Publications


Macroscopic non-classical states and terahertz quantum processing in room-temperature diamond

Nature Photonics (2011)

KC Lee, BJ Sussman, MR Sprague, P Michelberger, KF Reim, J Nunn, NK Langford, PJ Bustard, D Jaksch, IA Walmsley

The nature of the transition between the familiar classical, macroscopic world and the quantum, microscopic one continues to be poorly understood. Expanding the regime of observable quantum behaviour to large-scale objects is therefore an exciting open problem. In macroscopic systems of interacting particles, rapid thermalization usually destroys any quantum coherence before it can be measured or used at room temperature. Here, we demonstrate quantum processing in the vibrational modes of a macroscopic diamond sample under ambient conditions. Using ultrafast Raman scattering, we create an extended, highly non-classical state in the optical phonon modes of bulk diamond. Direct measurement of phonon coherence and correlations establishes the non-classical nature of the crystal dynamics. These results show that optical phonons in diamond provide a unique opportunity for the study of large-scale quantum behaviour, and highlight the potential for diamond as a micro-photonic quantum processor capable of operating at terahertz rates.


Entangling Macroscopic Diamonds at Room Temperature

Science 334 (2011) 1253-1256

KC Lee, MR Sprague, BJ Sussman, J Nunn, NK Langford, X-M Min, T Champion, P Michelberger, KF Reim, D England, D Jaksch, IA Walmsley

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.


Single-photon-level quantum memory at room temperature

Physical Review Letters 107 (2011)

KF Reim, P Michelberger, KC Lee, J Nunn, NK Langford, IA Walmsley

Room-temperature, easy-to-operate quantum memories are essential building blocks for future long distance quantum information networks operating on an intercontinental scale, because devices like quantum repeaters, based on quantum memories, will have to be deployed in potentially remote, inaccessible locations. Here we demonstrate controllable, broadband and efficient storage and retrieval of weak coherent light pulses at the single-photon level in warm atomic cesium vapor using the robust far off-resonant Raman memory scheme. We show that the unconditional noise floor of this technically simple quantum memory is low enough to operate in the quantum regime, even in a room-temperature environment. © 2011 American Physical Society.


Space-time coupling of shaped ultrafast ultraviolet pulses from an acousto-optic programmable dispersive filter

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS 28 (2011) 58-64

DJ McCabe, DR Austin, A Tajalli, S Weber, IA Walmsley, B Chatel


Spatio-temporal focusing of an ultrafast pulse through a multiply scattering medium

NATURE COMMUNICATIONS 2 (2011) ARTN 447

DJ McCabe, A Tajalli, DR Austin, P Bondareff, IA Walmsley, S Gigan, B Chatel


Real-World Quantum Sensors: Evaluating Resources for Precision Measurement

Physical Review Letters 107 (2011) 113603-113603

N Thomas-Peter, B Smith, A Datta, L Zhang, U Dorner, IA Walmsley


Quantum random bit generation using stimulated Raman scattering

Optics Express 19 (2011) 25173-25180

PJ Bustard, D Moffatt, R Lausten, G Wu, IA Walmsley, BJ Sussman

Random number sequences are a critical resource in a wide variety of information systems, including applications in cryptography, simulation, and data sampling. We introduce a quantum random number generator based on the phase measurement of Stokes light generated by amplification of zero-point vacuum fluctuations using stimulated Raman scattering. This is an example of quantum noise amplification using the most noise-free process possible: near unitary quantum evolution. The use of phase offers robustness to classical pump noise and the ability to generate multiple bits per measurement. The Stokes light is generated with high intensity and as a result, fast detectors with high signal-to-noise ratios can be used for measurement, eliminating the need for single-photon sensitive devices. The demonstrated implementation uses optical phonons in bulk diamond. © 2011 Optical Society of America.


