# Publications by Joshua Nunn

## Broadband single-photon-level memory in a hollow-core photonic crystal fibre

NATURE PHOTONICS **8** (2014) 287-291

## 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.

## 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.

## 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.

## Scalable photonic quantum networks

Optics InfoBase Conference Papers (2013)

A scalable photonic quantum network will facilitate the preparation of distributed quantum correlations among many light beams, allowing a new regime of state complexity to be accessed, and enabling new quantum-enhanced applications. Such a network can be constructed by means of linear optical operations on pure-state quantum light beams, measurement by efficient photodetectors, and storage in a photonic quantum memory. © OSA 2013.

## Scalable photonic quantum networks

Optics InfoBase Conference Papers (2013)

A scalable photonic quantum network will facilitate the preparation of distributed quantum correlations among many light beams, allowing a new regime of state complexity to be accessed, and enabling new quantum-enhanced applications. Such a network can be constructed by means of linear optical operations on pure-state quantum light beams, measurement by efficient photodetectors, and storage in a photonic quantum memory. © OSA 2013.

## Surpassing the conventional heisenberg limit using classical resources

CLEO: QELS_Fundamental Science, CLEO:QELS FS 2013 (2013)

We report the experimental estimation of the Kerr coefficient of an optical fiber. Using bright classical fields and shot-noise-limited detection, we show the precision scaling of 1=N3=2 in a room-temperature, all-optical system. © 2013 Optical Society of America.

## Storage of light in a hollow-core photonic-crystal fibre

CLEO: QELS_Fundamental Science, CLEO:QELS FS 2013 (2013)

We report the storage and retrieval of broadband optical pulses using a Raman interaction in a room-temperature ensemble of cesium atoms confined in a hollow-core photonic-crystal fibre. © 2013 Optical Society of America.

## Quantum memories and large-scale quantum coherence based on Raman interactions

2013 IEEE Photonics Society Summer Topical Meeting Series, PSSTMS 2013 (2013) 173-174

Applied research into quantum technologies and fundamental research into the foundations of quantum mechanics run hand in hand, since our understanding of quantum correlations both advances, and is advanced by, our ability to control large quantum systems. The off-resonant Raman interaction of light with material systems provides a powerful tool both for quantum information processing, and for accessing macroscopic non-classical states of matter. We describe a recent demonstration of entanglement between the motion of separated diamond crystals at room temperature, and the implementation of quantum memories in cesium vapour that can store and retrieve photons on demand with a time-bandwidth product exceeding 2000, both based on Raman scattering. © 2013 IEEE.

## Compact entanglement distillery using realistic quantum memories

Physical Review A - Atomic, Molecular, and Optical Physics **88** (2013)

We adopt the beam-splitter model for losses to analyze the performance of a recent compact continuous-variable entanglement distillation protocol implemented using realistic quantum memories. We show that the decoherence undergone by a two-mode squeezed state while stored in a quantum memory can strongly modify the results of the preparatory step of the protocol. We find that the well-known method for locally increasing entanglement, phonon subtraction, may not result in entanglement gain when losses are taken into account. Thus, we investigate the critical number mc of phonon subtraction attempts from the matter modes of the quantum memory. If the initial state is not de-Gaussified within mc attempts, the protocol should be restarted to obtain any entanglement increase. Moreover, the condition m c>1 implies an additional constraint on the subtraction beam-splitter interaction transmissivity, viz., it should be about 50% for a wide range of protocol parameters. Additionally, we consider the average entanglement rate, which takes into account both the unavoidable probabilistic nature of the protocol and its possible failure as a result of a large number of unsuccessful subtraction attempts. We find that a higher value of the average entanglement can be achieved by increasing the subtraction beam-splitter interaction transmissivity. We conclude that the compact distillation protocol with the practical constraints coming from realistic quantum memories allows a feasible experimental realization within existing technologies. © 2013 American Physical Society.

