Daniel Ljunggren

Experiments on long wavelength (1550 nm) "plug and play" quantum cryptography systems.

An interferometric quantum cryptographic system at 1550nm wavelength using gated InGaAs Avalanche Photo Diodes as single-photon receivers is demonstrated for a transmission distance up to 40 km.

Bright, single-spatial-mode source of frequency non-degenerate, polarization-entangled photon pairs using periodically poled KTP

We use two perpendicular crystals of periodically-poled KTP to directly generate polarization-entangled photon pairs, the majority of which are emitted into a single Gaussian spatial mode. The signal and idler photons have wavelengths of 810 nm and 1550 nm, respectively, and the photon-pair generation rate is 1.2x107 sec-1 for a pump power of 62 mW. The apparatus is compact, flexible, and easily to use.

Optimal focusing for maximal collection of entangled narrow-band photon pairs into single-mode fibers

We present a theoretical and experimental investigation of the emission characteristics and the flux of photon pairs generated by spontaneous parametric downconversion in quasi-phase matched bulk crystals for the use in quantum communication sources. We show that, by careful design, one can attain well defined modes close to the fundamental mode of optical fibers and obtain high coupling efficiencies also for bulk crystals, these being more easily aligned than crystal waveguides. We distinguish between singles coupling,

Experimental long wavelength quantum cryptography: from single-photon transmission to key extraction protocols

{\ensuremath{\gamma}}_{\mathrm{s}}

Highly efficient source for indistinguishable single photons of controlled shape

and

Narrowband polarization-entangled photon pairs distributed over a WDM link for qubit networks

{\ensuremath{\gamma}}_{\mathrm{i}}

Single photons made-to-measure

, conditional coincidence,

Cavity-based single-photon sources

{\ensuremath{\mu}}_{\mathrm{i}\ensuremath{\mid}\mathrm{s}}

Theory and experiment of entanglement in a quasi-phase-matched two-crystal source

, and pair coupling,

Experiments on long wavelength (1550 nm) "plug and play" quantum cryptography systems

{\ensuremath{\gamma}}_{\mathrm{c}}