Cryptography Reference
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used for practical applications. In order for quantum communications sys-
tems to become widely used, it will probably be necessary to develop quan-
tum repeaters that can extend the range of quantum key distribution systems
and correct for errors in the transmission of quantum information. One of the
most promising approaches for the development of quantum repeaters is the
use of linear optical techniques [1,2] to implement quantum logic gates, com-
bined with optical storage loops to implement a quantum memory device for
single photons [3-5]. In this chapter, we describe several prototype quantum
logic gates [6-8], a single-photon source [9], and a single-photon memory de-
vice [3,4] that we have recently demonstrated. A four-qubit encoding [5] that
allows these devices to be combined to implement a quantum repeater will
also be described.
The past development of quantum key distribution has been strongly
influenced by the need to overcome a variety of practical challenges, and
the future development of the field will probably be determined by the chal-
lenges that remain. As a result, we begin in Section 6.2 with a brief review of
the challenges facing the development of quantum communications systems,
both past and future. In Section 6.3, we describe the basic operation of prob-
abilistic quantum logic gates based on linear optics techniques, along with
experimental results from several devices of that kind. The development of
quantum repeaters will also require a source of single photons and a quantum
memory device, and demonstrations of prototype devices of that kind are de-
scribed in Section 6.4. A proposed implementation [5] of a quantum repeater
using a combination of these devices is outlined in Section 6.5, followed by a
summary in Section 6.6.
6.2 Challenges in Quantum
Communications
Quantum key distribution systems have evolved over the past 15 years in
response to a number of technical challenges that limited their performance
at the time. As a result, it may be useful to review briefly the past development
of quantum key distribution systems and to discuss the remaining challenges
that seem likely to determine the future development of the field of quantum
communications.
At one time, the only known method for quantum key distribution was
based on the use of the polarization states of single photons. In addition to
introducing the BB84 and B92 protocols, Bennett et al. also performed the
first experimental demonstration of quantum key distribution using photon
polarization states in a tabletop experiment [10]. But the use of single-photon
polarizations was considered to be a major obstacle to practical applications
at the time, since the state of polarization of a photon will change in a time-
dependent way as it propagates through an optical fiber. In response to this
problem, we developed a feedback loop [11,12] that automatically compen-
sated for the change in polarization of the photons. The system alternated
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