Alex McMillan

Narrowband high-fidelity all-fibre source of heralded single photons at 1570 nm.

An all-fibre heralded single photon source operating at 1570 nm has been demonstrated. The device generates correlated photon pairs, widely spaced in frequency, through four-wave mixing in a photonic crystal fibre. Separation of the pair photons and narrowband filtering is all achieved in fibre. The output heralded single photon rate was 9.2 x 10(4) per second, with a counts-to-accidentals ratio of 10.4 and a heralding fidelity of 52 %. Furthermore, narrowband filtering ensured that the output single photon state was near time-bandwidth limited with a coherence length of 4 ps. Such a source is well suited to quantum information processing applications.

Multicolor quantum metrology with entangled photons

Entangled photons can be used to make measurements with an accuracy beyond that possible with classical light. While most implementations of quantum metrology have used states made up of a single color of photons, we show that entangled states of two colors can show supersensitivity to optical phase and path length by using a photonic crystal fiber source of photon pairs inside an interferometer. This setup is relatively simple and robust to experimental imperfections. We demonstrate sensitivity beyond the standard quantum limit and show superresolved interference fringes using entangled states of two, four, and six photons.

Two-photon interference between disparate sources for quantum networking

Quantum networks involve entanglement sharing between multiple users. Ideally, any two users would be able to connect regardless of the type of photon source they employ, provided they fulfill the requirements for two-photon interference. From a theoretical perspective, photons coming from different origins can interfere with a perfect visibility, provided they are made indistinguishable in all degrees of freedom. Previous experimental demonstrations of such a scenario have been limited to photon wavelengths below 900 nm, unsuitable for long distance communication, and suffered from low interference visibility. We report two-photon interference using two disparate heralded single photon sources, which involve different nonlinear effects, operating in the telecom wavelength range. The measured visibility of the two-photon interference is 80 ± 4%, which paves the way to hybrid universal quantum networks.

Effects of self- and cross-phase modulation on photon purity for four-wave-mixing photon pair sources

We consider the effect of self-phase modulation and cross-phase modulation on the joint spectral amplitude of photon pairs generated by spontaneous four-wave mixing. In particular, the purity of a heralded photon from a pair is considered, in the context of schemes that aim to maximise the purity and minimise correlation in the joint spectral amplitude using birefringent phase-matching and short pump pulses. We find that non-linear phase modulation effects will be detrimental, and will limit the quantum interference visibility that can be achieved at a given generation rate. An approximate expression for the joint spectral amplitude with phase modulation is found by considering the group velocity walk-off between each photon and the pump, but neglecting the group-velocity dispersion at each wavelength. The group-velocity dispersion can also be included with a numerical calculation, and it is shown that it only has a small effect on the purity for the realistic parameters considered.

Demonstrating an absolute quantum advantage in direct absorption measurement

Engineering apparatus that harness quantum theory promises to offer practical advantages over current technology. A fundamentally more powerful prospect is that such quantum technologies could out-perform any future iteration of their classical counterparts, no matter how well the attributes of those classical strategies can be improved. Here, for optical direct absorption measurement, we experimentally demonstrate such an instance of an absolute advantage per photon probe that is exposed to the absorbative sample. We use correlated intensity measurements of spontaneous parametric downconversion using a commercially available air-cooled CCD, a new estimator for data analysis and a high heralding efficiency photon-pair source. We show this enables improvement in the precision of measurement, per photon probe, beyond what is achievable with an ideal coherent state (a perfect laser) detected with 100% efficient and noiseless detection. We see this absolute improvement for up to 50% absorption, with a maximum observed factor of improvement of 1.46. This equates to around 32% reduction in the total number of photons traversing an optical sample, compared to any future direct optical absorption measurement using classical light.

Sub-Shot-Noise Transmission Measurement Enabled by Active Feed-Forward of Heralded Single Photons

J. Sabines-Chesterking,1 R. Whittaker,1 S. K. Joshi,2 P. M. Birchall,1 P. A. Moreau,1 A. McMillan,1 H. V. Cable,1 J. L. O’Brien,1 J. G. Rarity,1 and J. C. F. Matthews1 Quantum Engineering Technology Labs, H. H. Wills Physics Laboratory and Department of Electrical & Electronic Engineering, University of Bristol, BS8 1FD, UK. Institute for Quantum Optics and Quantum Information (IQOQI) Austrian Academy of Sciences, Boltzmanngasse 3, A-1090 Vienna, Austria (Dated: November 24, 2016)

Four-Wave Mixing in Single-Mode Optical Fibers

Abstract The efficient generation of single photon and entangled photon states is of considerable interest both for fundamental studies of quantum mechanics and practical applications, such as quantum communications and computation. It is now well known that correlated pairs of photons suitable for such applications can be generated directly in a guided mode of an optical fiber through the nonlinear process of spontaneous four-wave mixing. Detection of one photon of the pair can be used to herald the presence of the other, in order to realise a probabilistic heralded single photon source. Alternatively, both photons can be used directly as an entangled photon pair if the source is designed such that the two photons are correlated in one or more of their degrees of freedom. This chapter provides an overview of the progress that has been made into the development of photon sources based on four-wave mixing in optical fibers. A theoretical model of four-wave mixing is described in Section 12.2, which demonstrates how the dispersion characteristics of an optical fiber influence the properties of the photon pair state that is generated. Section 12.3 focusses on heralded single photon sources operating in both the anomalous and normal dispersion regimes of optical fiber, and highlights several experimental demonstrations of this type of source. Section 12.4 discusses the concept of non-classical interference and the parameters of the generated photons that can influence the interference visibility. Section 12.5 expands upon this discussion to consider two different approaches for preparing photons in pure states that have been used to demonstrate high visibility two-photon interference. Section 12.6 describes several different experimental implementations of entangled photon pair sources. Finally, two practical applications using fiber-based photon sources are presented, with an all-fiber, quantum controlled-NOT gate discussed in Section 12.7, and the potential to use photonic fusion to build up large photonic cluster states outlined in Section 12.8.

Generation of Narrowband 1550 nm Photons in the Anomalous Dispersion Region of a Birefringent PCF

We present simulation results along with measured spectral and dispersion data for a highly birefringent PCF designed to produce naturally narrowband photons, at telecoms wavelengths, from a 1064 nm pump through cross-polarised four-wave mixing.

Fibre source of heralded single photons at 1550nm

We demonstrate a bright fibre source of heralded single photons at 1550 nm with detected rates greater than 10 kilocounts per second. Photon generation and separation is performed in spliced fibre components.

Experimentally exploring compressed sensing quantum tomography

In the light of the progress in quantum technologies, the task of verifying the correct functioning of processes and obtaining accurate tomographic information about quantum states becomes increasingly important. Compressed sensing, a machinery derived from the theory of signal processing, has emerged as a feasible tool to perform robust and significantly more resource-economical quantum state tomography for intermediate-sized quantum systems. In this work, we provide a comprehensive analysis of compressed sensing tomography in the regime in which tomographically complete data is available with reliable statistics from experimental observations of a multi-mode photonic architecture. Due to the fact that the data is known with high statistical significance, we are in a position to systematically explore the quality of reconstruction depending on the number of employed measurement settings, randomly selected from the complete set of data, and on different model assumptions. We present and test a complete prescription to perform efficient compressed sensing and are able to reliably use notions of model selection and cross-validation to account for experimental imperfections and finite counting statistics. Thus, we establish compressed sensing as an effective tool for quantum state tomography, specifically suited for photonic systems.