flexible polymer chip

To facilitate the implementation of large scale photonic quantum walks, we have developed a polymer waveguide platform capable of robust, polarization insensitive single mode guiding over a broad range of visible and near- infrared wavelengths. These devices have considerable elasticity, which we exploit to enable tuning of optical behaviour by precise mechanical deformations. In this work, we investigate pairs of beamsplitters arranged as interferometers. These systems demonstrate stable operation over a wide range of phases and reflectivities. We discuss device performance, and present an outlook on flexible polymer chips supporting large, reconfigurable optical circuits.

physics.app-ph↗