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).
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).
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.
Summary form only given. Generation of correlated photon pairs by spontaneous parametric four-wave mixing (FWM) in photonic crystal fibres (PCF) opens the way for many applications, particularly for fibre-based quantum communication devices. In degenerate FWM, two pump photons at frequency ωρare annihilated to produce simultaneously a signal photon at ωsand an idler photon at ωi, such as ωs+ ωi= 2ωp(energy conservation). This process occurs with high efficiency only if the phase-matching condition (i.e. momentum conservation) is satisfied [1].The group-velocity dispersion (GVD) of the silica-core PCF used in our experiments (fig.1.a, inset) has been experimentally determined by a time-of-flight measurement for wavelengths from 740 nm to 930 nm. The result is consistent with the analytical calculation provided by Saitoh and Koshiba [2]. Particularly, we estimate the zero-dispersion wavelength of the PCF: λZDW = 834 nm. From this GVD measurement, we deduce the phasematching curve given in fig.1.a, on which we note that there is a vertical asymptote at λp - 832 nm. Thus, FWM should not be observed if the pump wavelength λp is lower than this limit value. Our experimental observations are very different from this prediction. Indeed, by injecting inside the fundamental mode of a 1.3-m-long PCF hyperbolic secant picosecond pulses (with an intensity-FWHM duration of about 2 ps) at λinj = 815 nm with a 80-MHz repetition rate, we actually observe a spontaneous parametric FWM phenomenon (with sidebands generated around 925 nm and 745 nm), and moreover with a clear frequency dissymmetry with respect to the injected pulse frequency (ωinj = 2ππc/λinj) (fig.1.b): generated signal and idler frequencies ωs and ωi do not satisfy ωs + ωi = 2ωinj.A systematic study of this frequency-dissymmetric phenomenon shows that it occurs only when the injected pulse peak power (Pinj) is higher than a threshold value Pth, and that Pth increases when λinj decreases from λZDW. This variation of Pth as a function of λinj can be understood by considering the key role played by self-phase modulation (SPM) in the observed phenomenon. Indeed, the frequency-dissymmetric signal and idler waves are generated only when the spectral broadening (due to SPM) of the injected pulses is sufficient to reach a threshold wavelength λth (i.e. a threshold frequency ωth) of the order of 832 nm. This value corresponds to the position of the vertical asymptote in the phase-matching curve. Finally, we observe that ωs + ωi - 2ωth when Pinj Pth. All these observations highlight the fact that, when the injected pulse peak power Pinj is high enough, spectral broadening by SPM is sufficient to get a part of the injected pulse spectrum higher than the threshold value λth, and thus this part of the spectrum can play the role of pump in an efficient FWM process (i.e. with the phasematching condition satisfied). It is important to note that this phenomenon advantageously presents an extension of the spectral range on which an efficient FWM process can occur: instead of being restricted to injected pulse wavelengths higher than the zero-dispersion wavelength λZDW of the fibre (or at least, higher than a threshold wavelength λth of the order of λZDW), experimenters can work with lower wavelengths, providing the availability of a high enough peak power and/or interaction length to sufficiently broaden (by SPM) the injected pulse spectrum.
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.
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.
We study the generation of photon pairs by spontaneous four-wave mixing (SFWM) in photonic crystal optical fibers. We show that it is possible to engineer two-photon states with specific spectral entanglement properties suitable for quantum information processing applica- tions. In this work, we concentrate on inner-loop phasematching, for which signal and idler photons can be generated at a small spectral separation from the pump, while sufficiently removed from the pump to ensure the absence of background photons produced by spontaneous Raman scattering. We present specific source designs which permit both symmetric and asymmetric factorable states, which allow herald- ing of pure-state, single-photon wavepackets without the need for spectral post-filtering. Additionally, we present numerical results for the conversion efficiency, where we compare sources based on inner- and outer-loop phasematching.
Based on the dispersion property of a given photonic crystal fiber (PCF), we study how to directly generate photon pairs with minimized frequency correlation via pulse-pumped spontaneous four-wave mixing. After illustrating why the intensity correlation function g (2) of individual signal (idler) photons can be used to reliably characterize the frequency correlation of photon pairs, we numerically investigate the dependence of g (2) under various kinds of experimental conditions. The results show that to minimize the frequency correlation, the experimental parameters should be properly optimized by balancing the influences of the high-order dispersion and the intrinsic sinc oscillation of phase matching function, apart from the satisfaction of the specified phase matching condition and the usage of transform-limited pump pulses. To verify the calculated results, we conduct two series of experiments by regulating the pump to respectively satisfy the asymmetric and symmetric group velocity matching conditions in our 0.6 m-long PCF. In both cases, the measured values of g (2) are less than the calculated results due to the inhomogeneity of the PCF; however, the experimental results qualitatively agree with the numerical simulations. Our investigation is very useful in fiber-based quantum state engineering.