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Fiber Transport of Entangled Photonic Qudits

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Optical quantum communication (QC) has the potential to address the demands of high-speed and maximally-secure communication. In optical QC, information is encoded onto the state of photons. By using entangled photonic states, QC protocols can support fundamentally secure communication for applications which cannot use classical information. To increase the information throughput, it is desirable to encode more information per photon with high-dimensional entangled states of dimension $d$ (qudits). Generating high-dimensional entangled qudits at telecommunication wavelengths is particularly advantageous because existing fiber-optic infrastructure can be used for QC. Specifically, QC is well-suited for photonic transport in the O-band (1260--1360 nm) or C-band (1530--1565 nm) where transmission loss in fiber in minimal. Encoding information in temporal modes is desirable due to resilience of these states after propagation---an essential characteristic for long-distance communications. In this thesis, we generate high-dimensional time-bin-entangled states in the O-band using spontaneous four-wave mixing (SFWM) in single-mode optical fiber (SMF). We generate and verify entangled qubits (d = 2), qutrits (d = 3), and ququarts (d = 4) with 100-ps time-bin separation by performing quantum state tomography (QST). We make all required two-dimensional projective measurements for QST by mapping any two time bins onto orthogonal polarizations. We verify the generation of states with fidelities ~99%, ~ 97%, and ~ 93% for maximally entangled qubit, qutrit, and ququart states, respectively. This is the first tomography performed on a time-bin-entangled ququart state. We also generate various other qubit and qutrit entangled states with fidelities ~96% and ~94%, respectively. The ability to distribute the entanglement over fiber is essential for long-distance QC and information transfer. To demonstrate the ability to distribute these states over long distances, we expand the experiment to generate and distribute one photon of the two-photon entangled states over 25 km of standard SMF (Corning SMF-28). Using the same measurement system, we verify maximally time-bin entangled qubits and qutrits, with 200-ps separated time bins, over this distance with fidelities of ~98% and ~92%, respectively. This is the first distribution of time-bin-entangled qutrits over at least 25 km and first quantum state tomography performed on distributed time-bin entanglement. We also verify the distribution of other entangled qubits and qutrits over 25 km of SMF-28 with respective fidelities of ~98% and ~95%. Finally, we characterize noise generated by the measurement system for QST and propose modifications to allow for the generation, distribution, and measurement of entangled qudits in the C-band.

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  • 04/11/2018
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