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Theory and experiment of entanglement in a quasi-phase-matched two-crystal source
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.
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.
2006 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Phys. Rev. A, ISSN 1050-2947, Vol. 73, no 3, 032326- p.Article in journal (Refereed) Published
Abstract [en]

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).

Place, publisher, year, edition, pages
2006. Vol. 73, no 3, 032326- p.
Keyword [en]
quantum cryptography, photon pairs, generation, ktiopo4
National Category
Telecommunications
Identifiers
URN: urn:nbn:se:kth:diva-9064DOI: 10.1103/PhysRevA.73.032326ISI: 000236467500055Scopus ID: 2-s2.0-33645053534OAI: oai:DiVA.org:kth-9064DiVA: diva2:14618
Note
QC 20100908Available from: 2006-02-10 Created: 2006-02-10 Last updated: 2017-12-14Bibliographically 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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