Optimal focusing for maximal collection of entangled narrowband photon pairs into singlemode fibers
Abstract
We present a theoretical and experimental investigation of the emission characteristics and the flux of photon pairs generated by spontaneous parametric downconversion in quasiphase 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, {gamma}{sub s} and {gamma}{sub i}, conditional coincidence, {mu}{sub is}, and pair coupling, {gamma}{sub c}, and show how each of these parameters can be maximized by varying the focusing of the pump mode and the fibermatched modes using standard optical elements. Specifically we analyze a periodically poled KTPcrystal pumped by a 532 nm laser creating photon pairs at 810 nm and 1550 nm. Numerical calculations lead to coupling efficiencies above 93% at optimal focusing, which is found by the geometrical relation L/z{sub R} to be {approx_equal}1 to 2 for the pump mode and {approx_equal}2 to 3 for the fibermodes, where L is the crystal length and z{sub R} is the Rayleighrange of the modeprofile. These results are independent on L. By showing that the singlemodemore »
 Authors:

 Department of Microelectronics and Information Technology, Royal Institute of Technology, KTH, Electrum 229, SE164 40 Kista (Sweden)
 Publication Date:
 OSTI Identifier:
 20786264
 Resource Type:
 Journal Article
 Journal Name:
 Physical Review. A
 Additional Journal Information:
 Journal Volume: 72; Journal Issue: 6; Other Information: DOI: 10.1103/PhysRevA.72.062301; (c) 2005 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 10502947
 Country of Publication:
 United States
 Language:
 English
 Subject:
 74 ATOMIC AND MOLECULAR PHYSICS; CONVERSION; COUPLING; CRYSTALS; DATA TRANSMISSION; DESIGN; EFFICIENCY; EMISSION; FOCUSING; LASER RADIATION; OPTICAL FIBERS; PERIODICITY; PHOTONS; QUANTUM COMPUTERS; QUANTUM ENTANGLEMENT; QUANTUM MECHANICS; QUBITS; WAVEGUIDES
Citation Formats
Ljunggren, Daniel, and Tengner, Maria. Optimal focusing for maximal collection of entangled narrowband photon pairs into singlemode fibers. United States: N. p., 2005.
Web. doi:10.1103/PHYSREVA.72.0.
Ljunggren, Daniel, & Tengner, Maria. Optimal focusing for maximal collection of entangled narrowband photon pairs into singlemode fibers. United States. doi:10.1103/PHYSREVA.72.0.
Ljunggren, Daniel, and Tengner, Maria. Thu .
"Optimal focusing for maximal collection of entangled narrowband photon pairs into singlemode fibers". United States. doi:10.1103/PHYSREVA.72.0.
@article{osti_20786264,
title = {Optimal focusing for maximal collection of entangled narrowband photon pairs into singlemode fibers},
author = {Ljunggren, Daniel and Tengner, Maria},
abstractNote = {We present a theoretical and experimental investigation of the emission characteristics and the flux of photon pairs generated by spontaneous parametric downconversion in quasiphase 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, {gamma}{sub s} and {gamma}{sub i}, conditional coincidence, {mu}{sub is}, and pair coupling, {gamma}{sub c}, and show how each of these parameters can be maximized by varying the focusing of the pump mode and the fibermatched modes using standard optical elements. Specifically we analyze a periodically poled KTPcrystal pumped by a 532 nm laser creating photon pairs at 810 nm and 1550 nm. Numerical calculations lead to coupling efficiencies above 93% at optimal focusing, which is found by the geometrical relation L/z{sub R} to be {approx_equal}1 to 2 for the pump mode and {approx_equal}2 to 3 for the fibermodes, where L is the crystal length and z{sub R} is the Rayleighrange of the modeprofile. These results are independent on L. By showing that the singlemode bandwidth decreases {proportional_to}1/L, we can therefore design the source to produce and couple narrow bandwidth photon pairs well into the fibers. Smaller bandwidth means both less chromatic dispersion for long propagation distances in fibers, and that telecom Bragg gratings can be utilized to compensate for broadened photon packetsa vital problem for timemultiplexed qubits. Longer crystals also yield an increase in fiber photon flux {proportional_to}{radical}(L), and so, assuming correct focusing, we can only see advantages using long crystals.},
doi = {10.1103/PHYSREVA.72.0},
journal = {Physical Review. A},
issn = {10502947},
number = 6,
volume = 72,
place = {United States},
year = {2005},
month = {12}
}