KEYWORDS: Quantum state engineering, Quantum optics, Quantum communications, Current controlled current source, Quantum efficiency, Photon polarization, Quantum information processing
Efficiently creating optical quantum states, both simple (e.g., pure single-photon states) and complex (e.g., polarization-entangled but spectrally unentangled photon pairs), remains an experimental challenge. We report on a novel method that allows for efficiently preparing certain classes of states: by weakly driving repeated downconversion in a cavity, we can pseudo-deterministically add photons to a state, preparing Fock states of definite photon number. We discuss expected performance and experimental limitations, including the difficulty of creating pure photons at a high rate. Additionally, we report on our progress in engineering high-rate spatio-spectrally unentangled downconversion, a key technology for optical quantum information processing, and propose a novel 4-photon experimental scheme to test the intrinsic indistinguishability of the photons from this source.
KEYWORDS: Sensors, Optical fibers, Cryogenics, Single photon detectors, Reflection, Antireflective coatings, Signal attenuation, Signal to noise ratio, Electronics, Reflectivity
Visible light photon counters (VLPCs) and solid-state photomultipliers (SSPMs) are high-efficiency single-photon detectors which have multi-photon counting capability. While both the VLPCs and the SSPMs have inferred internal quantum efficiencies above 93%, the actual measured values for both the detectors were in fact limited to less than 88%, attributed to in-coupling losses. We are currently improving this overall detection efficiency via a) custom anti-reflection coating the detectors and the in-coupling fibers, b) implementing a novel cryogenic design to reduce transmission losses and, c) using low-noise electronics to obtain a better signal-to-noise ratio.
A source of single photons allows secure quantum key distribution, in addition, to being a critical resource for linear optics quantum computing. We describe our progress on deterministically creating single photons from spontaneous parametric downconversion, an extension of the Pittman, Jacobs and Franson scheme [Phys. Rev A, v66, 042303 (2002)]. Their idea was to conditionally prepare single photons by measuring one member of a spontaneously emitted photon pair and storing the remaining conditionally prepared photon until a predetermined time, when it would be "deterministically" released from storage. Our approach attempts to improve upon this by recycling the pump pulse in order to decrease the possibility of multiple-pair generation, while maintaining a high probability of producing a single pair. Many of the challenges we discuss are central to other quantum information technologies, including the need for low-loss optical storage, switching and detection, and fast feed-forward control.
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