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Optimal focusing for maximal collection of entangled narrow-band photon pairs into single-mode fibers
KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
2005 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 72, no 6, 062301- p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2005. Vol. 72, no 6, 062301- p.
Keyword [en]
Crystals, Light propagation, Optical devices, Optical waveguides, Photons, Quantum theory, Chromatic dispersion, Coupling efficiencies, Entangled narrow-band photon pairs, Parametric downconversion
National Category
Other Engineering and Technologies
Identifiers
URN: urn:nbn:se:kth:diva-9065DOI: 10.1103/PhysRevA.72.062301ISI: 000234334900030Scopus ID: 2-s2.0-28844486690OAI: oai:DiVA.org:kth-9065DiVA: diva2:14619
Note
QC 20100913Available from: 2006-02-10 Created: 2006-02-10 Last updated: 2010-09-13Bibliographically approved
In thesis
1. Entanglement in quantum communication: preparation and characterization of photonic qubits
Open this publication in new window or tab >>Entanglement in quantum communication: preparation and characterization of photonic qubits
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

At the heart of quantum physics lies the principle of superposition, and at the heart of information theory lies the bit. Perhaps the most useful property of quantum systems is that they can be loaded with information bits, so-called qubits, that are indefinitely both 0 and 1 until a measurement is made. Another consequence is that several qubits can become entangled, which is manifested by the non-classical correlations between such quantum systems when measured in all possible bases. Within the rapidly progressing fields of quantum information and quantum communication these quantum effects are utilized to perform tasks such as quantum computing and quantum cryptography.

In this thesis we present experimental and theoretical work using single photon sources to prepare ``flying'' photonic qubits. We describe work using mainly quasi-phase-matched nonlinear crystals to generate beams of entangled photon pairs, that are either encoded in polarization at near-visible wavelengths, or in time at optical fiber telecommunication wavelengths (1550 nm). The optical fiber is the medium used for transporting the qubits over a long distance, and it is therefore essential to couple the photons well into the fibers. By focusing the beams optimally, we have investigated how this problem can meet the requirement of creating photons of a narrow frequency bandwidth and a high photon flux. Furthermore, we have generated truly single photons that are heralded by an electrical signal. As a result of modifying the statistics of such sources we have been able to show the effect of photon antibunching. In two separate works, we have implemented a quantum key distribution system based on faint laser pulses at the telecom wavelength of 1550 nm, as well as protocols based on entanglement for performing authentication of key distribution in quantum cryptography.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. xv, 93 p.
Series
Trita-MVT, ISSN 0348-4467 ; 2006:1
National Category
Telecommunications
Identifiers
urn:nbn:se:kth:diva-616 (URN)91-7178-254-0 (ISBN)
Public defence
2006-02-23, Sal C1, Electrum, Isafjordsgatan 20-26, Kista, 10:00
Opponent
Supervisors
Note
QC 20100909Available from: 2006-02-10 Created: 2006-02-10 Last updated: 2010-09-09Bibliographically approved
2. Photonic Qubits for Quantum Communication: Exploiting photon-pair correlations; from theory to applications
Open this publication in new window or tab >>Photonic Qubits for Quantum Communication: Exploiting photon-pair correlations; from theory to applications
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

For any communication, the conveyed information must be carried by some physical system. If this system is a quantum system rather than a classical one, its behavior will be governed by the laws of quantum mechanics. Hence, the properties of quantum mechanics, such as superpositions and entanglement, are accessible, opening up new possibilities for transferring information. The exploration of these possibilities constitutes the field of quantum communication. The key ingredient in quantum communication is the qubit, a bit that can be in any superposition of 0 and 1, and that is carried by a quantum state. One possible physical realization of these quantum states is to use single photons. Hence, to explore the possibilities of optical quantum communication, photonic quantum states must be generated, transmitted, characterized, and detected with high precision. This thesis begins with the first of these steps: the implementation of single-photon sources generating photonic qubits. The sources are based on photon-pair generation in nonlinear crystals, and designed to be compatible with fiber optical communication systems. To ensure such a compatibility and to create a high-quality source, a theoretical analysis is made, optimizing the coupling of the photons into optical fibers. Based on the theoretical analysis, a heralded single-photon source and a two-crystal source of entangled photons-pairs are experimentally implemented. The source of entangled photons is further developed into a compact source with a narrow bandwidth compatible with standard telecommunication wavelength-division multiplexers, and even further developed to a more stable one-crystal source. The sources are to be used for quantum communication in general and quantum cryptography in particular. Specifically, a heralded single-photon source is implemented and then used for a full test of a decoy-state quantum cryptography protocol.

Place, publisher, year, edition, pages
Stockholm: KTH, 2008. xii, 79 p.
Series
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2008:13
Keyword
quantum communication, photon-pair sources, entanglement
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-4798 (URN)978-91-7415-005-6 (ISBN)
Public defence
2008-06-13, Sal D, Forum, KTH Kista, Isafjordsgatan 39, Kista, 10:00
Opponent
Supervisors
Note
QC 20100914Available from: 2008-06-04 Created: 2008-06-04 Last updated: 2010-09-14Bibliographically approved

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