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.

Coherent storage of temporally multimode light using a spin-wave atomic frequency comb memory


M Guendogan, M Mazzera, PM Ledingham, M Cristiani, H de Riedmatten

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

Optics Express 21 (2013) 22657-22670

B Calkins, PL Mennea, AE Lita, BJ Metcalf, WS Kolthammer, 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.

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


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

Entangbling - quantum correlations in diamond


IA Walmsley, IEEE

Quantum metrology without quantum entanglement

Modern Physics Letters B 26 (2012)

A Datta, A Shaji

We scrutinize the role of quantum entanglement in quantum metrology and discuss recent advances in nonlinear quantum metrology that allow improved scalings of the measurement precision with respect to the available resources. Such schemes can surpass the conventional Heisenberg limited scaling of 1/N of quantum enhanced metrology. Without investing in the preparation of entangled states, we review how systems with intrinsic nonlinearities such as BoseEinstein condensates and light-matter interfaces can provide improved scaling in single parameter estimation. © 2012 World Scientific Publishing Company.

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

Nature Photonics 6 (2012) 41-44

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.

Adaptive slit beam shaping for direct laser written waveguides.

Opt Lett 37 (2012) 470-472

PS Salter, A Jesacher, JB Spring, BJ Metcalf, N Thomas-Peter, RD Simmonds, NK Langford, IA Walmsley, MJ Booth

We demonstrate an improved method for fabricating optical waveguides in bulk materials by means of femtosecond laser writing. We use an LC spatial light modulator (SLM) to shape the beam focus by generating adaptive slit illumination in the pupil of the objective lens. A diffraction grating is applied in a strip across the SLM to simulate a slit, with the first diffracted order mapped onto the pupil plane of the objective lens while the zeroth order is blocked. This technique enables real-time control of the beam-shaping parameters during writing, facilitating the fabrication of more complicated structures than is possible using nonadaptive methods. Waveguides are demonstrated in fused silica with a coupling loss to single-mode fibers in the range of 0.2 to 0.5 dB and propagation loss <0.4 dB/cm.

Turning classical states quantum with linear optics and photon counting

2012 Conference on Lasers and Electro-Optics, CLEO 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. © 2012 OSA.

Synchronizing single photons with quantum memories

2012 Conference on Lasers and Electro-Optics, CLEO 2012 (2012)

J Nunn, NK Langford, T Champion, MR Sprague, PS Michelberger, KC Lee, XM Jin, D England, WS Kolthammer, IA Walmsley

Without deterministic single photon sources, multiphoton rates fall exponentially with the number of photons required, making practical photonics unfeasible. We show how quantum memories improve multiphoton rates by many orders of magnitude. © 2012 OSA.

Generalized multishearing interferometry for the complete multidimensional characterization of optical beams and ultrashort pulses

2012 Conference on Lasers and Electro-Optics, CLEO 2012 (2012)

AS Wyatt, J Biegert, IA Walmsley

We demonstrate increased accuracy and precision in the reconstruction of the multidimensional phase of electromagnetic fields based on multiple spectral shearing interferometry measurements made with shears of an arbitrary magnitude. © 2012 OSA.

Experimental realization of light with time-separated correlations by rephasing amplified spontaneous emission.

Physical review letters 109 (2012) 093602-

PM Ledingham, WR Naylor, JJ Longdell

Amplified spontaneous emission is a common noise source in active optical systems, it is generally seen as being an incoherent process. Here we excite an ensemble of rare earth ion dopants in a solid with a π pulse, resulting in amplified spontaneous emission. The application of a second π pulse leads to a coherent echo of the amplified spontaneous emission that is correlated in both amplitude and phase. For small optical thicknesses, we see evidence that the amplified spontaneous emission and its echo are entangled.

Quantum storage of a photonic polarization qubit in a solid

Physical Review Letters 108 (2012)

M Gündoǧan, 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. © 2012 American Physical Society.

Quantum diamonds

Optics and Photonics News 23 (2012) 34-41

J Nunn, I Walmsley

Compact Continuous-Variable Entanglement Distillation


A Datta, L Zhang, J Nunn, NK Langford, A Feito, MB Plenio, IA Walmsley

Controlled-NOT gate operating with single photons


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

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

Compact continuous-variable entanglement distillation

Physical Review Letters 108 (2012)

A Datta, L Zhang, J Nunn, NK Langford, A Feito, MB Plenio, IA Walmsley