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Characterization and Modification of Fiber-based Photon Pair Sources

Jennifer Erskine

Published Nov 14, 2018• 0 citations
Publisher

Abstract

Non-classical light sources are a fundamental building block of quantum photonic technologies. As these photonic technologies require higher numbers of sources and more specific source properties, it becomes increasingly important to characterize and manipulate these sources effectively. This thesis consists of three main projects, all relating to non-classical sources of light. First, we present a method for the rapid measurement of the joint spectral intensity of fiber-based photon pair sources. This method extends the concept of Stimulated Emission Tomography, using a chirped, broadband seed beam to stimulate the four wave mixing interaction. The use of the broadband seed, generated through supercontinuum generation, allows for measurements on the few second timescale and requires only a single pump laser to achieve high resolution joint spectra. In the second project, we use this characterization tool to test a variety of different fiber-based photon pair sources. We use three different modification approaches (bending, squeezing, and tapering) to induce changes in the joint spectral properties of the photon pair sources. We show that each of these modifications has some impact on the joint spectra measured. The resulting joint spectra are very complex, highlighting the importance of performing measurements rather than relying on calculations alone. Lastly, we demonstrate a fast switch for the manipulation of single photons. The switch uses the optical Kerr effect to rotate the polarization state of single photons at ultrafast timescales. The implementation of this switch is experimentally straightforward, using a commercial, single mode fiber as the Kerr medium and nJ level pump powers. We operate at an 80 MHz repetition rate and measure 97% switching efficiency, picosecond level switching speed, and approximately 800:1 signal to noise ratio from the operation.

