Pure state single photons are at the heart of optical quantum technologies. We have previously reported the generation of photon pairs by four-wave mixing in a single-mode photonic crystal fibre (PCF) and the use of a new birefringent phase matching scheme which results in the generation of narrowband signal photons, around 0.2 nm in bandwidth, and a factorizable photon pair state with no requirement for spectral filtering, leading to increased count rates. Our source is bright, polarized, and tuneable by variable laser wavelength and fibre parameters. Hong-Ou-Mandel (HOM) interference is observed between photons from two of these sources, a requirement for optical quantum information tasks.
Pure state single photons are at the heart of optical quantum technologies. We have previously reported the generation of photon pairs by four-wave mixing in a single-mode photonic crystal fibre (PCF) and the use of a new birefringent phase matching scheme which results in the generation of narrowband signal photons, around 0.2 nm in bandwidth, and a factorizable photon pair state with no requirement for spectral filtering, leading to increased count rates. Our source is bright, polarized, and tuneable by variable laser wavelength and fibre parameters. Hong-Ou-Mandel (HOM) interference is observed between photons from two of these sources, a requirement for optical quantum information tasks.
Single photons are at the heart of optical quantum technologies such as quantum cryptography, quantum computing and quantum metrology. In this paper we report on the creation of photon pairs via four-wave-mixing (FWM) in birefringent photonic crystal fibres (PCF). It has recently been suggested that for thoroughly engineering the phase matching condition of a birefringent PCF, photon pairs can by created in an intrinsically pure state and narrow band. This can be achieved for cross-polarized phase matching, where two pump photons in the slow axis are converted into a pair of photons, polarized in the orthogonal fast axis. The aim is to produce such intrinsically pure and narrow-band photons and hence to achieve high non-classical interference visibility without any requirement for spectral filtering. In order to experimentally test the purity of our heralded single photons we performed a Hong-Ou-Mandel experiment using heralded signal photons generated in separate PCF sources.
Summary form only given. Spontaneous four wave mixing (FWM) in photonic crystal fibre (PCF) is a promising approach to improved sources of photon pairs for quantum information applications. When pumped by a bright pulsed laser, the χ3nonlinearity of silica can produce correlated photons between a signal and idler wavelength equally spaced above and below the pump in frequency [1]. In PCF, the microstructure surrounding the silica core can be chosen to engineer the dispersion of the fibre, which allows control of the phase-matched signal and idler wavelengths, and even over the degree of spectral correlation between signal and idler - in particular, avoiding all spectral correlation can result in heralded photons in an intrinsically pure quantum state, without the usual need for lossy filtering [2]. Other advantages of PCF include a high brightness with moderate pump power due to the small guided mode area and long interaction length, and good coupling efficiency to single mode fibre for detection because the photons are generated in a similar mode shape.Multi-partite entangled states have many applications across quantum information, but cluster or graph states have recieved particular attention due to the potential for scalable, universal quantum computing based on measurement of a large cluster state [3]. In this paper, we discuss experimental methods for the generation of entanglement between four qubits, using two sources of polarization entangled photon pairs from PCFs setup in Sagnac loops, and quantum interference at a polarizing beamsplitter (Fig. 1a) to perform a fusion operation [4, 5]. We present results for 3 and 4 photon GHZ states with respective fidelities of 75% and 67%, and for a `star-cluster' state equivalent to a rotated GHZ state (Fig. 1b and c). Using this state, we demonstrate logic gates in the measurement based model of quantum computing [6]. We will also discuss progress in using interferometric path degrees of freedom for two of the photons to extend the cluster state with additional qubits (Fig 1d).
