# Publications

## Multi-photon quantum interference in a multi-port integrated photonic device

ArXiv (0)

BJ Metcalf, N Thomas-Peter, JB Spring, D Kundys, MA Broome, P Humphreys, X-M Jin, M Barbieri, WS Kolthammer, JC Gates, BJ Smith, NK Langford, PGR Smith, IA Walmsley

Increasing the complexity of quantum photonic devices is essential for many optical information processing applications to reach a regime beyond what can be classically simulated, and integrated photonics has emerged as a leading platform for achieving this. Here, we demonstrate three-photon quantum operation of an integrated device containing three coupled interferometers, eight spatial modes and many classical and nonclassical interferences. This represents a critical advance over previous complexities and the first on-chip nonclassical interference with more than two photonic inputs. We introduce a new scheme to verify quantum behaviour, using classically characterised device elements and hierarchies of photon correlation functions. We accurately predict the device's quantum behaviour and show operation inconsistent with both classical and bi-separable quantum models. Such methods for verifying multiphoton quantum behaviour are vital for achieving increased circuit complexity. Our experiment paves the way for the next generation of integrated photonic quantum simulation and computing devices.

## High-Fidelity Polarization Storage in a Gigahertz Bandwidth Quantum Memory

ArXiv (0)

DG England, PS Michelberger, TFM Champion, KF Reim, KC Lee, MR Sprague, X-M Jin, NK Langford, WS Kolthammer, J Nunn, IA Walmsley

We demonstrate a dual-rail optical Raman memory inside a polarization interferometer; this enables us to store polarization-encoded information at GHz bandwidths in a room-temperature atomic ensemble. By performing full process tomography on the system we measure up to 97\pm1% process fidelity for the storage and retrieval process. At longer storage times, the process fidelity remains high, despite a loss of efficiency. The fidelity is 86\pm4% for 1.5 \mu s storage time, which is 5,000 times the pulse duration. Hence high fidelity is combined with a large time-bandwidth product. This high performance, with an experimentally simple setup, demonstrates the suitability of the Raman memory for integration into large-scale quantum networks.

## On-chip, photon-number-resolving, telecom-band detectors for scalable photonic information processing

ArXiv (0)

T Gerrits, N Thomas-Peter, JC Gates, AE Lita, BJ Metcalf, B Calkins, NA Tomlin, AE Fox, AL Linares, JB Spring, NK Langford, RP Mirin, PGR Smith, IA Walmsley, SW Nam

Integration is currently the only feasible route towards scalable photonic quantum processing devices that are sufficiently complex to be genuinely useful in computing, metrology, and simulation. Embedded on-chip detection will be critical to such devices. We demonstrate an integrated photon-number resolving detector, operating in the telecom band at 1550 nm, employing an evanescently coupled design that allows it to be placed at arbitrary locations within a planar circuit. Up to 5 photons are resolved in the guided optical mode via absorption from the evanescent field into a tungsten transition-edge sensor. The detection efficiency is 7.2 \pm 0.5 %. The polarization sensitivity of the detector is also demonstrated. Detailed modeling of device designs shows a clear and feasible route to reaching high detection efficiencies.

## Managing photons for quantum information processing

PHILOS T ROY SOC A 361 (2003) 1493-1506

AB U'Ren, E Mukamel, K Banaszek, IA Walmsley

We study distinguishing information in the context of photonic quantum interference tailored for practical implementations of quantum information processing schemes. In particular, we consider the character of single-photon states optimized for multiple-source interference experiments and for experiments relying on Bell-state measurement and arrive at specific design criteria for photons produced by parametric down-conversion. Such states can be realistically implemented with available technology. We describe a novel method for characterizing the mode structure of single photons, and demonstrate it in the context of coherent light.

## Quantum limits of stochastic cooling of a bosonic gas

PHYSICAL REVIEW A 67 (2003) ARTN 061401

D Ivanov, S Wallentowitz, IA Walmsley

## Photon counting with a loop detector.

Opt Lett 28 (2003) 52-54

K Banaszek, IA Walmsley

We propose a design for a photon-counting detector that is capable of resolving multiphoton events. The basic element of the setup is a fiber loop, which traps the input field with the help of a fast electro-optic switch. A single weakly coupled avalanche photodiode is used to detect small portions of the signal field extracted from the loop. We analyze the response of the loop detector to an arbitrary input field and discuss both the reconstruction of the photon-number distribution of an unknown field from the count statistics measured in the setup and the application of the detector in conditional-state preparation.

