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Quantum Computing, Communication, and Simulation

State-engineered photon pair generation in graded-index multimode optical fibers (Conference Presentation)

Hamed Pourbeyram, Arash Mafi

Published Mar 14, 2018• 0 citations
Publisher

Abstract

We investigate by theory ultra broadband photon pair generation in graded index multimode optical fibers (GIMFs). It has been shown how the unique dispersive properties of GIMFs provide an opportunity to generate state engineered photon pairs. Our findings hint on interesting features such as: - GIMFs provide can be used as an ultra tunable source of state engineered photon pairs. - Quantum correlations are independent of fiber mode and group number and only depend on spectral separation from the pump. - It is possible to simultaneously generate uncorrelated and correlated photon pair in the same fiber.

DOI
10.1117/12.2289462

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We investigate by theory ultra broadband photon pair generation in graded index multimode optical fibers (GIMFs). It has been shown how the unique dispersive properties of GIMFs provide an opportunity to generate state engineered photon pairs. Our findings hint on interesting features such as: - GIMFs provide can be used as an ultra tunable source of state engineered photon pairs. - Quantum correlations are independent of fiber mode and group number and only depend on spectral separation from the pump. - It is possible to simultaneously generate uncorrelated and correlated photon pair in the same fiber.
We study theoretically the generation of photon pairs with controlled spectral correlations via the four-wave mixing process in graded-index multimode optical fibers (GIMFs). We show that the quantum correlations of the generated photons in GIMFs can be preserved over a wide spectral range for a tunable pump source. Therefore, GIMFs can be utilized as quantum-state-preserving tunable sources of photons. In particular, we have shown that it is possible to generate factorable two-photon states, which allow for heralding of pure-state single photons without the need for narrowband spectral post filtering. We also elaborate on the possibility of simultaneously generating correlated and uncorrelated photon pairs in the same optical fiber.
In this dissertation, multimode optical fibers (MMFs) have been studied as a robust and practical platform for nonlinear optics and quantum optics applications, where by experimental investigations and theoretical calculations we present the quantum and nonlinear virtue of MMFs as a versatile tool for future applications. Our investigations can be summarized in three parts: At first, we have investigated stimulated Raman scattering (SRS) in a long MMF, where more than 20 orders of cascaded Raman peaks have been generated. Observation of beam clean-up despite the highly multimode nature of the fiber and such strong nonlinear interaction despite the large fiber core size are distinguishable results of our work. Next, we have studied four-wave mixing (FWM) via intermodal phase-matching in both step-index and graded-index MMFs (GIMFs). A rigorous and general approach in the theoretical study is presented where some of the commonly used, yet inaccurate approximations are avoided. Experimentally, FWM side-bands with minimal contamination from SRS, and residual pump photons have been generated from quantum vacuum and amplified more than 11 orders of magnitude. Finally, in the last part of this dissertation, we have done a theoretical study of photon-pair generation in a multimode optical fiber (both step-index and GIMF) via intermodal-FWM as a source of state-engineered photon-pairs.
We present a study of spontaneously arisen spatially entangled photon pairs via intermodal four-wave mixing in a graded-index multimode optical fiber. Unique dispersive features of the fiber allow spectral indistinguishability of two different phase-matched processes, producing entangled pairs of spatial qubits. The bases are realized as superpositions of orthogonal transverse fiber modes having opposite parities. In particular, we take into consideration the spectral properties of the processes by examining their joint spectral amplitudes. It is shown that illuminating graded-index optical fiber with different pump wavelengths has an impact upon efficiency parameters accordingly the degree of spatial entanglement and gives rise to photon pairs with various spectral purities. Photons with higher spectral purity enable desired single-photon based interactions to take place, whereas photons with lower spectral purity exhibit hybrid entanglement in frequency and transverse mode. We also discuss Wigner function formalism and parity-displacement-based realization to characterize spatial properties of the states, as well as to verify quantum entanglement through a violation of Clauser-Horne-Shimony-Holt inequality.
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.