Optimal design for universal multiport interferometers

OPTICA 3 (2016) 1460-1465

WR Clements, PC Humphreys, BJ Metcalf, WS Kolthammer, IA Walmsley

Two-photon quantum walk in a multimode fiber.

Science Advances 2 (2016) e1501054-

H Defienne, M Barbieri, IA Walmsley, BJ Smith, S Gigan

Multiphoton propagation in connected structures-a quantum walk-offers the potential of simulating complex physical systems and provides a route to universal quantum computation. Increasing the complexity of quantum photonic networks where the walk occurs is essential for many applications. We implement a quantum walk of indistinguishable photon pairs in a multimode fiber supporting 380 modes. Using wavefront shaping, we control the propagation of the two-photon state through the fiber in which all modes are coupled. Excitation of arbitrary output modes of the system is realized by controlling classical and quantum interferences. This report demonstrates a highly multimode platform for multiphoton interference experiments and provides a powerful method to program a general high-dimensional multiport optical circuit. This work paves the way for the next generation of photonic devices for quantum simulation, computing, and communication.

Cavity-Enhanced Room-Temperature Broadband Raman Memory.

Physical review letters 116 (2016) 090501-

DJ Saunders, JHD Munns, TFM Champion, C Qiu, KT Kaczmarek, E Poem, PM Ledingham, IA Walmsley, J Nunn

Broadband quantum memories hold great promise as multiplexing elements in future photonic quantum information protocols. Alkali-vapor Raman memories combine high-bandwidth storage, on-demand readout, and operation at room temperature without collisional fluorescence noise. However, previous implementations have required large control pulse energies and have suffered from four-wave-mixing noise. Here, we present a Raman memory where the storage interaction is enhanced by a low-finesse birefringent cavity tuned into simultaneous resonance with the signal and control fields, dramatically reducing the energy required to drive the memory. By engineering antiresonance for the anti-Stokes field, we also suppress the four-wave-mixing noise and report the lowest unconditional noise floor yet achieved in a Raman-type warm vapor memory, (15±2)×10^{-3} photons per pulse, with a total efficiency of (9.5±0.5)%.

Editorial: Building Quantum Networks


IA Walmsley, J Nunn

Nonclassicality Criteria in Multiport Interferometry.

Physical review letters 117 (2016) 213602-

L Rigovacca, C Di Franco, BJ Metcalf, IA Walmsley, MS Kim

Interference lies at the heart of the behavior of classical and quantum light. It is thus crucial to understand the boundaries between which interference patterns can be explained by a classical electromagnetic description of light and which, on the other hand, can only be understood with a proper quantum mechanical approach. While the case of two-mode interference has received a lot of attention, the multimode case has not yet been fully explored. Here we study a general scenario of intensity interferometry: we derive a bound on the average correlations between pairs of output intensities for the classical wavelike model of light, and we show how it can be violated in a quantum framework. As a consequence, this violation acts as a nonclassicality witness, able to detect the presence of sources with sub-Poissonian photon-number statistics. We also develop a criterion that can certify the impossibility of dividing a given interferometer into two independent subblocks.

Efficient and pure femtosecond-pulse-length source of polarization-entangled photons.

Optics express 24 (2016) 10869-10879

MM Weston, HM Chrzanowski, S Wollmann, A Boston, J Ho, LK Shalm, VB Verma, MS Allman, SW Nam, RB Patel, S Slussarenko, GJ Pryde

We present a source of polarization entangled photon pairs based on spontaneous parametric downconversion engineered for frequency uncorrelated telecom photon generation. Our source provides photon pairs that display, simultaneously, the key properties for high-performance quantum information and fundamental quantum science tasks. Specifically, the source provides for high heralding efficiency, high quantum state purity and high entangled state fidelity at the same time. Among different tests we apply to our source we observe almost perfect non-classical interference between photons from independent sources with a visibility of (100 ± 5)%.

Quantum Correlations from the Conditional Statistics of Incomplete Data.