## Direct measurement of the spatial Wigner function with area-integrated detection.

Opt Lett 28 (2003) 1317-1319

E Mukamel, K Banaszek, IA Walmsley, C Dorrer

We demonstrate experimentally a novel technique for characterizing transverse spatial coherence by using the Wigner distribution function. The method is based on the measurement of interference between a pair of rotated and displaced replicas of the input beam with an area-integrating detector, and it provides an optimal signal-to-noise ratio in regimes when array detectors are not available. We analyze the quantum-optical picture of the presented measurement for single-photon signals and discuss possible applications in quantum information processing.

## Engineering photonic entanglement

(2003) 141-146

AB U'Ren, M De La Cruz, IA Walmsley, R Erdmann

We discuss the use of quasi-phase-matched downconversion to engineer the amount and type of space-time entanglement present in two-photon states. We also discuss a quantum positioning scheme based on a source of multiple entangled photons and report the implementation of this scheme for the case of two entangled photons.

## Measuring ultrafast pulses in the near-ultraviolet using spectral phase interferometry for direct electric field reconstruction

JOURNAL OF MODERN OPTICS 50 (2003) 179-184

P Londero, ME Anderson, C Radzewicz, C Iaconis, IA Walmsley

## Quantum oracles and the optical Bernstein-Vazirani algorithm

OPTICS FOR THE QUALITY OF LIFE, PTS 1 AND 2 4829 (2003) 618-619

I Walmsley, P Londero, C Dorrer, M Anderson, S Wallentowitz, K Banaszek

## Controlling the dephasing of vibrational wavepackets in potassium dimers

ULTRAFAST PHENOMENA XIII 71 (2003) 82-84

I Walmsley, P Londero, M Fitch, S Wallentowitz, L Waxer, C Radzewicz

## Measuring ultrafast pulses in the near-ultraviolet using spectral phase interferometry for direct electric field reconstruction

JOURNAL OF MODERN OPTICS 50 (2003) 179-184

P Londero, ME Anderson, C Radzewicz, C Iaconis, IA Walmsley

## Simplified field wave equations for the nonlinear propagation of extremely short light pulses

PHYSICAL REVIEW A 66 (2002) ARTN 013811

VG Bespalov, SA Kozlov, YA Shpolyanskiy, IA Walmsley

## Applications of nonlinear optics in ultrashort pulse metrology and vice versa

(2002) 225-226

IA Walmsley, E Kosik, C Dorrer, RW Boyd, G Piredda

We discuss recent developments in the characterization of ultrashort optical pulses in the space-time domain and the attosecond regime. These methods may be applied to the study of the dynamics of the field in nonlinear interactions.

## Mind at light speed: A new kind of intelligence

NATURE 416 (2002) 477-477

I Walmsley

## Quantum-enhanced interferometry with large heralded photon-number states

ArXiv (0)

GS Thekkadath, ME Mycroft, BA Bell, CG Wade, A Eckstein, DS Phillips, RB Patel, A Buraczewski, AE Lita, T Gerrits, SW Nam, M Stobińska, AI Lvovsky, IA Walmsley

Quantum phenomena such as entanglement can improve fundamental limits on the sensitivity of a measurement probe. In optical interferometry, a probe consisting of $N$ entangled photons provides up to a $\sqrt{N}$ enhancement in phase sensitivity compared to a classical probe of the same energy. Here, we employ high-gain parametric down-conversion sources and photon-number-resolving detectors to perform interferometry with novel heralded quantum probes of sizes up to $N=8$ (i.e. measuring up to 16-photon coincidences). Our probes are created by injecting heralded photon-number states into an interferometer, and in principle provide quantum-enhanced phase sensitivity even in the presence of significant optical loss. Our work paves the way towards quantum-enhanced interferometry using large entangled photonic states.