Quantum Correlations using Strong Optical Pulses in Rare Earth Ion Doped Crystals

2011 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) (2011)

PM Ledingham, JJ Longdell, IEEE


Quantum correlations using strong optical pulses in rare earth ion doped crystals

Optics InfoBase Conference Papers (2011)

PM Ledingham, JJ Longdell

We use photon echo based protocols with cryogenic rare earth ion dopants to create photon streams with time separated correlations. Theoretically, these streams are non-classically correlated. We present progress toward realizing this correlation. © 2010 Optical Society of America.


Vectorial phase retrieval for linear characterization of attosecond pulses

Physical Review Letters 107 (2011)

O Raz, O Schwartz, D Austin, AS Wyatt, A Schiavi, O Smirnova, B Nadler, IA Walmsley, D Oron, N Dudovich

The waveforms of attosecond pulses produced by high-harmonic generation carry information on the electronic structure and dynamics in atomic and molecular systems. Current methods for the temporal characterization of such pulses have limited sensitivity and impose significant experimental complexity. We propose a new linear and all-optical method inspired by widely used multidimensional phase retrieval algorithms. Our new scheme is based on the spectral measurement of two attosecond sources and their interference. As an example, we focus on the case of spectral polarization measurements of attosecond pulses, relying on their most fundamental property-being well confined in time. We demonstrate this method numerically by reconstructing the temporal profiles of attosecond pulses generated from aligned CO2 molecules. © 2011 American Physical Society.


Quantum metrology with imperfect states and detectors

Physical Review A 83 (2011) 6

A Datta, L Zhang, N Thomas-Peter, U Dorner, B Smith, IA Walmsley


Single-photon-level quantum memory at room temperature.

Phys Rev Lett 107 (2011) 053603-

KF Reim, P Michelberger, KC Lee, J Nunn, NK Langford, IA Walmsley

Room-temperature, easy-to-operate quantum memories are essential building blocks for future long distance quantum information networks operating on an intercontinental scale, because devices like quantum repeaters, based on quantum memories, will have to be deployed in potentially remote, inaccessible locations. Here we demonstrate controllable, broadband and efficient storage and retrieval of weak coherent light pulses at the single-photon level in warm atomic cesium vapor using the robust far off-resonant Raman memory scheme. We show that the unconditional noise floor of this technically simple quantum memory is low enough to operate in the quantum regime, even in a room-temperature environment.


Accuracy measurements and improvement for complete characterization of optical pulses from nonlinear processes via multiple spectral-shearing interferometry

Opt. Express OSA 19 (2011) 25

AS Wyatt, A Grün, PK Bates, O Chalus, J Biegert, IA Walmsley

We demonstrate that multiple spectral-shearing interferometry increases the precision and accuracy of measurements of the spectral phase of a complex pulse (time-bandwidth product $=$ 125) arising from self-phase modulation in a gas filled capillary. We verify that the measured interferometric phase is accurate to 0.1 rad across the full bandwidth by checking the consistency between the spectral phases of each individual shear measurement. The accuracy of extracting pulse parameters (group delay dispersion, pulse duration and peak intensity) for single shear measurements were verified to better than 10% by comparison with the multishear reconstruction. High order space-time coupling is quantified across a single transverse dimension, verifying the suitability of such pulses for use in strong field experiments.


Optimal experiment design for minimal tomography

Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference: 2010 Laser Science to Photonic Applications, CLEO/QELS 2010 (2010)

J Nunn, BJ Smith, G Puentes, JS Lundeen, IA Walmsley

Given an experimental set-up and a fixed number of measurements, how should one take data in order to optimally reconstruct the state of a quantum system? We show how to calculate the optimal design explicitly. © 2010 Optical Society of America.


Amplification of impulsively excited molecular rotational coherence.

