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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Characterization of a fiber based heralded single photon source at telecom wavelength

Nuno A. Silva, Armando N. Pinto

Published Nov 18, 2013• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.2026203

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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.
We investigate theoretically the spontaneous four-wave mixing (FWM) process that occurs in optical waveguides, as a source of quantum correlated photon-pairs. We consider that the waveguide used to implement the spontaneous FWM process presents a high value of nonlinear parameter, γ = 93.4 W-1m-1, and a non-negligible value of loss coefficient, α = 133.3 dB/m. Moreover, the theoretical model also consider the Raman scattering that inevitably accompanies the FWM process, and generates time-uncorrelated (noise) photon pairs. We use the coincident-to-accidental ratio (CAR) as a figure of merit of the photon pair source, and we were able to observe a CAR of the order of 65 in a high loss regime. After, we use the time-correlated photon pairs generated by the spontaneous FWM process to implement a heralded single photon source at waveguide output. In this scenario, the detection of one photon of the pair heralds the presence of the other photon. The quality of the source was studied by the evaluation of the second order coherence function for one photon of the pair conditioned by the detection of its twin photon. We observe that the presence of a high loss coefficient tends to improve the quality of the photon source, when compared with the lossless regime, even considering the Raman noise photons. We obtain a value for the conditional second order coherence function of the order of 0.11 in absence of loss, and a value of 0.03 for a loss coefficient of 133.3 dB/m.
We demonstrate a source of photon pairs with widely separated wavelengths, 810 and 1548 nm, generated through spontaneous four-wave mixing in a microstructured fiber. The second-order autocorrelation function g((2))(0) was measured to confirm the nonclassical nature of a heralded single-photon source constructed from the fiber. The microstructured fiber presented herein has the interesting property of generating photon pairs with wavelengths suitable for a quantum repeater able to link free-space channels with fiber channels, as well as for a high-quality telecommunication wavelength heralded single photon source. It also has the advantage of potentially low-loss coupling into standard optical fiber. These reasons make this photon pair source particularly interesting for long-distance quantum communication.
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.
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.
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){g}^{(2)}(0)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.
We experimentally demonstrate a heralded single photon source at 1290 nm by exploiting the spontaneous four wave mixing in a taper-drawn micro/nano-fiber (MNF). Because the frequency detuning between the pump and heralded single photons is ∼58  THz, the contamination by Raman scattering is significantly reduced at room temperature. Since the MNF is naturally connected to standard single mode fibers via fiber tapers, the source would be compatible with the existing fiber networks. When the emission rate of heralded signal photons is about 4.6 kHz, the measured second-order intensity correlation function g(2)(0) is 0.017±0.002, which is suppressed by a factor of more than 55, relative to the classical limit.