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Optical Fiber Communication Conference

Long wavelength quantum cryptography and entanglement manipulations

Anders Karlsson, Mohamed Bourennane, Daniel Ljunggren, Iulia Ghiu, Rubens Viana Ramos

Published Jun 13, 2001• 0 citations
OpenAlex

Abstract

We will review ongoing work at KTH on quantum communication, discussing in particular two projects; long wavelength quantum cryptography, and entangled state manipulations. In order to extend quantum communications to longer distances it is essential to be able to use the low loss telecommunication window at 1550 nm. We, and recently others have for some time worked on quantum cryptography systems in this wavelength region. Much of the effort has been devoted to the investigation of the InGaAs avalanche photo diodes used as single-photon counters, and examples of the performances and systems implications will be given. As a second topic, for quantum communication beyond point-to-point transmission, it is of some interest to investigate the possibilities offered by multi-particle entangled states. Recently, very interesting results are emerging on purification using linear optics manipulations. We will discuss recent theory work on three-particle entangled states of the inequivalent GHZ-classes and the W-classes and the possibility to do entangled-state-assisted transformations between these classes, as well as to consider possible applications.

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We will review ongoing work at KTH on quantum communication, discussing in particular two projects; long wavelength quantum cryptography, and entangled state manipulations. In order to extend quantum communications to longer distances it is essential to be able to use the low loss telecommunication window at 1550 nm. We, and recently others have for some time worked on quantum cryptography systems in this wavelength region. Much of the effort has been devoted to the investigation of the InGaAs avalanche photo diodes used as single-photon counters, and examples of the performances and systems implications will be given. As a second topic, for quantum communication beyond point-to-point transmission, it is of some interest to investigate the possibilities offered by multi-particle entangled states. Recently, very interesting results are emerging on purification using linear optics manipulations. We will discuss recent theory work on three-particle entangled states of the inequivalent GHZ-classes and the W-classes and the possibility to do entangled-state-assisted transformations between these classes, as well as to consider possible applications.
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.
Quantum cryptography is a technique for establishing a fundamentally secure key for cryptography using the encoding of the key on single photon states in such a way that an eavesdropper cannot acquire any information about the key without impairing detectable errors in the communication channel [1]. In the recent years there has been a remarkable progress in experimental implementations at 850 nm and 1300 nm wavelengths with secret key generation at kbits/s up to 48 km distance. However, for 1550 nm wavelength, where fiber losses are as smallest, progress has been hampered by the lack of good single photon detectors.
Quantum mechanics allows a number of communications protocols that would not be possible classically, including provably-secure messages (quantum cryptography), increased channel capacity, and the ability to transmit an unknown quantum state (quantum teleportation). This tutorial will provide an introductory description of what appear to be the most promising applications of quantum communications, including quantum cryptography and quantum repeaters. The most commonly-used methods for quantum cryptography will be described, including the advantages and disadvantages of each in practical applications. The maximum range of current quantum cryptography systems is limited by the fact that optical amplifiers would destroy the quantum coherence of single photons and cannot be used in these kinds of systems. One potential solution to this problem is the transmission of single photons in free space, which would allow a global system for quantum cryptography based on a network of satellites and ground stations. Free-space quantum cryptography systems of that kind have now been demonstrated over relatively short distances. The range of quantum cryptography systems in optical fibers can also be extended using quantum repeaters that make use of entanglement purification and quantum teleportation. These same techniques could also be used to implement a “quantum Internet” for the transmission of qubits from one quantum computer to another. The practical challenges that will have to be met in order to implement quantum repeaters and quantum teleportation over large distances will be discussed.
Summary form only given. Quantum cryptography is a technique for establishing a fundamentally secure cryptographic key, using the encoding of the key on single photon pulses. In the recent years here has been a remarkable progress in experimental implementations at 1300 nm wavelength. However, for 1550 nm wavelength, there are few results, as the performance of single photon detectors for 1550 nm has been considered insufficient. We report on the implementation of a 1550 nm stable plug-and-play interferometric scheme for quantum cryptography, and for simplicity, we have only implemented a two-state encoding.
One of our major target is quantum key distribution (QKD,) which is closest to the practical use. Though QKD can be performed only with current technology, there still remain many thing to be improved. One of the most important devices that determine the system performance is photon detectors. It limits the transmission distance in optical fiber communication wavelength. A photon detector combining the two avalanche photon diodes (APD) has been demonstrated for qubit discrimination in 1550 nm. Spikes accompanied with the signals in gated-mode were canceled by balanced output from the two APDs. The balanced APD photon detector shows very low dark count rate (2x10-7) counts/pulse) with 10 % detection efficiency. We have also developed a highly stable interferometer on a planer-lightwave-circuit (PLC.) We have achieved single-photon interference over 150 km using time-division interferometers for quantum cryptography, which were composed of the two integrated-optic asymmetric Mach-Zehnder interferometers, and the balanced gated-mode photon detectors. The observed visibility was more than 80 % after 150-km transmission, which refers to a quantum bit error rate of less than 10 %. We will also describe temperature insensible QKD system and high speed (100 bps) key transmission over 40 km fiber.