Phys Rev Lett 104 (2010) 193902-

PJ Bustard, BJ Sussman, IA Walmsley

We propose a scheme for preparation of high-coherence molecular dynamics which are phase stable with respect to ultrashort pulses. We experimentally demonstrate an example of this scheme using a phase-independent, nanosecond-duration, pump pulse to prepare a rotational coherence in molecular hydrogen. This rotational coherence is made phase stable with respect to a separate source of ultrashort pulses by seeding. The coherence is used to generate spectral broadening of femtosecond probe radiation by molecular phase modulation.


Optimal experiment design for quantum state tomography: Fair, precise, and minimal tomography

Physical Review A - Atomic, Molecular, and Optical Physics 81 (2010)

J Nunn, BJ Smith, G Puentes, IA Walmsley, JS Lundeen

Given an experimental setup and a fixed number of measurements, how should one take data to optimally reconstruct the state of a quantum system? The problem of optimal experiment design (OED) for quantum state tomography was first broached by Kosut. Here we provide efficient numerical algorithms for finding the optimal design, and analytic results for the case of 'minimal tomography'. We also introduce the average OED, which is independent of the state to be reconstructed, and the optimal design for tomography (ODT), which minimizes tomographic bias. Monte Carlo simulations confirm the utility of our results for qubits. Finally, we adapt our approach to deal with constrained techniques such as maximum-likelihood estimation. We find that these are less amenable to optimization than cruder reconstruction methods, such as linear inversion. © 2010 The American Physical Society.


Quantum memories

EUROPEAN PHYSICAL JOURNAL D 58 (2010) 1-22

C Simon, M Afzelius, J Appel, AB de la Giroday, SJ Dewhurst, N Gisin, CY Hu, F Jelezko, S Kroll, JH Muller, J Nunn, ES Polzik, JG Rarity, H De Riedmatten, W Rosenfeld, AJ Shields, N Skoeld, RM Stevenson, R Thew, IA Walmsley, MC Weber, H Weinfurter, J Wrachtrup, RJ Young


Entanglement quantification from incomplete measurements: Applications using photon-number-resolving weak homodyne detectors

New Journal of Physics 12 (2010)

G Puentes, A Datta, A Feito, J Eisert, MB Plenio, IA Walmsley

The certificate of success for a number of important quantum information processing protocols, such as entanglement distillation, is based on the difference in the entanglement content of the system quantum states before and after the protocol. In such cases, effective bounds need to be placed on the entanglement of non-local states consistent with statistics obtained from local measurements. In this paper, we study numerically the ability of a hybrid homodyne detector that combines phase sensitivity and photon-number resolution to set accurate bounds on the entanglement content of two-mode quadrature squeezed states without the need for full state tomography. We show that it is possible to set tight lower bounds on the entanglement of a family of two-mode degaussified states using only a few measurements. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.


Towards high-speed optical quantum memories

Nature Photonics 4 (2010) 218-221

KF Reim, J Nunn, VO Lorenz, BJ Sussman, KC Lee, NK Langford, D Jaksch, IA Walmsley

Quantum memories, capable of controllably storing and releasing a photon, are a crucial component for quantum computers1 and quantum communications2. To date, quantum memories3-6 have operated with bandwidths that limit data rates to megahertz. Here we report the coherent storage and retrieval of sub-nanosecond low-intensity light pulses with spectral bandwidths exceeding 1GHz in caesium vapour. The novel memory interaction takes place through a far off-resonant two-photon transition in which the memory bandwidth is dynamically generated by a strong control field7,8. This should allow data rates more than 100 times greater than those of existing quantum memories. The memory works with a total efficiency of 15%, and its coherence is demonstrated through direct interference of the stored and retrieved pulses. Coherence times in hot atomic vapours are on the order of microseconds, the expected storage time limit for this memory. © 2010 Macmillan Publishers Limited. All rights reserved.


All-electrical coherent control of the exciton states in a single quantum dot

PHYSICAL REVIEW B 82 (2010) ARTN 241301

AB de la Giroday, AJ Bennett, MA Pooley, RM Stevenson, N Skoeld, RB Patel, I Farrer, DA Ritchie, AJ Shields

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