Cryptography Reference
In-Depth Information
3.2.4 Higher Dimensional Entanglement
for Quantum Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.2.5 Entanglement-Based Quantum Cryptography . . . . . . . . . . . . . . . . 62
3.2.5.1 Adopted BB84 Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.2.5.2 An Entanglement-Based Quantum
Cryptography Prototype System . . . . . . . . . . . . . . . . . . . . 64
3.2.6 Toward a Global Quantum Communication
Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
3.2.6.1 Free-Space Distribution of Quantum
Entanglement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
3.2.6.2 Quantum Communications in Space . . . . . . . . . . . . . . . . 73
3.3 Conclusion and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
3.1 Introduction
Quantum communication and quantum computation are novel methods of
information transfer and information processing, all fundamentally based
on the principles of quantum physics. The performances outdo their classical
counterparts in many aspects [1,2]. In almost all quantum communication and
quantum computation schemes, quantum entanglement [3] plays a decisive
role. In essence, an entangled system can carry all information (e.g., on their
polarization properties) only in their correlations, while no individual subsys-
tem carries any information. This leads to correlations that are much stronger
than classically allowed [89, 100], which is a powerful resource for informa-
tion processing. It is therefore important to be able to generate, manipulate,
and distribute entanglement as accurately and as efficiently as possible.
Successful demonstrations of quantum communication protocols started
with photon experiments in 1992 and include quantum cryptography [4,5],
the simultaneous distribution of a cryptographic key that is ultimately secured
by the laws of quantum physics; later followed quantum dense coding [6,7],
a protocol to double the classically allowed capacity of a communication
channel by encoding two bits of information per bit sent, and finally quantum
teleportation [8,9], the remote transfer of an arbitrary quantum state between
distant locations.
Since these early achievements, the field of quantum communication, or
more generally quantum information processing, has very much advanced.
New schemes and techniques allow the generation and manipulation of en-
tangled photon pairs and even of four-photon states with much higher effi-
ciency and precision [10,11]. Also, the distances over which entanglement can
be distributed are regularly pushed further. Owing to new protocols one can
now achieve the successive use of teleported states and also the teleportation
of entanglement via entanglement swapping (see Section 3.2.1). An impor-
tant method for distributing pure entangled states even over noisy channels
