| 1. |
- Ljunggren, Daniel, et al.
(författare)
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A source of entangled photon-pairs : Optimizing emission in two quasi-phasematched crystals
- 2004
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Ingår i: Quantum Communication, Measurement And Computing. - 0735402167 ; , s. 354-357
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Konferensbidrag (refereegranskat)abstract
- We extend our earlier results [1] of a bright source of non-degenerate polarization entangled photon-pairs by making a thorough theoretical and experimental investigation of its single-mode emission and state-decoherence. Using two perpendicular bulk crystals of periodically poled KTP, the source directly creates the polarization-entangled state (\HH> + \VV>)/root2 with signal and idler wavelengths of 810 nm and 1550 nm, respectively. In order to optimize the coupling into single-mode fiber we have developed a numerical algorithm which determines the emission mode and the fiber coupling efficiency for a given crystal length and focussing of the pump. For a 4.5 mm long crystal the optimal focussing of the pump is to a beam waist radius of 14 mum. With these focussing conditions the computer calculations give the lowest M-2 value for the emission mode, M-2 approximate to 3, and the best possible fiber coupling efficiency, gamma approximate to 75%. Using the theoretical results to optimize our experimental setup we obtained photon count rates of 1.4 MHz singles and 12 kHz coincidences, and visibilities H, V: 99.6% and 45degrees: 94.2%.
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| 2. |
- Ljunggren, Daniel, et al.
(författare)
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Optimal focusing for maximal collection of entangled narrow-band photon pairs into single-mode fibers
- 2005
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 72:6, s. 062301-
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Tidskriftsartikel (refereegranskat)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(s) and gamma(i), conditional coincidence, mu(i vertical bar s), and pair coupling, gamma(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(R) to be approximate to 1 to 2 for the pump mode and approximate to 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 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 root L, and so, assuming correct focusing, we can only see advantages using long crystals.
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| 3. |
- Ljunggren, Daniel, et al.
(författare)
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Theory and experiment of entanglement in a quasi-phase-matched two-crystal source
- 2006
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 73:3, s. 032326-
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Tidskriftsartikel (refereegranskat)abstract
- We report results regarding a source of polarization entangled photon pairs created by the process of spontaneous parametric downconversion in two orthogonally oriented, periodically poled, bulk KTiOPO4 crystals. The source emits light colinearly at the nondegenerate wavelengths of 810 and 1550 nm, and is optimized for a single-mode optical fiber collection and long-distance quantum communication. The configuration favors long crystals, which promote a high photon-pair production rate at a narrow bandwidth, together with a high pair probability in fibers. The quality of entanglement is limited by chromatic dispersion, which we analyze by determining the output state. We find that such a decoherence effect is strongly material dependent, providing for long crystals an upper bound on the visibility of the coincidence fringes of 41% for KTiOPO4, and zero for LiNbO3. The best obtained raw visibility, when canceling decoherence with an extra piece of crystal, was 91 +/- 0.2%, including background counts. We confirm by a violation of the CHSH-inequality (S=2.679 +/- 0.004 at 55 s(-1/2) standard deviations) and by complete quantum state tomography that the fibers carry high-quality entangled pairs at a maximum rate of 55x10(3) s(-1) THz(-1) mW(-1).
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| 4. |
- Pelton, Matthew, et al.
(författare)
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Bright, single-spatial-mode source of frequency non-degenerate, polarization-entangled photon pairs using periodically poled KTP
- 2004
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Ingår i: Optics Express. - 1094-4087. ; 12:15, s. 3573-3580
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Tidskriftsartikel (refereegranskat)abstract
- 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 1.2 x 10(7) sec(-1) for a pump power of 62 mW. The apparatus is compact, flexible, and easily to use.
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| 5. |
- Sauge, Sebastien, et al.
