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Photonic Qubits for Quantum Communication: Exploiting photon-pair correlations; from theory to applications
KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
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 [en]
quantum communication, photon-pair sources, entanglement
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-4798ISBN: 978-91-7415-005-6 (print)OAI: oai:DiVA.org:kth-4798DiVA: diva2:14014
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
List of papers
1. 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
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2004 (English)In: Optics Express, 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: 2015-03-30Bibliographically approved
2. 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: 2010-09-13Bibliographically approved
3. 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: 2010-09-13Bibliographically approved
4. 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
5. Narrowband polarization-entangled photon pairs distributed over a WDM link for qubit networks
Open this publication in new window or tab >>Narrowband polarization-entangled photon pairs distributed over a WDM link for qubit networks
Show others...
2007 (English)In: Optics Express, ISSN 1094-4087, Vol. 15, no 11, 6926-6933 p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
Fiber optic networks, Light polarization, Phase matching, Wavelength, Wavelength division multiplexing, Photon detectors, Quantum communication, Signal gating, Wavelength division multiplexers
National Category
Telecommunications
Identifiers
urn:nbn:se:kth:diva-8636 (URN)10.1364/OE.15.006926 (DOI)000247240300047 ()2-s2.0-34249730595 (Scopus ID)
Note
QC 20100914Available from: 2008-06-04 Created: 2008-06-04 Last updated: 2012-03-22Bibliographically approved
6. Experimental Decoy-State Quantum Key Distribution with a Sub-Poissionian Heralded Single-Photon Source
Open this publication in new window or tab >>Experimental Decoy-State Quantum Key Distribution with a Sub-Poissionian Heralded Single-Photon Source
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2008 (English)In: Physical Review Letters, ISSN 0031-9007, Vol. 100, no 9, 090501- p.Article in journal (Refereed) Published
Abstract [en]

We have experimentally demonstrated a decoy-state quantum key distribution scheme (QKD) with a heralded single-photon source based on parametric down-conversion. We used a one-way Bennett-Brassard 1984 protocol with a four states and one-detector phase-coding scheme, which is immune to recently proposed time-shift attacks, photon-number splitting attacks, and can also be proven to be secure against Trojan horse attacks and any other standard individual or coherent attacks. In principle, the setup can tolerate the highest losses or it can give the highest secure key generation rate under fixed losses compared with other practical schemes. This makes it a quite promising candidate for future quantum key distribution systems.

Keyword
Parameter estimation, Photons, Security of data, Quantum key distribution, Single-photon sources, Trojan horse attacks
National Category
Telecommunications
Identifiers
urn:nbn:se:kth:diva-8637 (URN)10.1103/PhysRevLett.100.090501 (DOI)000253764500007 ()2-s2.0-40849118365 (Scopus ID)
Note
QC 20100914Available from: 2008-06-04 Created: 2008-06-04 Last updated: 2012-03-22Bibliographically approved
7. A single-crystal source of path-polarization entangled photons at non-degenerate wavelengths
Open this publication in new window or tab >>A single-crystal source of path-polarization entangled photons at non-degenerate wavelengths
2008 (English)In: Optics Express, ISSN 1094-4087, Vol. 16, no 13, 9701-9707 p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
Photons, Polarization, 1550 nm, Entangled photon pairs, Narrow bands, Polarization-entangled photons, Quasi-phase-matched (QPM)
National Category
Telecommunications
Identifiers
urn:nbn:se:kth:diva-8638 (URN)10.1364/OE.16.009701 (DOI)000257563900050 ()2-s2.0-46149127578 (Scopus ID)
Note
QC 20100914. Uppdaterad från submitted till published (20100914). Tidigare titel: A single-crystal source of phase-polarization entangled photons at non-degenerate wavelengthsAvailable from: 2008-06-04 Created: 2008-06-04 Last updated: 2010-09-14Bibliographically approved

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