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Title: Optimal focusing for maximal collection of entangled narrow-band photon pairs into single-mode 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 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, {gamma}{sub s} and {gamma}{sub i}, conditional coincidence, {mu}{sub i|s}, 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 fiber-matched modes using standard optical elements. Specifically we analyze a periodically poled KTP-crystal 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 fiber-modes, where L is the crystal length and z{sub R} is the Rayleigh-range of the mode-profile. These results are independent on L. By showing that the single-modemore » 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 packets--a vital problem for time-multiplexed 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.« less

Authors:
;  [1]
  1. Department of Microelectronics and Information Technology, Royal Institute of Technology, KTH, Electrum 229, SE-164 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 1050-2947
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 narrow-band photon pairs into single-mode fibers. United States: N. p., 2005. Web. doi:10.1103/PHYSREVA.72.0.
Ljunggren, Daniel, & Tengner, Maria. Optimal focusing for maximal collection of entangled narrow-band photon pairs into single-mode fibers. United States. doi:10.1103/PHYSREVA.72.0.
Ljunggren, Daniel, and Tengner, Maria. Thu . "Optimal focusing for maximal collection of entangled narrow-band photon pairs into single-mode fibers". United States. doi:10.1103/PHYSREVA.72.0.
@article{osti_20786264,
title = {Optimal focusing for maximal collection of entangled narrow-band photon pairs into single-mode 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 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, {gamma}{sub s} and {gamma}{sub i}, conditional coincidence, {mu}{sub i|s}, 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 fiber-matched modes using standard optical elements. Specifically we analyze a periodically poled KTP-crystal 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 fiber-modes, where L is the crystal length and z{sub R} is the Rayleigh-range of the mode-profile. These results are independent on L. By showing that the single-mode 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 packets--a vital problem for time-multiplexed 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 = {1050-2947},
number = 6,
volume = 72,
place = {United States},
year = {2005},
month = {12}
}