Publications


Mapping coherence in measurement via full quantum tomography of a hybrid optical detector

Nature Photonics 6 (2012) 364-368

L Zhang, HB Coldenstrodt-Ronge, A Datta, G Puentes, JS Lundeen, XM Jin, BJ Smith, MB Plenio, IA Walmsley

Quantum states and measurements exhibit wave-like (continuous) or particle-like (discrete) character. Hybrid discrete-continuous photonic systems are key to investigating fundamental quantum phenomena1-3, generating superpositions of macroscopic states4, and form essential resources for quantum-enhanced applications5 such as entanglement distillation6,7 and quantum computation8, as well as highly efficient optical telecommunications9,10. Realizing the full potential of these hybrid systems requires quantum-optical measurements sensitive to non-commuting observables such as field quadrature amplitude and photon number11-13. However, a thorough understanding of the practical performance of an optical detector interpolating between these two regions is absent. Here, we report the implementation of full quantum detector tomography, enabling the characterization of the simultaneous wave and photon-number sensitivities of quantum-optical detectors. This yields the largest parameterization to date in quantum tomography experiments, requiring the development of novel theoretical tools. Our results reveal the role of coherence in quantum measurements and demonstrate the tunability of hybrid quantum-optical detectors. © 2012 Macmillan Publishers Limited. All rights reserved.


Continuous phase stabilization and active interferometer control using two modes

JOURNAL OF MODERN OPTICS 59 (2012) 42-45

G Jotzu, TJ Bartley, HB Coldenstrodt-Ronge, BJ Smith, IA Walmsley


Continuous phase stabilization and active interferometer control using two modes

Journal of Modern Optics 59 (2012) 42-45

G Jotzu, TJ Bartley, HB Coldenstrodt-Ronge, BJ Smith, IA Walmsley

We present a computer-based active interferometer stabilization method that can be set to an arbitrary phase difference and does not rely on modulation of the interfering beams. The scheme utilizes two orthogonal modes propagating through the interferometer with a constant phase difference between them to extract a common relative phase and generate a linear feedback signal. Switching times of 50ms over a range of 0-6π radians at 632.8 nm are experimentally demonstrated. The relative interferometer phase can be stabilized up to several days to within ± 3°. © 2011 Copyright Taylor and Francis Group, LLC.


Mapping coherence in measurement via full quantum tomography of a hybrid optical detector

Nature Photonics (2012)

L Zhang, HB Coldenstrodt-Ronge, A Datta, G Puentes, JS Lundeen, X-M Jin, BJ Smith, MB Plenio, IA Walmsley


Multiphoton state engineering by heralded interference between single photons and coherent states

Phys. Rev. A American Physical Society 86 (2012) 043820-043820

TJ Bartley, G Donati, JB Spring, X-M Jin, M Barbieri, A Datta, BJ Smith, 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.


High-performance single-photon generation with commercial-grade optical fiber

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

C Söller, O Cohen, BJ Smith, IA Walmsley, C Silberhorn

High-quality quantum sources are of paramount importance for the implementation of quantum technologies. We present here a heralded single-photon source based on commercial-grade polarization-maintaining optical fiber. The heralded photons exhibit a purity of at least 0.84 and an unprecedented heralding efficiency into a single-mode fiber of 85%. The birefringent phase-matching condition of the underlying four-wave mixing process can be controlled mechanically to optimize the wavelength tuning needed for interfacing multiple sources, as is required for large-scale entanglement generation. © 2011 American Physical Society.


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 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


Integrated photonic sensing

New Journal of Physics 13 (2011)

N Thomas-Peter, NK Langford, A Datta, L Zhang, BJ Smith, JB Spring, BJ Metcalf, HB Coldenstrodt-Ronge, M Hu, J Nunn, IA Walmsley

Loss is a critical roadblock to achieving photonic quantum-enhanced technologies. We explore a modular platform for implementing integrated photonics experiments and consider the effects of loss at different stages of these experiments, including state preparation, manipulation and measurement. We frame our discussion mainly in the context of quantum sensing and focus particularly on the use of loss-tolerant Holland-Burnett states for optical phase estimation. In particular, we discuss spontaneous four-wave mixing in standard birefringent fibre as a source of pure, heralded single photons and present methods of optimizing such sources. We also outline a route to programmable circuits that allows the control of photonic interactions even in the presence of fabrication imperfections and describe a ratiometric characterization method for beam splitters, which allows the characterization of complex circuits without the need for full process tomography. Finally, we present a framework for performing state tomography on heralded states using lossy measurement devices. This is motivated by a calculation of the effects of fabrication imperfections on precision measurement using Holland-Burnett states. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.


