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Entanglement in quantum communication: preparation and characterization of photonic qubits
KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
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: urn:nbn:se:kth:diva-616ISBN: 91-7178-254-0 (print)OAI: oai:DiVA.org:kth-616DiVA: diva2:14623
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
List of papers
1. Characterization of an asynchronous source of heralded single photons generated at a wavelength of 1550 nm
Open this publication in new window or tab >>Characterization of an asynchronous source of heralded single photons generated at a wavelength of 1550 nm
(English)Article in journal (Refereed) In press
Abstract [en]

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.

National Category
Telecommunications
Identifiers
urn:nbn:se:kth:diva-9063 (URN)
Note
QC 20100908Available from: 2006-02-10 Created: 2006-02-10 Last updated: 2010-09-08Bibliographically approved
2. Theory and experiment of entanglement in a quasi-phase-matched two-crystal source
Open this publication in new window or tab >>Theory and experiment of entanglement in a quasi-phase-matched two-crystal source
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).

Keyword
quantum cryptography, photon pairs, generation, ktiopo4
National Category
Telecommunications
Identifiers
urn:nbn:se:kth:diva-9064 (URN)10.1103/PhysRevA.73.032326 (DOI)000236467500055 ()2-s2.0-33645053534 (Scopus ID)
Note
QC 20100908Available from: 2006-02-10 Created: 2006-02-10 Last updated: 2017-12-14Bibliographically approved
3. Optimal focusing for maximal collection of entangled narrow-band photon pairs into single-mode fibers
Open this publication in new window or tab >>Optimal focusing for maximal collection of entangled narrow-band photon pairs into single-mode fibers
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.

Keyword
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:nbn:se:kth:diva-9065 (URN)10.1103/PhysRevA.72.062301 (DOI)000234334900030 ()2-s2.0-28844486690 (Scopus ID)
Note
QC 20100913Available from: 2006-02-10 Created: 2006-02-10 Last updated: 2017-12-14Bibliographically approved
4. Bright, single-spatial-mode source of frequency non-degenerate, polarization-entangled photon pairs using periodically poled KTP
Open this publication in new window or tab >>Bright, single-spatial-mode source of frequency non-degenerate, polarization-entangled photon pairs using periodically poled KTP
Show others...
2004 (English)In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 12, no 15, 3573-3580 p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
Crystals, Light polarization, Multiphoton processes, Optical pumping, Probability, Quantum cryptography, Quantum theory, Single mode fibers
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-9066 (URN)10.1364/OPEX.12.003573 (DOI)000222908900035 ()2-s2.0-4644290314 (Scopus ID)
Note
QC 20100901Available from: 2006-02-10 Created: 2006-02-10 Last updated: 2017-12-14Bibliographically approved
5. Authority-based user authentication in quantum key distribution
Open this publication in new window or tab >>Authority-based user authentication in quantum key distribution
2000 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 62, no 2, 022305- p.Article in journal (Refereed) Published
Abstract [en]

 We propose secure protocols for user authenticated quantum key distribution on jammable public channels between two parties, Alice and Bob. Via an arbitrator, Trent, these protocols provide data integrity and mutual identification of the messenger and recipient. The first three are based on single-photon generation and detection. The first and second require (initially) an unjammable channel between the arbitrator and each party. The third requires one broadcast from the arbitrator, disclosing what type of deterministic modification of the states sent through the quantum channel was done by him. The fourth and fifth protocols are based on two-particle entanglement with a preselection of nonorthogonal superpositions of Bell states. These two protocols also require one broadcast from the arbitrator disclosing the type of entangled state in each sending.

Keyword
Communication channels (information theory), Cryptography, Data privacy, Network protocols, Photons, Security of data
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-9067 (URN)10.1103/PhysRevA.62.022305 (DOI)000088683400030 ()
Note
QC 20100831Available from: 2006-02-10 Created: 2006-02-10 Last updated: 2017-12-14Bibliographically approved
6. Experimental long wavelength quantum cryptography: from single-photon transmission to key extraction protocols
Open this publication in new window or tab >>Experimental long wavelength quantum cryptography: from single-photon transmission to key extraction protocols
Show others...
2000 (English)In: Journal of Modern Optics, ISSN 0950-0340, E-ISSN 1362-3044, Vol. 47, 563-579 p.Article in journal (Refereed) Published
Abstract [en]

We present experiments on long wavelength (λ = 1.55 μm) 'plug and play' quantum cryptography systems. We discuss the performance of single-photon detectors at λ = 1.55 μm. Furthermore, we address the full implementation of the quantum cryptography protocol, discussing in detail the implementation of protocols for error correction and privacy amplification needed to get a secure key. We illustrate the theory with examples from a full software simulation to show the performance of the complete protocol in terms of final secure key creation rate

Keyword
Error correction, Light transmission, Optical fibers, Photodiodes, Photons, Quantum optics, Wavelength division multiplexing
National Category
Telecommunications
Identifiers
urn:nbn:se:kth:diva-9068 (URN)10.1080/09500340008244060 (DOI)000085234900031 ()
Note
QC 20100909Available from: 2006-02-10 Created: 2006-02-10 Last updated: 2017-12-14Bibliographically approved

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