Summary form only given. Single-photon sources are key components in applications of photonic quantum technologies such as quantum key distribution (QKD) [1]. One way of realizing single-photon sources is generation of photon pairs (PP) using spontaneous four-wave mixing (FWM): two photons from a pump p annihilate and create two side-band photons at frequencies determined partly by the energy conservation 2ωρ= ω1+ ω2, where ωp,ω1,ω2are the frequencies of the pump and the two side-bands, respectively, and partly by the phase-matching condition. PP generated spontaneously arrive at indeterministic times but even so, they are useful for QKD because one of the photons can be heralded by detecting the other. The heralded photons are then used for transmitting the quantum key.We propose a higher order mode scheme for generating PP in silica fibers; the side-band photons are generated at center wavelength λ1 = 810 nm in the linearly polarized LP02-mode and at λ2 = 1550 nm in the LP01mode, respectively, by pumping in the LP11-mode at λ7, = 1064 nm, where laser sources are readily available. The 810 nm photon is detectable with relatively cheap and efficient silicon-based single-photon detectors, and the heralded 1550 nm photon is mode-matched with transmission fibers in the low-loss spectral window. We design a silica fiber that phase matches these modes and wavelengths while it also reduces the impacts of core radius fluctuations (CRF) caused by fabrication imperfections; the fiber design is inspired by B. P.-P. Kuo et al. [3] and a sketch of the refractive index profile is shown in Fig. 1(a). The scalar wave equation is solved using appropriate Sellmeier data for Germanium-doped silica and the calculated effective indexes for the three modes versus wavelength are shown in Fig. 1(b) together with the core and cladding refractive indexes; the inset shows the wavelength configuration of the pump and two side-bands; phase matching is achieved when the effective indexes of the three modes lie on a straight line [2]. The nonlinear overlap of the waves in this configuration is approximately 18% of the corresponding overlap where all waves are in the LP01-mode.
Summary form only given. Single-photon sources are key components in applications of photonic quantum technologies such as quantum key distribution (QKD) [1]. One way of realizing single-photon sources is generation of photon pairs (PP) using spontaneous four-wave mixing (FWM): two photons from a pump p annihilate and create two side-band photons at frequencies determined partly by the energy conservation 2ωρ= ω1+ ω2, where ωp,ω1,ω2are the frequencies of the pump and the two side-bands, respectively, and partly by the phase-matching condition. PP generated spontaneously arrive at indeterministic times but even so, they are useful for QKD because one of the photons can be heralded by detecting the other. The heralded photons are then used for transmitting the quantum key.We propose a higher order mode scheme for generating PP in silica fibers; the side-band photons are generated at center wavelength λ1 = 810 nm in the linearly polarized LP02-mode and at λ2 = 1550 nm in the LP01mode, respectively, by pumping in the LP11-mode at λ7, = 1064 nm, where laser sources are readily available. The 810 nm photon is detectable with relatively cheap and efficient silicon-based single-photon detectors, and the heralded 1550 nm photon is mode-matched with transmission fibers in the low-loss spectral window. We design a silica fiber that phase matches these modes and wavelengths while it also reduces the impacts of core radius fluctuations (CRF) caused by fabrication imperfections; the fiber design is inspired by B. P.-P. Kuo et al. [3] and a sketch of the refractive index profile is shown in Fig. 1(a). The scalar wave equation is solved using appropriate Sellmeier data for Germanium-doped silica and the calculated effective indexes for the three modes versus wavelength are shown in Fig. 1(b) together with the core and cladding refractive indexes; the inset shows the wavelength configuration of the pump and two side-bands; phase matching is achieved when the effective indexes of the three modes lie on a straight line [2]. The nonlinear overlap of the waves in this configuration is approximately 18% of the corresponding overlap where all waves are in the LP01-mode.
We model the spectral quantum-mechanical purity of heralded single photons from a photon-pair source based on nondegenerate spontaneous four-wave mixing taking the impact of distributed dispersion fluctuations into account. The considered photon-pair-generation scheme utilizes pump-pulse walk-off to produce pure heralded photons and phase matching is achieved through the dispersion properties of distinct spatial modes in a few-mode silica step-index fiber. We show that fiber-core-radius fluctuations in general severely impact the single-photon purity. Furthermore, by optimizing the fiber design we show that generation of single photons with very high spectral purity is feasible even in the presence of large core-radius fluctuations. At the same time, contamination from spontaneous Raman scattering is greatly mitigated by separating the single-photon frequency by more than 32 THz from the pump frequency. (C) 2017 Optical Society of America
We model the spectral quantum-mechanical purity of heralded single photons from a photon-pair source based on nondegenerate spontaneous four-wave mixing taking the impact of distributed dispersion fluctuations into account. The considered photon-pair-generation scheme utilizes pump-pulse walk-off to produce pure heralded photons and phase matching is achieved through the dispersion properties of distinct spatial modes in a few-mode silica step-index fiber. We show that fiber-core-radius fluctuations in general severely impact the single-photon purity. Furthermore, by optimizing the fiber design we show that generation of single photons with very high spectral purity is feasible even in the presence of large core-radius fluctuations. At the same time, contamination from spontaneous Raman scattering is greatly mitigated by separating the single-photon frequency by more than 32 THz from the pump frequency.
Single photon sources are a key element for quantum computing, quantum key distribution (QKD) and quantum communications. In particular, producing single photons at telecommunications wavelengths is valuable for QKD protocols and would enable realizing the quantum internet. The preferred method for their generation has long been spontaneous down conversion in bulk crystals, which suffers from connection loss to fiber networks. In-fiber spontaneous four-wave mixing provides a viable alternative as a photon pair source due to being compatible with existing fiber networks. We present an all-fiber photon pair source based on degenerate four-wave mixing in a 400 m Highly-Nonlinear fiber, with signal and idler wavelengths generated at 1552.5 nm and 1557 nm respectively. The source consists of CW pump laser operating at 1554.75 nm, which is slightly detuned from the zero group velocity dispersion wavelength into the normal dispersion regime. After pair generation in the highly-nonlinear fiber, three arrayed waveguide gratings are employed to spatially separate signal and idler, and provides a 120 dB pump power reduction. Firstly the source is modelled and experimentally characterized in the well known classical regime of stimulated four-wave mixing. The effect of fiber cooling on spontaneous Raman scattering is modelled and characterized, and a 30% reduction in spontaneous emission is found when cooling the fiber to -77°C. In the low power regime the coincidence to accidental count ratio is simulated and measured. An increase in the coincidence to accidental count ratio is observed when cooling the fiber.
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.
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.
Four-wave mixing in optical fibers has been proven to have many applications within processing of classical optical signals. In addition, recent developments in multimode fibers have made it possible to achieve the necessary phase-matching for efficient four-wave mixing over a very wide bandwidth. Thus, the combination of multimode fiber optics and four-wave mixing is very attractive for various applications. This is especially the case for applications in quantum communication, for example in photon-pair generation. This is the subject of this work, where we discuss the impact of fluctuations in core radius on the quality of the heralded single-photon states and demonstrate experimental results of intermodal spontaneous four-wave mixing for photon-pair generation.