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.


Complete characterization of weak-homodyne photon-number-resolving detectors: Applications to non-classical photonic state reconstructions

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

HB Coldenstrodt-Ronge, G Puentes, O Cohen, F Noriega, X Yang, JS Lundeen, BJ Smith, IA Walmsley

A novel detector combining phase sensitivity and photon-number resolution is experimentally characterized by measuring its positive-operator-value measurement set. Direct application to tomographic reconstruction of heralded single-photon states is presented. ©2010 Optical Society of America.


Phase-controlled photonic quantum circuits in laser written integrated optics

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

N Thomas-Peter, BJ Smith, D Kundys, PGR Smith, IA Walmsley

We present a direct UV-written integrated photonic circuit with on-chip phase control through a thermo-optic phase shifter. An arbitrary beam splitter and a two-photon NOON state are demonstrated with high visibility interference. © 2010 Optical Society of America.


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.


Two-photon interference and commutation relations

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

BJ Smith, N Thomas-Peter, IA Walmsley

We experimentally demonstrate spectral-temporal two-photon interference at a beam splitter with pure-state separable photons. This shows the bosonic nature of light, characterizing creation and annihilation operators. © 2010 Optical Society of America.


COMPONENTS FOR MULTI-PHOTON NON-CLASSICAL STATE PREPARATION AND MEASUREMENT

LASER SPECTROSCOPY (2010) 148-157

G Puentes, L Zhang, H Coldenstrodt-Ronge, O Cohen, BJ Smith, N Thomas-Peter, IA Walmsley


Full characterization of quantum optical detectors

Pacific Rim Conference on Lasers and Electro-Optics, CLEO - Technical Digest (2009)

HB Coldenstrodt-Ronge, AP Worsley, JS Lundeen, A Feito, KL Pregnell, C Silberhorn, PJ Mosley, BJ Smith, G Puentes, N Thomas-Peter, TC Ralph, J Eisert, MB Plenio, IA Walmsley

Full characterization of detectors in the quantum regime is presented. We determine the POVM elements of a photon-number-resolving detector. A method for precise calibration of detector efficiency is demonstrated. © 2009 IEEE.


Compact coupler designs for quantum optical circuits produced by direct UV writing

CLEO/Europe - EQEC 2009 - European Conference on Lasers and Electro-Optics and the European Quantum Electronics Conference (2009)

DO Kundys, JC Gates, CBE Gawith, BJ Smith, JS Lundeen, LA Walmsley, PGR Smith


Quantum-enhanced phase estimation in the presence of loss

CLEO/Europe - EQEC 2009 - European Conference on Lasers and Electro-Optics and the European Quantum Electronics Conference (2009)

R Demkowicz-Dobrzański, M Kacprowicz, W Wasilewski, K Banaszek, U Dorner, BJ Smith, JS Lundeen, A Ian Walmsley


Photon pair generation in birefringent optical fibers

Optics Express 17 (2009) 23589-23602

BJ Smith, P Mahou, O Cohen, JS Lundeen, IA Walmsley

We study both experimentally and theoretically the generation of photon pairs by spontaneous four-wave mixing (SFWM) in standard biréfringent optical fibers. The ability to produce a range of two-photon spectral states, from highly correlated (entangled) to completely factorable, by means of cross-polarized biréfringent phase matching, is explored. A simple model is developed to predict the spectral state of the photon pair which shows how this can be adjusted by choosing the appropriate pump bandwidth, fiber length and birefringence. Spontaneous Raman scattering is modeled to determine the tradeoff between SFWM and background Raman noise, and the predicted results are shown to agree with experimental data. © 2009 Optical Society of America.

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