# Publications

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

Nature communications **5** (2014) 5584-

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.

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

Optics letters **39** (2014) 6142-6145

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.

## Nonclassical light manipulation in a multiple-scattering medium.

Optics letters **39** (2014) 6090-6093

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

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.

## Quantum discord in quantum information theory - From strong subadditivity to the mother protocol

Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) **6745 LNCS** (2014) 188-197

Positivity of quantum discord is shown to be equivalent to the strong sub additivity of the von Neumann entropy. This leads to a connection between the mother protocol of quantum information theory [17] and quantum discord. We exploit this to show that discord is a measure coherence in the performance of the mother protocol. Since the mother protocol is a unification of an important class of problems (those that are bipartite, unidirectional and memoryless), we show discord to be a measure of coherence in these protocols. Our work generalizes an earlier operational interpretation of discord provided in terms of quantum state merging [10]. © 2014 Springer-Verlag Berlin Heidelberg.

## Heralded single photon storage in a room-temperature, broadband quantum memory

2014 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) (2014)

## Manipulating a non-classical state of light propagating through a multiply scattering medium

2014 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) (2014)

## Characterizing the variation of propagation constants in multicore fiber.

Optics express **22** (2014) 25689-25699

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).

## Photon-pair generation in photonic crystal fibre with a 1.5 GHz modelocked VECSEL

OPTICS COMMUNICATIONS **327** (2014) 39-44

## Storage of up-converted telecom photons in a doped crystal

NEW JOURNAL OF PHYSICS **16** (2014) ARTN 113021

## Quantum teleportation on a photonic chip

NATURE PHOTONICS **8** (2014) 770-774

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

Optics InfoBase Conference Papers (2014)

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-

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.

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

Physical Review Letters **113** (2014)

© 2014 American Physical Society. 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.

## Towards scalable photonics via quantum storage

Proceedings of SPIE - The International Society for Optical Engineering **8636** (2013)

Single photons are a vital resource for optical quantum information processing. efficient and deterministic single photon sources do not yet exist, however. To date, experimental demonstrations of quantum processing primitives have been implemented using non-deterministic sources combined with heralding and/or postselection. Unfortunately, even for eight photons, the data rates are already so low as to make most experiments impracticable. It is well known that quantum memories, capable of storing photons until they are needed, are a potential solution to this 'scaling catastrophe'. Here, we analyze two protocols for generating multiphoton states using quantum memories, showing how the production rates can be enhanced by many orders of magnitude. We identify the time-bandwidth product as a key figure of merit in this connection. © 2013 SPIE.

## Sequential path entanglement for quantum metrology

Scientific Reports **3** (2013)

Path entanglement is a key resource for quantum metrology. Using path-entangled states, the standard quantum limit can be beaten, and the Heisenberg limit can be achieved. However, the preparation and detection of such states scales unfavourably with the number of photons. Here we introduce sequential path entanglement, in which photons are distributed across distinct time bins with arbitrary separation, as a resource for quantum metrology. We demonstrate a scheme for converting polarization Greenberger-Horne-Zeilinger entanglement into sequential path entanglement. We observe the same enhanced phase resolution expected for conventional path entanglement, independent of the delay between consecutive photons. Sequential path entanglement can be prepared comparably easily from polarization entanglement, can be detected without using photon-number-resolving detectors, and enables novel applications.

## Direct observation of sub-binomial light

Physical Review Letters **110** (2013)

Nonclassical states of light are necessary resources for quantum technologies such as cryptography, computation and the definition of metrological standards. Observing signatures of nonclassicality generally requires inferring either the photon number distribution or a quasiprobability distribution indirectly from a set of measurements. Here, we report an experiment in which the nonclassical character of families of quantum states is assessed by direct inspection of the outcomes from a multiplexed photon counter. This scheme does not register the actual photon number distribution; the statistics of the detector clicks alone serve as a witness of nonclassicality, as proposed by Sperling et al.. Our work paves a way for the practical characterization of increasingly sophisticated states and detectors. © 2013 American Physical Society.

## Spin-wave storage of single photon level light fields in a doped solid

Optics InfoBase Conference Papers (2013)

We present optical storage experiments with weak coherent states and heralded single photons by using the atomic frequency comb technique in the long-lived hyperfine levels in an ensemble of Pr3+ ions doped into a solid. © OSA 2013.

## Vanishing quantum discord is not necessary for completely positive maps

PHYSICAL REVIEW A **87** (2013) ARTN 042301

## QUANTUM DISCORD AS A RESOURCE IN QUANTUM COMMUNICATION

INTERNATIONAL JOURNAL OF MODERN PHYSICS B **27** (2013) ARTN 1345041