In this paper, we use the Wigner function in order to determine the photon number statistics of a heralded single-photon source constructed from a photon-pair source using four-wave mixing in optical fibers. Having this result, we discuss the performance of a quantum key distribution system when this kind of single-photon source is employed.
In this paper, we use the Wigner function in order to determine the photon number statistics of a heralded single-photon source constructed from a photon-pair source using four-wave mixing in optical fibers. Having this result, we discuss the performance of a quantum key distribution system when this kind of single-photon source is employed.
Starting from the derived expression of the Bogoliubov transformation for a broadband pulsed pump, we theoretically characterize the heralded single-photon source, which is based on the signal and idler photon pairs produced in optical fiber via spontaneous four-wave mixing by a pulsed pump. The second-order intensity correlation function, heralding efficiency, indistinguishability, and brightness of the heralded single photons as a function of source parameters, are analyzed and discussed. The validity of the theoretical analysis and calculations are confirmed by the experimental results. The investigations are useful for optimizing the source parameters and for developing a single-photon source suitable for quantum-information processing.
Single photon sources are fundamental elements for quantum information science, mainly for quantum key distribution (QKD) applications. Nevertheless, on-demand single photon sources are difficult to obtain, and their implementation in a practical application, such as QKD, demands further developments. A different approach to a single photon source is based on the generation of quantum correlated photon-pairs. In that case, the detection of one photon of the pair heralds the presence of the second photon. In this work, we present a heralded single photon source based on the spontaneous four-wave mixing (FWM) process in optical fibers. We characterize theoretically the heralded photon source by means of the conditional second-order coherence function, gc(2) , with gc(2) = 0 for a perfect single photon source. We observe the nonclassical nature of the source, with gc(2) ≪ 1 far below the classical limit. Our results also show that the Raman scattering in general degrades the quality of the source, due to the generation of uncorrelated photons. However, our findings show that in some frequency regimes we are able to observe a gc(2) ≈ 10−3. Moreover, our analysis shows that the propagation of the photons in a standard single mode fiber (SSMF) does not change significantly the value of gc(2), for distances of L < 25/α, with α the loss coefficient of the SSMF. This results can help to guide the development of single photon sources for quantum information applications.
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.
We present a single-photon source based on the stimulated four-wave mixing process with adjustable linear state of polarization. This source allowed us to generate non-orthogonal states of polarization, that can be used for quantum key distribution experiments. The average number of photon counts as a function of the angle of the analyzer was obtained for two non-orthogonal linear states of polarization. The results show an accurate detection after propagation through a standard single-mode optical fiber with a length equal to 20 km. We also employed a master/slave configuration between two avalanche photodiodes, in order to check the probability of our single-photon source to emit pulses with more than one photon, for an average number of photons per pulse, μ ∼ 0.2, and consequently the probability of a photon to choose one of the paths. Results show that our experimental scheme is suitable for polarization-encoding experiments.
A single-photon source based on the stimulated four-wave mixing (SFWM) process in optical fibers is presented. At the output of the source, the state of polarization (SOP) of the photons can be adjusted in order to obtain any linear polarization. A theoretical model to describe the average photon counts recorded in the avalanche photodiodes (APDs) is presented. The experimental results show an accurate detection of two non-orthogonal linear SOPs after propagation through a 60 km quantum channel, and good agreement with theory. This source, operating in a low power regime, can be used for quantum key distribution (QKD) using polarization-encoding in quantum communications.
We present an experimental characterization of a heralded single-photon source based on spontaneous four-wave mixing in a single-mode microstructure fiber. We measure the second-order correlation function g(2)(0) to be far below the classical, Poissonian limit of 1 over a broad spectral range. With single-spatial-mode output, such a spectrally bright and broad fiber-based heralded single-photon source is suitable for a variety of quantum-information applications.