# Publications

## 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.

## 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.

## 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.

## Non-Classical States Of Light: Toward Scalable Photonic Quantum Networks

2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) (2013)

## On-chip low loss heralded source of pure single photons

Optics Express **21** (2013) 13522-13532

A key obstacle to the experimental realization of many photonic quantum-enhanced technologies is the lack of low-loss sources of single photons in pure quantum states. We demonstrate a promising solution: generation of heralded single photons in a silica photonic chip by spontaneous four-wave mixing. A heralding efficiency of 40%, corresponding to a preparation efficiency of 80% accounting for detector performance, is achieved due to efficient coupling of the low-loss source to optical fibers. A single photon purity of 0:86 is measured from the source number statistics without narrow spectral filtering, and confirmed by direct measurement of the joint spectral intensity. We calculate that similar high-heralded-purity output can be obtained from visible to telecom spectral regions using this approach. On-chip silica sources can have immediate application in a wide range of single-photon quantum optics applications which employ silica photonics. © 2013 Optical Society of America.

## Boson sampling on a photonic chip.

Science **339** (2013) 798-801

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.

## High-efficiency Bragg Grating Enhanced On-chip Photon-number-resolving Detectors

2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS EUROPE AND INTERNATIONAL QUANTUM ELECTRONICS CONFERENCE (CLEO EUROPE/IQEC) (2013)

## Direct observation of sub-binomial light

2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS EUROPE AND INTERNATIONAL QUANTUM ELECTRONICS CONFERENCE (CLEO EUROPE/IQEC) (2013)

## Measuring nonlocal coherence with weak-field homodyne detection

2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS EUROPE AND INTERNATIONAL QUANTUM ELECTRONICS CONFERENCE (CLEO EUROPE/IQEC) (2013)

## 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.

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

NEW JOURNAL OF PHYSICS **15** (2013) ARTN 045012

## Attosecond Sampling of Arbitrary Optical Waveforms

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

Optics Express **21** (2013) 22657-22670

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.

## Mutual interferometric characterization of a pair of independent electric fields.

Opt Lett **38** (2013) 5299-5302

We demonstrate a novel interferometric characterization scheme that allows the complete reconstruction of two interfering electric fields. The phase profiles of both beams, and their relative phase, can be retrieved simultaneously as a function of any degree of freedom in which it is possible to shear one of the beams. The method has applications in wavefront sensing or ultrashort-pulse measurement, especially also in the domain of extreme light sources where it is difficult to generate a reference field or to replicate the beam in order to perform a self-referencing measurement. We demonstrate the technique experimentally by measuring simultaneously two ultrashort pulses in a single laser shot.

## 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) ARTN 055013

## Quantum enhanced multiple phase estimation.

Phys Rev Lett **111** (2013) 070403-

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.

## Linear optical quantum computing in a single spatial mode.

Phys Rev Lett **111** (2013) 150501-

We present a scheme for linear optical quantum computing using time-bin-encoded qubits in a single spatial mode. We show methods for single-qubit operations and heralded controlled-phase (cphase) gates, providing a sufficient set of operations for universal quantum computing with the Knill-Laflamme-Milburn [Nature (London) 409, 46 (2001)] scheme. Our protocol is suited to currently available photonic devices and ideally allows arbitrary numbers of qubits to be encoded in the same spatial mode, demonstrating the potential for time-frequency modes to dramatically increase the quantum information capacity of fixed spatial resources. As a test of our scheme, we demonstrate the first entirely single spatial mode implementation of a two-qubit quantum gate and show its operation with an average fidelity of 0.84±0.07.

## 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.