DOI
10.20381/ruor-22678

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Non-classical light sources are a fundamental building block of quantum photonic technologies. As these photonic technologies require higher numbers of sources and more specific source properties, it becomes increasingly important to characterize and manipulate these sources effectively. This thesis consists of three main projects, all relating to non-classical sources of light. First, we present a method for the rapid measurement of the joint spectral intensity of fiber-based photon pair sources. This method extends the concept of Stimulated Emission Tomography, using a chirped, broadband seed beam to stimulate the four wave mixing interaction. The use of the broadband seed, generated through supercontinuum generation, allows for measurements on the few second timescale and requires only a single pump laser to achieve high resolution joint spectra. In the second project, we use this characterization tool to test a variety of different fiber-based photon pair sources. We use three different modification approaches (bending, squeezing, and tapering) to induce changes in the joint spectral properties of the photon pair sources. We show that each of these modifications has some impact on the joint spectra measured. The resulting joint spectra are very complex, highlighting the importance of performing measurements rather than relying on calculations alone. Lastly, we demonstrate a fast switch for the manipulation of single photons. The switch uses the optical Kerr effect to rotate the polarization state of single photons at ultrafast timescales. The implementation of this switch is experimentally straightforward, using a commercial, single mode fiber as the Kerr medium and nJ level pump powers. We operate at an 80 MHz repetition rate and measure 97% switching efficiency, picosecond level switching speed, and approximately 800:1 signal to noise ratio from the operation.
Photon pair sources have wide ranging applications in a variety of quantum photonic experiments and protocols. Many of these protocols require well controlled spectral correlations between the two output photons. However, due to low cross-sections, measuring the joint spectral properties of photon pair sources has historically been a challenging and time-consuming task. Here, we present an approach for the real-time measurement of the joint spectral properties of a fiber-based four wave mixing source. We seed the four wave mixing process using a broadband chirped pulse, studying the stimulated process to extract information regarding the spontaneous process. In addition, we compare stimulated emission measurements with the spontaneous process to confirm the technique's validity. Joint spectral measurements have taken many hours historically and several minutes with recent techniques. Here, measurements have been demonstrated in 5-30 s depending on resolution, offering substantial improvement. Additional benefits of this approach include flexible resolution, large measurement bandwidth, and reduced experimental overhead.
Controlling the spatial and spectral–temporal properties of photon pairs produced in artificially structured materials is fundamental to the realization of numerous photonic quantum information applications. Tailoring the joint spectral properties of photon pairs is of particular importance for applications relying on time–energy entanglement, high-visibility interference, and heralding. Yet measuring the joint spectral properties is a time-consuming task requiring coincidence counting, typically resulting in low-resolution spectra with a poor signal-to-noise ratio. In this work we capture the joint spectral correlations of photon pairs that would be produced in optical fibers with unprecedented speed, resolution, and signal-to-noise ratio, using a scheme based on stimulated four-wave mixing. We also illustrate that this technique can be used in engineering joint spectral correlations, making it a powerful tool for studying quantum states.
In quantum photonics, the requirement for photon pairs with specific quantum states has led to a demand for a fast, high resolution and accurate characterisation of photon pair sources. However, current quantum methods of characterisation suffer from limited accuracy and resolution, and only consist of intensity measurements that prevent access to phase-sensitive measurement photon pairs. A promising tool that addresses these challenges, uses the classical analogue of nonlinear processes to stimulate photon generation, yielding much higher count rates that allows for a higher resolution and accurate photon pair source characterisation. Furthermore, this classical measurement allows for an innovative method to perform full phase-sensitive quantum tomography of photon pair sources that was previous thought to be experimentally challenging to obtain. This thesis examines and compares the quantum and classical method of characterisation of spectral correlations in χ^3 nonlinear devices; namely two integrated silicon nanowires, and a highly nonlinear fibre. In the first study, we use stimulated nonlinear process to confirm the speed-up of characterisation of photon pairs and demonstrate that additional resolution is gained when compared to the traditional coincidence measurements with no increase in measurement time. By applying this technique with phase-sensitive amplification to another identical silicon nanowire, the first phase sensitive measurements are presented showing details that are otherwise hidden in traditional intensity measurements. Furthermore, phase-sensitive measurement of a highly nonlinear fibre shows that phase-sensitive measurements have excellent sensitivity to small features when compared to the traditional intensity measurements.
Ultrafast quantum light pulses with durations of 1ps and below show great promise as information carriers in quantum communication and computation. In future they may also be used to probe physical processes at ultrashort timescales with a resolution beyond the limits of Heisenberg uncertainty. This thesis focuses on the creation and manipulation of quantum light pulses with second order nonlinear optical processes in optical waveguides. In chapter 1. Introduction, we lead the reader towards this work's topic by giving a brief qualitative overview over the EPR paradox, quantum entanglement, three-wave-mixing and quantum light pulses. Chapter 2. Basic concepts familiarizes the reader with the physical concepts and mathematical tools underlying this thesis. In chapter 3. Spectral engineering, we discuss the requirements to produce separable photon pair states with group velocity matching and examine several nonlinear materials widely used for optical waveguide inscription for their suitability to group velocity matching in the telecom wavelength regime. In chapter 4. A PP-KTP waveguide as parametric downconversion source, we give the basic spontaneous parametric downconversion source setup, as well as some initial measurements to characterize the single photon detectors, to demonstrate the production of correlated photons, and to determine the phasematching properties of the PP-KTP source. In the following chapter, 5. Fiber spectrometer, we present the single-photon fiber spectrometer. We discuss the experimental setup and calibration of the device, and measure the joint spectrum of photon pairs from our PP-KTP source. Finally, we investigate the behavior of a joint spectrum measurement of a high mean photon number source with binary detectors. In 6. Two-mode squeezed vacuum source, we characterize spectral correlations of an ultrafast SPDC source with the second order correlation function g². We present the g² measurement results and show that we can control the spectral correlations to produce a two-mode squeezed vacuum state of light. We then determine the mean photon number and gain of the source. 7. Quantum pulse gate: While the previous chapters focus on the creation and characterization of ultrafast quantum pulses of light, we now propose a way to manipulate the mode structure of a given quantum light state. We discuss the concept of an active optical filter sensitive to spectral/temporal pulse shape: The quantum pulse gate. In the final chapter 8. Conclusion and outlook, we recapitulate the main results of this thesis and provide a few pointers towards possibilities for future research building on it.
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.