We report on the development and use of a high heralding-efficiency, single-mode-fiber coupled telecom-band source of entangled photons for quantum technology applications. The source development efforts consisted of theoretical and experimental efforts and we demonstrated a correlated-mode coupling efficiency of 97% ± 2%, the highest efficiency yet achieved for this type of system. We then incorporated these beneficial source development techniques in a Sagnac configured telecom-band entangled photon source that generates photon pairs entangled in both time/energy and polarization degrees of freedom. We made use of these highly desirable entangled states to investigate several promising quantum technologies.
We demonstrate photon-number discrimination using a novel semiconductor detector that utilizes a layer of self-assembled
InGaAs quantum dots (QDs) as an optically addressable floating gate in a GaAs/AlGaAs δ-doped field-effect
transistor. When the QDOGFET (quantum dot, optically gated, field-effect transistor) is illuminated, the internal gate
field directs the holes generated in the dedicated absorption layer of the structure to the QDs, where they are trapped.
The positively charged holes are confined to the dots and screen the internal gate field, causing a persistent change in the
channel current that is proportional to the total number of holes trapped in the QD ensemble. We use highly attenuated
laser pulses to characterize the response of the QDOGFET cooled to 4 K. We demonstrate that different photon-number
states produce well resolved changes in the channel current, where the responses of the detector reflect the Poisson
statistics of the laser light. For a mean photon number of 1.1, we show that decision regions can be defined such that
the QDOGFET determines the number (0, 1, 2, or ≥3) of detected photons with a probability of accuracy ≥83 % in a
single-shot measurement.
A novel system for ultra-long-distance quantum key distribution in optical fiber, incorporating ultra-low-noise transition-edge
sensor (TES) photodetectors, is described. Integration of the TES detectors into the system was facilitated with a
unique optically switched interferometer design. The performance of the system over 101 km of dark, single-mode fiber
at 1550 nm and a clock rate of 1 MHz is described. Secret-key bits were produced after error correction and privacy
amplification when using mean photon numbers of 0.01, 0.0148, 0.02, 0.0304, and 0.2 photons/pulse at the output of the
transmitter. At a mean photon number of 0.1 photons per pulse at the transmitter, a transmission line loss of 29.92 dB,
roughly equivalent to 150 km of optical fiber, could be tolerated and secret bits extracted from the transmitted key.
Quantum communications is fast becoming an important component of many applications in quantum information
science. Sharing quantum information over a distance among geographically separated nodes using photonic qubits
requires a reconfigurable transparent networking infrastructure that can support quantum information services. Using
quantum key distribution (QKD) as an example of a quantum communications service, we investigate the ability of fiber
networks to support both conventional optical traffic and single-photon quantum communications signals on a shared
infrastructure. The effect of Raman scattering from conventional channels on the quantum bit error rate (QBER) of a
QKD system is analyzed. Additionally, the potential impact and mitigation strategies of other transmission impairments
such as four-wave mixing, cross-phase modulation, and noise from mid-span optical amplifiers are discussed. We also
review recent trends toward the development of automated and integrated QKD systems which are important steps
toward reliable and manufacturable quantum communications systems.
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