The field of quantum optics offers the intriguing possibility of developing new, potentially ground-breaking technologies based on the laws of quantum physics. Single quanta of light, called photons, are at the heart of many such quantum optical technologies. They can serve, for example, as information carriers in quantum cryptography and in quantum-information processing. If such applications are to fulfill their enormous potential, it is essential to have sources available that provide photons in the required quantum states. An established way of generating states of single photons is based on photon-pair sources. In this approach, the detection of one photon of a pair heralds the presence of the partner photon, which can then be further processed. However, to generate heralded photons in pure quantum states, specific conditions have to be met, making the design and implementation of such sources challenging. This thesis presents two new photon-pair sources that allow the heralded generation of single photons with high purity. Both sources are based on optical fiber, which allows high collection efficiencies for the generated states and opens the possibility of easy integration into modern-day fiber networks. Chapter 1 provides an introduction to quantum optical technologies and gives a short overview of current approaches to generate pure single photons. Chapter 2 presents the basic theory on which the experimental work presented in Chapters 3 and 4 is based. It explains the concept of a spectrally broad single photon and how photon purity can be tested using Hong-Ou-Mandel (HOM) interference. Subsequently, the properties of photon-pair states and the heralded generation of single photons from such states are discussed. It is shown that for a heralded photon to be pure, the original photon pair has to be in a factorable state, that is, free of any correlations. The introduced theory is then applied to spontaneous four-wave mixing (SFWM) in optical fibers, a nonlinear process that allows the generation of photon pairs. It is shown how the factorability of the created pairs depends on the fiber's dispersion properties, and the necessary conditions for the generation of factorable states are derived. The chapter concludes with an analysis of the photon statistics of the four-wave mixing process, explaining how statistics and the g(2) correlation function can be used to evaluate the factorability of the SFWM output state. Chapters 3 and 4 contain the main results of this thesis. Chapter 3 presents the first spectrally decorrelated photon-pair source bridging the visible and telecommunication wavelength ranges. The photon pairs are generated by SFWM in a photonic crystal fiber (PCF). The chapter documents the procedure of how the PCF source was designed and fabricated, as well as its characterization based on spectral measurements and photon statistics. One photon of the pair state is generated in the visible at 513.5 nm, where it can be efficiently detected with silicon-based single-photon detectors. It serves as a herald for its broadband partner photon, which is emitted in the telecom wavelength range at 1547 nm. A spectral coincidence measurement shows that the heralded photon exhibits a purity of up to 0.82. At increased pump powers, even the heralded generation of higher photon-number states in the telecom band becomes feasible. In Chapter 4, we use commercial birefringent fiber to herald high-purity photons of picosecond duration at 848 nm. The presented two-source HOM interference experiment shows that the heralded unfiltered state has a purity of at least 0.73. This value is increased to 0.84 with the application of wavelength filters suppressing low-intensity side peaks in the spectrum. The heralded photons are coupled into a single-mode fiber with an efficiency of 85%, the highest reported value to date for factorable sources. It is also shown that a commercial polarization controller can be used to fine-tune the generated wavelengths. This facilitates matching photons from multiple sources, as required for optical quantum computing schemes. Chapter 5 summarizes all results and provides an outlook toward potential future projects.
The preparation of single photons in a pure quantum state is a subject of great interest in physics, enabling the control of light at an unprecedented level. The ease with which certain degrees of freedom of photon states, such as polarisation, can be manipulated, along with the inherent resilience of photons to decoherence, makes them well suited for use as qubits. Recent rapid developments in the transmission and processing of quantum information, as well as the likely technological impact of potential real-world applications such as quantum cryptography and quantum computation, mean that the demand for high performance single photon sources is likely to increase in the near future. One approach to producing single photon states, which are known to be in a well-defined spatio-temporal mode without destructively measuring them, is to take advantage of nonlinear optics. Nonlinear processes can be used to realise frequency conversion by generating a single, correlated pair of photons from an intense pump laser source. The detection of one of the photons from a pair can then be used to indicate the presence of the other photon in the pair, a procedure known as heralding. This thesis describes the development of a source of heralded single photons at 1550 nm, generated directly in the core of a photonic crystal fibre (PCF). By taking advantage of low loss fibre components for the required spectral filtering of the generated photon state, a heralding fidelity of 52% was achieved. The source was designed to be used with a picosecond pulsed fibre laser, making it relatively low cost and maintenance free. With 148 mW of average pump power a heralded output photon rate of 6.4 × 104 s-1 was observed, demonstrating the brightness of the source. The purity of the generated single photons was established by measuring non-classical interference, with a visibility of 70%, between the photons output from this source and a source based on a PPLN waveguide. The fabrication of a series of birefringent PCFs for the generation of spectrally pure state photons at 1550 nm is also discussed. These PCFs will be useful for incorporation in the next generation of high performance, fibre-based photon sources.
The paper reviews a new source of time-correlated photon pairs based on phase matched four-wave mixing in an optical fiber. This source has the potential to replace sources based on a non-linear birefringent crystals and waveguides. It generates linear polarised photon pairs in a narrow bandwidth, single circular mode which can be efficiently coupled into single mode fibres. We generate the parametric gain by pumping with a laser wavelength slightly below the zero dispersion wavelength, in the normal dispersion regime.
In this paper, we demonstrate a source of photon pairs based on four-wave-mixing in photonic crystal fibres. Careful engineering of the phase matching conditions in the fibres enables us to create photon pairs at 597 nm and 860 nm in an intrinsically factorable state showing no spectral correlations. This allows for heralding one photon in a pure state and hence renders narrow band filtering obsolete. The source is narrow band, bright and achieves an overall detection efficiency of up to 21% per photon. For the first time, a Hong-Ou-Mandel interference with unfiltered photons from separate fibre sources is presented.