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Optical Engineering

Quantum entanglement enables low-power optical communication to orbital ranges

John Lekki

Published Jan 1, 2008• 2 citations
Publisher

Abstract

The signal intensity as a function of range is considered for optical communication systems that utilize time-coincident pairs (or larger sets) of photons for information encoding. Two systems are examined: one that generates pairs of photons that have an entangled quantum state, and another where the pairs of photons are generated from a pair of pulsed photon sources. The signal intensity as a function of range is analyzed as a qualitative first-order approximation for these two techniques. For the first time, to this author's knowledge, it is shown that pairs of photons that share a quantum state, and hence have highly correlated momenta, can produce communication systems that have a high degree of noise immunity and are useful for ranges significantly beyond the collimated range of the transmitter, which up until now has been considered the maximum range. As an example, it is shown that a transmitter with a 1-m aperture and a pair of pulsed photon sources will be effective for about 200 km, while a transmitter with the same aperture and a quantum-entangled photon source will have an effective range greater than 60,000 km.

DOI
10.1117/1.2835043

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The signal intensity as a function of range is considered for optical communication systems that utilize time-coincident pairs (or larger sets) of photons for information encoding. Two systems are examined: one that generates pairs of photons that have an entangled quantum state, and another where the pairs of photons are generated from a pair of pulsed photon sources. The signal intensity as a function of range is analyzed as a qualitative first-order approximation for these two techniques. For the first time, to this author's knowledge, it is shown that pairs of photons that share a quantum state, and hence have highly correlated momenta, can produce communication systems that have a high degree of noise immunity and are useful for ranges significantly beyond the collimated range of the transmitter, which up until now has been considered the maximum range. As an example, it is shown that a transmitter with a 1-m aperture and a pair of pulsed photon sources will be effective for about 200 km, while a transmitter with the same aperture and a quantum-entangled photon source will have an effective range greater than 60,000 km.
We investigate entanglement-based quantum key distribution protocols, with particular emphasis on their efficiency under realistic conditions of satellite quantum communications, where performance is limited by the low power of a received signal and background radiation. We focus on scenarios where each photon pair is used to encode multiple qubits in order to optimally utilize the weak signal. By optimizing over the source intensity and the number of encoded qubits we study the theoretical information limit for the QKD efficiency. We show that the optimal efficiency is attained for finite entangled photons pair production probability which is in contrast to conventional communication efficiency maximized in the limit of vanishing signal strength. The multiqubit encoding can enhance the secret key rate by up to an order of magnitude compared to single-qubit schemes.
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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.
High-dimensional entanglement is an important physical resource for quantum communication. A basic issue for any communication scheme is how many shared bits two parties can extract subject to experimental noise. We determine the shared information that can be extracted from time-bin entangled photons using frame encoding. We consider photons generated by a general down-conversion source and also model losses, dark counts and the effects of multiple photons within each frame. Furthermore, we describe a procedure for including other imperfections such as after-pulsing, detector dead-times and jitter. The results are illustrated by deriving analytic expressions for the maximum information that can be extracted from high-dimensional time-bin entangled photons generated by down conversion. A key finding is that under realistic conditions and using standard SPAD detectors one can still choose the frame size so as to extract over 10 bits per photon. These results are thus useful for experiments on high-dimensional quantum-key distribution systems, but are not limited to such systems. For example, the results are also useful for determining the limits of fibre arrays or within time-multiplexing schemes.
A key goal of quantum communication is to determine the maximum number of bits shared between two quantum systems. An important example of this is in entanglement-based quantum key distribution (QKD) schemes. A realistic treatment of this general communication problem must take account of the nonideal nature of the entanglement source and the detectors. The aim of this paper is to give such a treatment. We obtain analytic expression for the mutual information in terms of experimental parameters. The results are applied to communication schemes that rely on spontaneous parametric down conversion to generate entangled photons. We show that our results can be applied to tasks such as calculating the optimal rate of bits per photon in high-dimensional time-bin-encoded QKD protocols (prior to privacy amplification). A key finding for such protocols is that, by using realistic experimental parameters, one can obtain over 10 bits per photon. We also show how our results can be applied to characterize the capacity of a fiber array and to quantify entanglement using mutual information.