dorsal/arxiv
View SchemaOptimal focusing for maximal collection of entangled narrow-band photon pairs into single-mode fibers
| Authors | Daniel Ljunggren, Maria Tengner |
|---|---|
| Categories | |
| ArXiv ID | quant-ph/0507046 |
| URL | https://arxiv.org/abs/quant-ph/0507046 |
| DOI | 10.1103/PhysRevA.72.062301 |
| Journal | Phys. Rev. A 72, 062301 (2005) |
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, conditional coincidence, and pair coupling, 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 94% at optimal focusing, which is found by the geometrical relation L/z_R to be ~ 1 to 2 for the pump mode and ~ 2 to 3 for the fiber-modes, where L is the crystal length and z_R is the Rayleigh-range of the mode-profile. These results are independent on L. By showing that the single-mode bandwidth decreases as 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 sqrt{L}, and so, assuming correct focusing, we can only see advantages using long crystals.
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"abstract": "We present a theoretical and experimental investigation of the emission\ncharacteristics and the flux of photon pairs generated by spontaneous\nparametric downconversion in quasi-phase matched bulk crystals for the use in\nquantum communication sources. We show that, by careful design, one can attain\nwell defined modes close to the fundamental mode of optical fibers and obtain\nhigh coupling efficiencies also for bulk crystals, these being more easily\naligned than crystal waveguides. We distinguish between singles coupling,\nconditional coincidence, and pair coupling, and show how each of these\nparameters can be maximized by varying the focusing of the pump mode and the\nfiber-matched modes using standard optical elements. Specifically we analyze a\nperiodically poled KTP-crystal pumped by a 532 nm laser creating photon pairs\nat 810 nm and 1550 nm. Numerical calculations lead to coupling efficiencies\nabove 94% at optimal focusing, which is found by the geometrical relation L/z_R\nto be ~ 1 to 2 for the pump mode and ~ 2 to 3 for the fiber-modes, where L is\nthe crystal length and z_R is the Rayleigh-range of the mode-profile. These\nresults are independent on L. By showing that the single-mode bandwidth\ndecreases as 1/L, we can therefore design the source to produce and couple\nnarrow bandwidth photon pairs well into the fibers. Smaller bandwidth means\nboth less chromatic dispersion for long propagation distances in fibers, and\nthat telecom Bragg gratings can be utilized to compensate for broadened photon\npackets--a vital problem for time-multiplexed qubits. Longer crystals also\nyield an increase in fiber photon flux proportional to sqrt{L}, and so,\nassuming correct focusing, we can only see advantages using long crystals.",
"arxiv_id": "quant-ph/0507046",
"authors": [
"Daniel Ljunggren",
"Maria Tengner"
],
"categories": [
"quant-ph"
],
"doi": "10.1103/PhysRevA.72.062301",
"journal_ref": "Phys. Rev. A 72, 062301 (2005)",
"title": "Optimal focusing for maximal collection of entangled narrow-band photon pairs into single-mode fibers",
"url": "https://arxiv.org/abs/quant-ph/0507046"
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