Single-photon subtraction (SPS) is useful for engineering optical quantum states and can be accomplished experimentally by heralding on the detection of one photon in the output port of a beamsplitter. Alternatively, conditioning on zero reflected photons modifies states by “zero-photon subtraction” (ZPS). Here we experimentally demonstrate that ZPS reduces the mean photon number of superpositions/mixtures of Fock states. The observed trends in attenuation show a dependence on the Mandel Q parameter for various input states, resulting in complementary behavior between SPS and ZPS. Theoretical results also show higher-order effects on the photon number distribution, beyond reduction in mean photon number.
Quantum cryptography systems can operate over relatively long distances in standard telecommunications fiber by
taking advantage of the low transmission losses in these fibers at 1.3 or 1.5 microns. Although there has been much
progress toward the development of highly efficient and low-noise detectors for these wavelengths, silicon avalanche
photodiodes currently offer superior single photon counting performance, but only at visible and near IR wavelengths
where the fiber transmission is poor. For ranges typical of local area networks, it is possible that a quantum key
distribution (QKD) system operating below 850nm could be optimal, even though standard telecommunications fiber
supports multiple optical modes at these wavelengths. We have recently developed an optical mode filter that allows
efficient higher order mode rejection from standard telecommunications fiber near 830nm. We have used this type of
filter to launch and recover QKD signals from a polarization-based system implementing the BB84 quantum
cryptography protocol. Here we present results from testing and operation in installed fiber links ranging up to 3km.
These results demonstrate that the filters can attenuate the higher order modes over 35dB while having a minimal
(<1dB) impact on the fundamental mode carrying the QKD signal.
We review an experimental demonstration of a simple irreversible circuit of two probabilistic exclusive-OR (XOR) gates for single-photon qubits. We describe the operation of the individual linear-optics gates and the overall circuit in terms of two-photon and three-photon quantum interference effects. We also discuss future plans for quantum circuits using single-photon qubits from stored parametric down-conversion sources.
Although there has been tremendous progress in the development of true “on-demand” single-photon sources, periodic or “pseudodemand” single-photon sources can be a sufficient resource for many optical quantum information processing applications. Here we review a recent experimental demonstration of a periodic single-photon source based on parametric down-conversion photon pairs, optical storage loops, and high-speed switching. We also review an experiment in which high speed switching and storage loops were used to implement a periodic quantum memory device for polarization-encoded single-photon qubits. Finally, we describe a method in which two of these periodic quantum memory devices are used to facilitate the production of a periodic source of entangled photon pairs. These experiments and proposals are all motivated within the context of linear optics quantum computing.
Ever since Knill, Laflamme and Milburn [Nature (London) 409, 46 (2001)] showed that nondeterministic quantum logic operations could be performed with linear optical elements, additional
photons (ancilla) and projective measurements, the idea of linear-optics quantum computation has attracted considerable interest. Our group has recently demonstrated several devices of this kind. We give an overview of recent experimental results, including the quantum parity check, the destructive controlled-NOT, and a cyclical quantum memory. The need for high-efficiency detection of single photons, and for detectors capable of distinguishing photon number will be discussed. Some experimental improvements towards meeting that need will be presented.
We describe an on-going experimental investigation of a newly predicted mechanism for the production of nonlinear interactions between two photons. This mechanism involves nonlocal cooperative effects between pairs of atoms and exists only at the quantum level. An optical approach to quantum computing based on this effect may have many practical advantages.
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