Physical review letters 117 (2016) 083601-083601

J Sperling, TJ Bartley, G Donati, M Barbieri, X-M Jin, A Datta, W Vogel, IA Walmsley

We study, in theory and experiment, the quantum properties of correlated light fields measured with click-counting detectors providing incomplete information on the photon statistics. We establish a correlation parameter for the conditional statistics, and we derive the corresponding nonclassicality criteria for detecting conditional quantum correlations. Classical bounds for Pearson's correlation parameter are formulated that allow us, once they are violated, to determine nonclassical correlations via the joint statistics. On the one hand, we demonstrate nonclassical correlations in terms of the joint click statistics of light produced by a parametric down-conversion source. On the other hand, we verify quantum correlations of a heralded, split single-photon state via the conditional click statistics together with a generalization to higher-order moments. We discuss the performance of the presented nonclassicality criteria to successfully discern joint and conditional quantum correlations. Remarkably, our results are obtained without making any assumptions on the response function, quantum efficiency, and dark-count rate of photodetectors.

Enhanced delegated computing using coherence

PHYSICAL REVIEW A 93 (2016) ARTN 032339

S Barz, V Dunjko, F Schlederer, M Moore, E Kashefi, IA Walmsley

A Cavity-Enhanced Room-Temperature Broadband Raman Memory


PM Ledingham, JHD Munns, SE Thomas, TFM Champion, C Qiu, KT Kaczmarek, A Feizpour, E Poem, IA Walmsley, J Nunn, DJ Saunders, IEEE

Directly comparing entanglement-enhancing non-Gaussian operations


TJ Bartley, IA Walmsley

Interfacing GHz-bandwidth heralded single photons with a warm vapour Raman memory


PS Michelberger, TFM Champion, MR Sprague, KT Kaczmarek, M Barbieri, XM Jin, DG England, WS Kolthammer, DJ Saunders, J Nunn, IA Walmsley

Broadband noise-free optical quantum memory with neutral nitrogen-vacancy centers in diamond

PHYSICAL REVIEW B 91 (2015) ARTN 205108

E Poem, C Weinzetl, J Klatzow, KT Kaczmarek, JHD Munns, TFM Champion, DJ Saunders, J Nunn, IA Walmsley

Precision metrology using weak measurements.

Physical review letters 114 (2015) 210801-

L Zhang, A Datta, IA Walmsley

Weak values and measurements have been proposed as a means to achieve dramatic enhancements in metrology based on the greatly increased range of possible measurement outcomes. Unfortunately, the very large values of measurement outcomes occur with highly suppressed probabilities. This raises three vital questions in weak-measurement-based metrology. Namely, (Q1) Does postselection enhance the measurement precision? (Q2) Does weak measurement offer better precision than strong measurement? (Q3) Is it possible to beat the standard quantum limit or to achieve the Heisenberg limit with weak measurement using only classical resources? We analyze these questions for two prototypical, and generic, measurement protocols and show that while the answers to the first two questions are negative for both protocols, the answer to the last is affirmative for measurements with phase-space interactions, and negative for configuration space interactions. Our results, particularly the ability of weak measurements to perform at par with strong measurements in some cases, are instructive for the design of weak-measurement-based protocols for quantum metrology.

Bad cavities for good memories: Suppression of four-wave mixing in Raman memories

Proceedings of Frontiers in Optics 2015, FIO 2015 (2015)

J Munns, TFM Champion, C Qiu, PM Ledingham, DJ Saunders, IA Walmsley, J Nunn

© 2015 Optical Society of America. Quantum memories enable the synchronisation of photonic operations. Raman memories are a promising platform, but are susceptible to four-wave mixing noise. We present a demonstration of a cavity-enhanced Raman memory, showing suppression of four-wave mixing.

Bad Cavities for Good Memories: Storing Broadband Photons with Low Noise


J Nunn, T Champion, J Munns, C Qiu, D Saunders, IA Walmsley, IEEE

Tomography of photon-number resolving continuous-output detectors


PC Humphreys, BJ Metcalf, T Gerrits, T Hiemstra, AE Lita, J Nunn, SW Nam, A Datta, WS Kolthammer, IA Walmsley

Solid State Spin-Wave Quantum Memory for Time-Bin Qubits.