## Large-alphabet time-frequency entangled quantum key distribution by means of time-to-frequency conversion.

Opt Express **21** (2013) 15959-15973

We introduce a novel time-frequency quantum key distribution (TFQKD) scheme based on photon pairs entangled in these two conjugate degrees of freedom. The scheme uses spectral detection and phase modulation to enable measurements in the temporal basis by means of time-to-frequency conversion. This allows large-alphabet encoding to be implemented with realistic components. A general security analysis for TFQKD with binned measurements reveals a close connection with finite-dimensional QKD protocols and enables analysis of the effects of dark counts on the secure key size.

## Entang-bling: Observing quantum correlations in room-temperature solids

Journal of Physics: Conference Series **442** (2013)

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. © Published under licence by IOP Publishing Ltd.

## 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.

## Efficient optical pumping and high optical depth in a hollow-core photonic-crystal fibre for a broadband quantum memory

New Journal of Physics **15** (2013)

The generation of large multiphoton quantum states - for applications in computing, metrology and simulation - requires a network of high-efficiency quantum memories capable of storing broadband pulses. Integrating these memories into a fibre offers a number of advantages towards realizing this goal: strong light-matter coupling at low powers, simplified alignment and compatibility with existing photonic architectures. Here, we introduce a large-core kagome-structured hollow-core fibre as a suitable platform for an integrated fibre-based quantum memory with a warm atomic vapour. We demonstrate, for the first time, efficient optical pumping in such a system, where 90 ± 1% of atoms are prepared in the ground state. We measure high optical depths (3 × 104) and narrow homogeneous linewidths (6 ± 2 MHz) that do not exhibit significant transit-time broadening, showing that we can prepare a Λ-level system in a pure state. Our results establish that kagome fibres are suitable for implementing a broadband, room-temperature quantum memory, as well as a range of nonlinear optical effects. © IOP Publishing and Deutsche Physikalische Gesellschaft.

## Enhancing multiphoton rates with quantum memories

Physical Review Letters **110** (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 nondeterministic 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 in detail the benefits of quantum memories for producing multiphoton states, showing how the production rates can be enhanced by many orders of magnitude. We identify the quantity ηB as the most important figure of merit in this connection, where η and B are the efficiency and time-bandwidth product of the memories, respectively. © 2013 American Physical Society.

## Strategies for enhancing quantum entanglement by local photon subtraction

Physical Review A - Atomic, Molecular, and Optical Physics **87** (2013)

Subtracting photons from a two-mode squeezed state is a well-known method to increase entanglement. We analyze different strategies of local photon subtraction from a two-mode squeezed state in terms of entanglement gain and success probability. We develop a general framework that incorporates imperfections and losses in all stages of the process: before, during, and after subtraction. By combining all three effects into a single efficiency parameter, we provide analytical and numerical results for subtraction strategies using photon-number-resolving and threshold detectors. We compare the entanglement gain afforded by symmetric and asymmetric subtraction scenarios across the two modes. For a given amount of loss, we identify an optimized set of parameters, such as initial squeezing and subtraction beam splitter transmissivity, that maximize the entanglement gain rate. We identify regimes for which asymmetric subtraction of different Fock states on the two modes outperforms symmetric strategies. In the lossless limit, subtracting a single photon from one mode always produces the highest entanglement gain rate. In the lossy case, the optimal strategy depends strongly on the losses on each mode individually, such that there is no general optimal strategy. Rather, taking losses on each mode as the only input parameters, we can identify the optimal subtraction strategy and required beam splitter transmissivities and initial squeezing parameter. Finally, we discuss the implications of our results for the distillation of continuous-variable quantum entanglement. © 2013 American Physical Society.

## Storage of light in a hollow-core photonic-crystal fibre

2013 Conference on Lasers and Electro-Optics, CLEO 2013 (2013)

We report the storage and retrieval of broadband optical pulses using a Raman interaction in a room-temperature ensemble of cesium atoms confined in a hollow-core photonic-crystal fibre. © 2013 The Optical Society.

## Surpassing the conventional heisenberg limit using classical resources

2013 Conference on Lasers and Electro-Optics, CLEO 2013 (2013)

We report the experimental estimation of the Kerr coefficient of an optical fiber. Using bright classical fields and shot-noise-limited detection, we show the precision scaling of 1/N3/2 in a room-temperature, all-optical system. © 2013 The Optical Society.