Single photons are essential resource for quantum communication and quantum information processing, which can carry quantum information to distant locations. A promising scheme for single photon generation is the heralded single photon source (HSPS), which is based on the generation of correlated photon pairs (CPPs). Utilizing the quantum correlation property of the CPPs, one photon of the CPP is detected providing an electrical signal to herald the other photon as a single photon output. Recently, 1.5 μm CPP generation through spontaneously four wave-mixing (SFWM) in fiber has focused much attention, which provides a practical way to realize 1.5 μm fiber-based HSPS. The quality of a HSPS is described by the preparation efficiency and g(2)(0). In the fiber-based HSPS, the preparation efficiency is determined by the loss of the filtering and splitting system and the noise photons generated by spontaneously Raman scattering (SpRS). Considering the impact of the SpRS can be reduced by cooling the fiber and optimizing the frequency detuning of filtering and splitting system, the loss of the filtering and splitting system may give a theoretical up-limit of the preparation efficiency. In this paper, using commercial fiber components, we realize a high quality HSPS based on cooled fiber with a preparation efficiency of 80% under a g(2)(0) of0.06, showing its great potential in the application of quantum information technology.
Single photons are essential resource for quantum communication and quantum information processing, which can carry quantum information to distant locations. A promising scheme for single photon generation is the heralded single photon source (HSPS), which is based on the generation of correlated photon pairs (CPPs). Utilizing the quantum correlation property of the CPPs, one photon of the CPP is detected providing an electrical signal to herald the other photon as a single photon output. Recently, 1.5 μm CPP generation through spontaneously four wave-mixing (SFWM) in fiber has focused much attention, which provides a practical way to realize 1.5 μm fiber-based HSPS. The quality of a HSPS is described by the preparation efficiency and g(2)(0). In the fiber-based HSPS, the preparation efficiency is determined by the loss of the filtering and splitting system and the noise photons generated by spontaneously Raman scattering (SpRS). Considering the impact of the SpRS can be reduced by cooling the fiber and optimizing the frequency detuning of filtering and splitting system, the loss of the filtering and splitting system may give a theoretical up-limit of the preparation efficiency. In this paper, using commercial fiber components, we realize a high quality HSPS based on cooled fiber with a preparation efficiency of 80% under a g(2)(0) of0.06, showing its great potential in the application of quantum information technology.
A high-quality heralded single-photon source (HSPS) at 1.5 μm is experimentally demonstrated based on spontaneous four wave-mixing in a piece of dispersion-shifted fiber cooled by liquid nitrogen. To improve the up-limit of the preparation efficiency of the HSPS, commercial dense wavelength-division multiplexing components are used to reduce the loss of the filtering and splitting system between the fiber and the single-photon detectors. As a result, a preparation efficiency of 80% is realized under a g (2) (0) of 0.06 in our HSPS experimental system, which is the best performance for the fiber-based HSPS at 1.5 μm reported so far.
In this paper, a heralded single photon source (HSPS) at 1.5 μm with two independent orthogonally polarized outputs is realized based on a piece of polarization maintaining dispersion shifted fiber (PM-DSF). The HSPS is based on two scalar spontaneous four wave mixing (SFWM) processes along the two fiber polarization axes, while two vector SFWM processes are suppressed due to the high birefringence in the PM-DSF. The preparation efficiencies of the two independent outputs are about 73.7% and 69.1%, respectively, under a second-order correlation function g(2)(0) of 0.059. The indistinguishability between the two independent heralded single photons is demonstrated by Hong-Ou-Mandel (HOM) interference with a visibility of 78.9%, showing its great potential in quantum optics experiments and applications of quantum information.
A 1.5 um synchronous heralded single photon source (HSPS) is experimentally demonstrated based on dispersion shifted fiber and commercial fiber components in the paper. Experimental results show that both the preparation efficiency and the conditional second order correlation function g2(0) increase with the pump light level. Between the two important parameters, tradeoff should be taken in order to obtain a high quality fiber-based synchronous HSPS. A synchronous HSPS with a prepare efficiency of >60% and g2(0)<0.06 is achieved, the multi-photon probability of which is reduced by a factor of more than 16 compared with the Poissonian light sources. The experimental results show a great potential of the fiber-based synchronous HSPS for quantum information applications.
Single photon sources (SPSs) play important roles in quantum communication and quantum information processing. Spontaneous four wave mixing (SFWM) in optical fibers provides a promising way to realize practical heralded single photon sources (HSPSs), since it is compatible with current techniques of optical communications. In this paper, two independent HSPSs at 1.5μm band are realized in one polarization maintaining dispersion shifted fiber (PM-DSF) utilizing its large birefringence. When pulsed pump light passes through an optical fiber, two kinds of SFWM will take place simultaneously. One is scalar processes, in which two annihilated pump photons and generated photon pair are all polarized along the same fiber polarization axis. The other is vector processes, in which two annihilated pump photons are polarized along different fiber polarization axes, either for the two photons of the generated pair. In the PM-DSF, the large birefringence generates obvious walk-off effect on the two pump polarization components, which leads to an effective suppression of the vector processes. Hence, by proper pump polarization, correlated photon pairs (CPPs) with different polarization directions can be generated independently by the two scalar processes, which can be used to realize two independent HSPSs. The indistinguishability of the heralded photons generated by the two independent sources is demonstrated by an experiment of Hong-Ou-Mandel (HOM) interference. Using a fiber coupler as the beam splitter, a visibility of HOM dip of 76% is achieved, showing their potential on quantum information.
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.