Milja Medic

Demonstration of a Quantum Controlled-NOT Gate in the Telecommunications Band

We present the first quantum controlled-not (cnot) gate realized using a fiber-based indistinguishable photon-pair source in the 1.55 microm telecommunications band. Using this free-space cnot gate, all four Bell states are produced and fully characterized by performing quantum-state tomography, demonstrating the gates unambiguous entangling capability and high fidelity. Telecom-band operation makes this cnot gate particularly suitable for quantum-information-processing tasks that are at the interface of quantum communication and linear optical quantum computing.

Fiber-based telecommunication-band source of degenerate entangled photons

We have constructed and experimentally characterized what we believe to be the first fiber-based source of degenerate polarization-entangled photon pairs in the telecommunication band. Our source design utilizes bichromatic pump pulses and an optical-fiber Sagnac loop aligned to deterministically separate degenerate photon pairs at a central wavelength. The source exhibits 0.997+/-0.006 fidelity with a maximally entangled state, measured using quantum state tomography. When reconfigured to produce identical photon pairs, the source exhibits a Hong-Ou-Mandel interference visibility of 0.97+/-0.04.

Characterization of fiber-generated entangled photon pairs with superconducting single-photon detectors

We demonstrate the suitability of fiber-generated entangled photon pairs for practical quantum communications in the telecom band by measuring their properties with superconducting single-photon detectors that produce negligible dark counts. The photon pairs are created in approximately 5-ps duration windows at 50 MHz rate while the detectors are operated in ungated free running mode. We obtain a coincidence to accidental-coincidence ratio >80 with raw photon-counting data, i.e., without making any post-measurement corrections. Using a previously demonstrated counter-propagating scheme we also produce polarization-entangled photon pairs at 50-MHz rate, which in coincidence detection directly yield two-photon interference with a fringe visibility >98%.

Multiple-qubit quantum state visualization

We present a method for graphically visualizing any two-qubit quantum state. This tool, based on the Poincaré sphere, provides an unambiguous, intuitive, and useful compliment to photonic state tomography.

Experimental Characterization of a Telecommunications-Band Quantum Controlled- not Gate

The quantum controlled-not gate is an example of the maximally entangling gate, which is a broad class of operations that are necessary for scalable linear optics quantum computation. Here, we characterize a telecommunications-wavelength (1550 nm) quantum controlled- not gate, and for the first time, experimentally bound its process fidelity by measuring its operation in two complementary polarization bases. The gates final process fidelity F is given by 91% ¿ F ¿ 95%.

Effects of polarization-dependent loss and fiber birefringence on photon-pair entanglement in fiber-optic channels

Quantum communication requires that photon-pair entanglement be preserved as the photons are distributed to remote locations. We model the effects of loss and birefringence on polarization-entangled photon pairs propagating in optical fibers.

Entangled photon polarimetry

We construct an entangled photon polarimeter capable of displaying an evolving quantum state in real time. We use it to record a 3 frame-per-second live video of a two-photon states transition from separability to entanglement.

Engineering a telecom-band controlled-NOT gate

We experimentally characterize a linear optics, telecom-band quantum controlled-NOT gate using a fiber-based source of degenerate photon pairs, and bound its process fidelity to 0.907 = Fp = 0.948.

Linear optics quantum process tomography

Linear optics quantum computation (LOQC) eliminates the need for a strongly nonlinear photon‐photon interaction to create entangling gate operation, instead replacing it with the fundamental nonlinearity of quantum measurement. The LOQC gates rely on nondeterministic performance conditioned on detection or nondetection of events in ancillary modes, and are normally characterized in the multiple photon basis in which they are designed to operate. By assuming that the underlying physical structure of an LOQC gate is linear, and therefore describable entirely by it’s action on single‐photon input states, it is possible to exponentially reduce the number of parameters necessary to characterize that gate.

Quantum Logic Gates with Fiber-Generated Entanglement in the Telecommunications Band

Quantum states and gates in the 1.5-micron wavelength range can leverage the existing telecommunications infrastructure for communications-based quantum information processing. We present the latest results on characterization of a telecommunications-wavelength linear optics quantum controlled-NOT gate. Article not available.