Simultaneous spatial characterization of two independent sources of high harmonic radiation.

Optics letters 39 (2014) 6142-6145

MM Mang, C Bourassin-Bouchet, IA Walmsley

We present the simultaneous spatial characterization of two independent sources of high harmonic radiation from a series of interferograms. Our technique transfers the necessity of replicating and shearing the test beam to a second, independent beam that may be easier to manipulate, and thus opens the possibility to characterize complex light sources. We demonstrate our technique by reconstructing the wavefronts of two high harmonic beams and use this information to study the spatial properties of different quantum paths.

A solid state spin-wave quantum memory for photonic time-Bin qubits

Optics InfoBase Conference Papers (2014)

Nonclassical light manipulation in a multiple-scattering medium.

Optics letters 39 (2014) 6090-6093

H Defienne, M Barbieri, B Chalopin, B Chatel, IA Walmsley, BJ Smith, S Gigan

We investigate the possibility of using a scattering medium as a highly multimode platform for implementing quantum walks. We demonstrate the manipulation of a single photon propagating through a strongly scattering medium using wavefront-shaping technique. Measurement of the scattering matrix allows the wavefront of the photon to be shaped to compensate the distortions induced by multiple scattering events. The photon can thus be directed coherently to a specific output mode. Using this approach, we show how entanglement of a single photon across different modes can be manipulated despite the enormous wavefront disturbance caused by the scattering medium.

Broadband single-photon-level memory in a hollow-core photonic crystal fibre

NATURE PHOTONICS 8 (2014) 287-291

MR Sprague, PS Michelberger, TFM Champion, DG England, J Nunn, X-M Jin, WS Kolthammer, A Abdolvand, PSJ Russell, IA Walmsley

Broadband single-photon-level memory in a hollow-core photonic crystal fibre

Nature Photonics 8 (2014) 287-291

J Nunn, XM Jin, WS Kolthammer, A Abdolvand, PSJ Russell, IA Walmsley

Storing information encoded in light is critical for realizing optical buffers for all-optical signal processing and quantum memories for quantum information processing. These proposals require efficient interaction between atoms and a well-defined optical mode. Photonic crystal fibres can enhance light-matter interactions and have engendered a broad range of nonlinear effects; however, the storage of light has proven elusive. Here, we report the first demonstration of an optical memory in a hollow-core photonic crystal fibre. We store gigahertz-bandwidth light in the hyperfine coherence of caesium atoms at room temperature using a far-detuned Raman interaction. We demonstrate a signal-to-noise ratio of 2.6:1 at the single-photon level and a memory efficiency of 27 ± 1%. Our results demonstrate the potential of a room-temperature fibre-integrated optical memory for implementing local nodes of quantum information networks. © 2014 Macmillan Publishers Limited. All rights reserved.

Characterizing the variation of propagation constants in multicore fiber.

Optics express 22 (2014) 25689-25699

PJ Mosley, I Gris-Sánchez, JM Stone, RJA Francis-Jones, DJ Ashton, TA Birks

We demonstrate a numerical technique that can evaluate the core-to-core variations in propagation constant in multicore fiber. Using a Markov Chain Monte Carlo process, we replicate the interference patterns of light that has coupled between the cores during propagation. We describe the algorithm and verify its operation by successfully reconstructing target propagation constants in a fictional fiber. Then we carry out a reconstruction of the propagation constants in a real fiber containing 37 single-mode cores. We find that the range of fractional propagation constant variation across the cores is approximately ± 2 × 10(-5).

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.

Quantum teleportation on a photonic chip

NATURE PHOTONICS 8 (2014) 770-774

BJ Metcalf, JB Spring, PC Humphreys, N Thomas-Peter, M Barbieri, WS Kolthammer, X-M Jin, NK Langford, D Kundys, JC Gates, BJ Smith, PGR Smith, IA Walmsley

Observing optical coherence across Fock layers with weak-field homodyne detectors.

Nature communications 5 (2014) 5584-

G Donati, TJ Bartley, X-M Jin, M-D Vidrighin, A Datta, M Barbieri, IA Walmsley

Quantum properties of optical modes are typically assessed by observing their photon statistics or the distribution of their quadratures. Both particle- and wave-like behaviours deliver important information and each may be used as a resource in quantum-enhanced technologies. Weak-field homodyne (WFH) detection provides a scheme that combines the wave- and particle-like descriptions. Here we show that it is possible to observe a wave-like property such as the optical coherence across Fock basis states in the detection statistics derived from discrete photon counting. We experimentally demonstrate these correlations using two WHF detectors on each mode of two classes of two-mode entangled states. Furthermore, we theoretically describe the response of WHF detection on a two-mode squeezed state in the context of generalized Bell inequalities. Our work demonstrates the potential of this technique as a tool for hybrid continuous/discrete-variable protocols on a phenomenon that explicitly combines both approaches.

Tradeoff in simultaneous quantum-limited phase and loss estimation in interferometry

PHYSICAL REVIEW A 89 (2014) ARTN 023845

PJD Crowley, A Datta, M Barbieri, IA Walmsley

Storing GHz bandwidth heralded single photons in a room-temperature raman memory: Efficiency and noise

Optics InfoBase Conference Papers (2014)

MR Sprague, K Kacmarek, D Saunders, WS Kolthammer, XM Jin, DG England, IA Walmsley

We store GHz-bandwidth heralded single photons in a room-temperature Raman memory, which is a crucial primitive for scalable quantum photonics. We discuss methods to suppress four-wave mixing noise, which accompanies the retrieved photons. © 2014 Optical Society of America.

