Publications


Recursive quantum detector tomography

NEW JOURNAL OF PHYSICS 14 (2012) ARTN 115005

L Zhang, A Datta, HB Coldenstrodt-Ronge, X-M Jin, J Eisert, MB Plenio, IA Walmsley


Turning classical states quantum with linear optics and photon counting

Optics InfoBase Conference Papers (2012)

TJ Bartley, G Donati, XM Jin, JB Spring, M Barbieri, BJ Smith, A Datta, L Zhang, IA Walmsley

We demonstrate a method to transmute classical light into a quantum state without invoking any nonlinear optical processes. Using a tunable beam splitter and photon number resolving measurement, we create a novel non-Gaussian state. © 2011 Optical Society of America.


Quantum storage of a photonic polarization qubit in a solid.

Physical review letters 108 (2012) 190504-

M Gündoğan, PM Ledingham, A Almasi, M Cristiani, H de Riedmatten

We report on the quantum storage and retrieval of photonic polarization quantum bits onto and out of a solid state storage device. The qubits are implemented with weak coherent states at the single photon level, and are stored for a predetermined time of 500 ns in a praseodymium doped crystal with a storage and retrieval efficiency of 10%, using the atomic frequency comb scheme. We characterize the storage by using quantum state tomography, and find that the average conditional fidelity of the retrieved qubits exceeds 95% for a mean photon number μ=0.4. This is significantly higher than a classical benchmark, taking into account the poissonian statistics and finite memory efficiency, which proves that our crystal functions as a quantum storage device for polarization qubits. These results extend the storage capabilities of solid state quantum light matter interfaces to polarization encoding, which is widely used in quantum information science.


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.


Turning classical states quantum with linear optics and photon counting

CLEO: Science and Innovations, CLEO_SI 2012 (2012)

TJ Bartley, G Donati, XM Jin, JB Spring, M Barbieri, BJ Smith, A Datta, L Zhang, IA Walmsley

We demonstrate a method to transmute classical light into a quantum state without invoking any nonlinear optical processes. Using a tunable beam splitter and photon number resolving measurement, we create a novel non-Gaussian state. © 2011 Optical Society of America.


Turning classical states quantum with linear optics and photon counting

CLEO: Applications and Technology, CLEO_AT 2012 (2012)

TJ Bartley, G Donati, XM Jin, JB Spring, M Barbieri, BJ Smith, A Datta, L Zhang, IA Walmsley

We demonstrate a method to transmute classical light into a quantum state without invoking any nonlinear optical processes. Using a tunable beam splitter and photon number resolving measurement, we create a novel non-Gaussian state. © 2011 Optical Society of America.


Controlled-NOT gate operating with single photons

APPLIED PHYSICS LETTERS 100 (2012) ARTN 211103

MA Pooley, DJP Ellis, RB Patel, AJ Bennett, KHA Chan, I Farrer, DA Ritchie, AJ Shields


Multipulse Addressing of a Raman Quantum Memory: Configurable Beam Splitting and Efficient Readout

Phys. Rev. Lett. American Physical Society 108 (2012) 263602-263602

KF Reim, J Nunn, X-M Jin, PS Michelberger, TFM Champion, DG England, KC Lee, WS Kolthammer, NK Langford, IA Walmsley


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


Quantum metrology with imperfect states and detectors

Physical Review A - Atomic, Molecular, and Optical Physics 83 (2011)

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

Quantum enhancements of precision in metrology can be compromised by system imperfections. These may be mitigated by appropriate optimization of the input state to render it robust, at the expense of making the state difficult to prepare. In this paper, we identify the major sources of imperfection of an optical sensor: input state preparation inefficiency, sensor losses, and detector inefficiency. The second of these has received much attention; we show that it is the least damaging to surpassing the standard quantum limit in a optical interferometric sensor. Further, we show that photonic states that can be prepared in the laboratory using feasible resources allow a measurement strategy using photon-number-resolving detectors that not only attain the Heisenberg limit for phase estimation in the absence of losses, but also deliver close to the maximum possible precision in realistic scenarios including losses and inefficiencies. In particular, we give bounds for the tradeoff between the three sources of imperfection that will allow true quantum-enhanced optical metrology. © 2011 American Physical Society.


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 discord and quantum computing - An appraisal

International Journal of Quantum Information 9 (2011) 1787-1805

A Datta, A Shaji

We discuss models of computing that are beyond classical. The primary motivation is to unearth the cause of non-classical advantages in computation. Completeness results from computational complexity theory lead to the identification of very disparate problems, and offer a kaleidoscopic view into the realm of quantum enhancements in computation. Emphasis is placed on the "power of one qubit" model, and the boundary between quantum and classical correlations as delineated by quantum discord. A recent result by Eastin on the role of this boundary in the efficient classical simulation of quantum computation is discussed. Perceived drawbacks in the interpretation of quantum discord as a relevant certificate of quantum enhancements are addressed. © 2011 World Scientific Publishing Company.


Building multimode quantum optical networks

Optics InfoBase Conference Papers (2011)

IA Walmsley, J Nunn, N Langford, A Datta, L Zhang, B Smith, N Thomas-Peter, J Spring, B Metcalf, D Englland, K Reim, P Michelberger, T Champion

Light offers a route to the generation of macroscopic quantum states based on both multiple photons (e.g. Schrödinger kittens) and multiple modes (e.g. Dicke-Werner). The combination of these approaches affords new possibilities in both fundamental physics and in technological applications. The routes to building scalabl networks embodying such systems from feasible laboratory resources will be discussed. © 2011 OSA.


Single-photon-level memory at room temperature

2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference, CLEO EUROPE/EQEC 2011 (2011)

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

Quantum memories capable of storing single photons are essential building blocks for quantum information processing, enabling the storage and transfer of quantum information over long distances [1]. Devices operating at room temperature can be deployed on a large scale and integrated into existing photonic networks, but so far warm quantum memories have been susceptible to noise at the single photon level [2]. Using a fundamentally different approach to quantum memories, i.e. the recently developed far off-resonant Raman memory scheme [3], we present a highly efficient room-temperature memory that is able to operate with a low unconditional noise floor in the quantum regime, something that no other room-temperature memory has been able to demonstrate before. The quantum memory is operated in warm caesium vapour, and the long-lived, 9.2 GHz hyperfine-split states of the caesium D2 line serve as ground and storage states. The laser fields are 15 GHz detuned from the excited state. © 2011 IEEE.


High-speed electrical control of a solid-state photonic quantum interface

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

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


Semiconductor Single Photon Devices for Quantum Information Processing

QUANTUM COMMUNICATION, MEASUREMENT AND COMPUTING (QCMC): THE TENTH INTERNATIONAL CONFERENCE 1363 (2011)

O Thomas, CL Salter, AJ Bennett, RM Stevenson, MA Pooley, MB Ward, RB Patel, AB de la Giroday, N Skold, I Farrer, CA Nicoll, DA Ritchie, AJ Shields


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.


Photonic quantum memories

2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference, CLEO EUROPE/EQEC 2011 (2011)

IA Walmsley, J Nunn, N Langford, K Reim, P Michelberger, T Champion

We survey the state of the art in photonic quantum memories, describing different approaches to the storage and manipulation of quantum light beams and pulses, and expected applications. © 2011 IEEE.

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