(författare)
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A single-crystal source of path-polarization entangled photons at non-degenerate wavelengths
- 2008
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Ingår i: Optics Express. - 1094-4087. ; 16:13, s. 9701-9707
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate a bright, narrowband, compact, quasi-phase-matched single-crystal source generating path-polarization-entangled photon pairs at 810 nm and 1550 nm at a maximum rate of 3 x 10(6) s(-1) THz(-1) mW(-1) after coupling to single-mode fiber, and with two-photon interference visibility above 90%. While the source can already be used to implement quantum communication protocols such as quantum key distribution, this work is also instrumental for narrowband applications such as entanglement transfer from photonic to atomic qubits, or entanglement of photons from independent sources.
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| 6. |
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| 7. |
- Sauge, Sebastien, et al.
(författare)
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Narrowband polarization-entangled photon pairs distributed over a WDM link for qubit networks
- 2007
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Ingår i: Optics Express. - 1094-4087. ; 15:11, s. 6926-6933
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Tidskriftsartikel (refereegranskat)abstract
- We present a bright, narrowband, portable, quasi-phase- matched two-crystal source generating polarization- entangled photon pairs at 809 nm and 1555 nm at a maximum rate of 1.2 x 10(6) s(-1) THz(-1) mW(-1) after coupling to single- mode fiber. The quantum channel at 1555 nm and the synchronization signal gating the single photon detector are multiplexed in the same optical fiber of length 27 km by means of wavelength division multiplexers (WDM) having 100 GHz (0.8 nm) spacing between channels. This implementation makes quantum communication applications compatible with current high-speed optical networks.
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| 8. |
- Sauge, Sebastien, et al.
(författare)
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Quantum communication in optical networks : an overview and selected recent results
- 2007
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Ingår i: ICTON 2007. - 9781424412488 ; , s. 30-33
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Konferensbidrag (refereegranskat)abstract
- We discuss recent work in quantum communication, and in some details present a bright, narrowband, portable, quasi-phase-matched two-crystal source generating polarization-entangled photon pairs at 809 nm and 1555 nm. We also show how the single-photon quantum channel at 1555 nm and a classical synchronization signal gating the single photon detector at the receiving side can be multiplexed in the same optical fiber of length 27 km by means of wavelength division multiplexers (WDM) having 100 GHz (0.8 nm) spacing between channels. This illustrates bow single-photon quantum communication applications is compatible with current high-speed optical networks.
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| 9. |
- Sauge, Sebastien, et al.
(författare)
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Single crystal source of polarization entangled photons at non-degenerate wavelengths
- 2008
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Ingår i: 2008 Conference On Lasers And Electro-Optics & Quantum Electronics And Laser Science Conference. - 9781557528599 ; , s. 2996-2997
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Konferensbidrag (refereegranskat)abstract
- We demonstrate a bright, narrowband, compact single-crystal source of polarization entangled photon pairs at non-degenerate wavelength. This work is instrumental for quantum key distribution and entanglement transfer from photonic to atomic qubits.
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| 10. |
- Tengner, Maria, et al.
(författare)
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Characterization of an asynchronous source of heralded single photons generated at a wavelength of 1550 nm
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Tidskriftsartikel (refereegranskat)abstract
- We make a thorough analysis of heralded single photon sources regarding how factors such as the detector gate-period, the photon rates, the fiber coupling efficiencies, and the system losses affect the performance of the source. In the course of this we give a detailed description of how to determine fiber coupling efficiencies from experimentally measurable quantities. We show that asynchronous sources perform, under most conditions, better than synchronous sources with respect to multiphoton events, but only for nearly perfect coupling efficiencies. We apply the theory to an asynchronous source of heralded single photons based on spontaneous parametric downconversion in a periodically poled, bulk, KTiOPO4 crystal. The source generates light with highly non-degenerate wavelengths of 810 nm and 1550 nm, where the 810 nm photons are used to announce the presence of the 1550 nm photons inside a single-mode optical fiber. For our setup we find the probability of having a 1550 nm photon present in the single-mode fiber, as announced by the 810 nm photon, to be 48%. The probability of multiphoton events is strongly suppressed compared to a Poissonian light source, giving highly sub-Poisson photon statistics.
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