Continuous-variable quantum computing in optical time-frequency modes using quantum memories.

Physical review letters 113 (2014) 130502-

PC Humphreys, WS Kolthammer, J Nunn, M Barbieri, A Datta, IA Walmsley

We develop a scheme for time-frequency encoded continuous-variable cluster-state quantum computing using quantum memories. In particular, we propose a method to produce, manipulate, and measure two-dimensional cluster states in a single spatial mode by exploiting the intrinsic time-frequency selectivity of Raman quantum memories. Time-frequency encoding enables the scheme to be extremely compact, requiring a number of memories that are a linear function of only the number of different frequencies in which the computational state is encoded, independent of its temporal duration. We therefore show that quantum memories can be a powerful component for scalable photonic quantum information processing architectures.

A solid state spin-wave quantum memory for photonic time-Bin qubits

Optics InfoBase Conference Papers Part F3-EQEC 2015 (2014)

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

Large-alphabet time-frequency entangled quantum key distribution by means of time-to-frequency conversion.

Opt Express 21 (2013) 15959-15973

J Nunn, LJ Wright, C Söller, L Zhang, IA Walmsley, BJ Smith

We introduce a novel time-frequency quantum key distribution (TFQKD) scheme based on photon pairs entangled in these two conjugate degrees of freedom. The scheme uses spectral detection and phase modulation to enable measurements in the temporal basis by means of time-to-frequency conversion. This allows large-alphabet encoding to be implemented with realistic components. A general security analysis for TFQKD with binned measurements reveals a close connection with finite-dimensional QKD protocols and enables analysis of the effects of dark counts on the secure key size.

Boson sampling on a photonic chip.

Science 339 (2013) 798-801

JB Spring, BJ Metcalf, PC Humphreys, WS Kolthammer, X-M Jin, M Barbieri, A Datta, N Thomas-Peter, NK Langford, D Kundys, JC Gates, BJ Smith, PGR Smith, IA Walmsley

Although universal quantum computers ideally solve problems such as factoring integers exponentially more efficiently than classical machines, the formidable challenges in building such devices motivate the demonstration of simpler, problem-specific algorithms that still promise a quantum speedup. We constructed a quantum boson-sampling machine (QBSM) to sample the output distribution resulting from the nonclassical interference of photons in an integrated photonic circuit, a problem thought to be exponentially hard to solve classically. Unlike universal quantum computation, boson sampling merely requires indistinguishable photons, linear state evolution, and detectors. We benchmarked our QBSM with three and four photons and analyzed sources of sampling inaccuracy. Scaling up to larger devices could offer the first definitive quantum-enhanced computation.

Measuring nonlocal coherence with weak-field homodyne detection


TJ Bartley, G Donati, X-M Jin, A Datta, M Barbieri, IA Walmsley, IEEE

Attosecond Sampling of Arbitrary Optical Waveforms


AS Wyatt, T Witting, A Schiavi, D Fabris, JP Marangos, JWG Tisch, IA Walmsley, IEEE

High quantum-efficiency photon-number-resolving detector for photonic on-chip information processing

Optics Express 21 (2013) 22657-22670

A Lamas-Linares, JB Spring, PC Humphreys, RP Mirin, JC Gates, PGR Smith, IA Walmsley, T Gerrits, SW Nam

The integrated optical circuit is a promising architecture for the realization of complex quantum optical states and information networks. One element that is required for many of these applications is a high-efficiency photon detector capable of photon-number discrimination. We present an integrated photonic system in the telecom band at 1550 nm based on UV-written silica-on-silicon waveguides and modified transition-edge sensors capable of number resolution and over 40 % efficiency. Exploiting the mode transmission failure of these devices, we multiplex three detectors in series to demonstrate a combined 79 % ± 2 % detection efficiency with a single pass, and 88 % ± 3 % at the operating wavelength of an on-chip terminal reflection grating. Furthermore, our optical measurements clearly demonstrate no significant unexplained loss in this system due to scattering or reflections. This waveguide and detector design therefore allows the placement of number-resolving single-photon detectors of predictable efficiency at arbitrary locations within a photonic circuit - a capability that offers great potential for many quantum optical applications. © 2013 Optical Society of America.

Quantum enhanced multiple phase estimation.

Phys Rev Lett 111 (2013) 070403-

PC Humphreys, M Barbieri, A Datta, IA Walmsley

We study the simultaneous estimation of multiple phases as a discretized model for the imaging of a phase object. We identify quantum probe states that provide an enhancement compared to the best quantum scheme for the estimation of each individual phase separately as well as improvements over classical strategies. Our strategy provides an advantage in the variance of the estimation over individual quantum estimation schemes that scales as O(d), where d is the number of phases. Finally, we study the attainability of this limit using realistic probes and photon-number-resolving detectors. This is a problem in which an intrinsic advantage is derived from the estimation of multiple parameters simultaneously.

Entang-bling: Observing quantum correlations in room-temperature solids

Journal of Physics: Conference Series 442 (2013)

K Reim, D England, D Jaksch

Quantum entanglement in the motion of macroscopic solid bodies has implications both for quantum technologies and foundational studies of the boundary between the quantum and classical worlds. Entanglement is usually fragile in room-temperature solids, owing to strong interactions both internally and with the noisy environment. We generated motional entanglement between vibrational states of two spatially separated, millimeter-sized diamonds at room temperature. By measuring strong nonclassical correlations between Raman-scattered photons, we showed that the quantum state of the diamonds has positive concurrence with 98% probability. Our results show that entanglement can persist in the classical context of moving macroscopic solids in ambient conditions. © Published under licence by IOP Publishing Ltd.