Physical review letters 114 (2015) 230501-

M Gündoğan, PM Ledingham, K Kutluer, M Mazzera, H de Riedmatten

We demonstrate the first solid-state spin-wave optical quantum memory with on-demand read-out. Using the full atomic frequency comb scheme in a Pr(3+):Y2SiO5 crystal, we store weak coherent pulses at the single-photon level with a signal-to-noise ratio >10. Narrow-band spectral filtering based on spectral hole burning in a second Pr(3+):Y2SiO5 crystal is used to filter out the excess noise created by control pulses to reach an unconditional noise level of (2.0±0.3)×10(-3) photons per pulse. We also report spin-wave storage of photonic time-bin qubits with conditional fidelities higher than achievable by a measure and prepare strategy, demonstrating that the spin-wave memory operates in the quantum regime. This makes our device the first demonstration of a quantum memory for time-bin qubits, with on-demand read-out of the stored quantum information. These results represent an important step for the use of solid-state quantum memories in scalable quantum networks.

Quantum optics: science and technology in a new light.

Science (New York, N.Y.) 348 (2015) 525-530

IA Walmsley

Light facilitates exploration of quantum phenomena that illuminate the basic properties of nature and also enables radical new technologies based on these phenomena. The critical features of quantum light that underpin the opportunities for discovery and application are exceptionally low noise and strong correlations. Rapid progress in both science and technology has been stimulated by adopting components developed for optical telecommunications and networking, such as highly efficient detectors, integrated photonic circuits, and waveguide- or nanostructure-based nonlinear optical devices. These provide the means to generate new quantum states of light and matter of unprecedented scale, containing many photons with quantum correlations across space and time. Notably, networks with only several tens of photons are already beyond what can be efficiently analyzed by current computers.

Ultrahigh and persistent optical depths of cesium in Kagomé-type hollow-core photonic crystal fibers.

Optics letters 40 (2015) 5582-5585

KT Kaczmarek, DJ Saunders, MR Sprague, WS Kolthammer, A Feizpour, PM Ledingham, B Brecht, E Poem, IA Walmsley, J Nunn

Alkali-filled hollow-core fibers are a promising medium for investigating light-matter interactions, especially at the single-photon level, due to the tight confinement of light and high optical depths achievable by light-induced atomic desorption (LIAD). However, until now these large optical depths could only be generated for seconds, at most once per day, severely limiting the practicality of the technology. Here we report the generation of the highest observed transient (>10(5) for up to a minute) and highest observed persistent (>2000 for hours) optical depths of alkali vapors in a light-guiding geometry to date, using a cesium-filled Kagomé-type hollow-core photonic crystal fiber (HC-PCF). Our results pave the way to light-matter interaction experiments in confined geometries requiring long operation times and large atomic number densities, such as generation of single-photon-level nonlinearities and development of single photon quantum memories.

Joint estimation of phase and phase diffusion for quantum metrology.

Nature communications 5 (2014) 3532-

MD Vidrighin, G Donati, MG Genoni, X-M Jin, WS Kolthammer, MS Kim, A Datta, M Barbieri, IA Walmsley

Phase estimation, at the heart of many quantum metrology and communication schemes, can be strongly affected by noise, whose amplitude may not be known, or might be subject to drift. Here we investigate the joint estimation of a phase shift and the amplitude of phase diffusion at the quantum limit. For several relevant instances, this multiparameter estimation problem can be effectively reshaped as a two-dimensional Hilbert space model, encompassing the description of an interferometer phase probed with relevant quantum states--split single-photons, coherent states or N00N states. For these cases, we obtain a trade-off bound on the statistical variances for the joint estimation of phase and phase diffusion, as well as optimum measurement schemes. We use this bound to quantify the effectiveness of an actual experimental set-up for joint parameter estimation for polarimetry. We conclude by discussing the form of the trade-off relations for more general states and measurements.