In practice, the quantum channel also will be affected by noise, and it will be hard to distinguish between noise and eavesdropping.
If Eve wants, she can intercept the quantum channel just to not allow Alice and Bob to communicate.
No amplifiers are used on the optical fiber carrying the quantum signal. Such devices would disrupt the communication in the same way an eavesdropper does. This implies, in turn, that QKD's range is limited.
Following the no-cloning theorem, QKD can provide only a 1:1 connection. So the number of links will increase N(N – 1)/2, as N represents the number of nodes.
Researchers have been developing such systems for more than a decade. The DARPA Quantum Network, which became fully operational in BBN's laboratory in October 2003, has been continuously running in six nodes, operating through the telecommunications fiber between Harvard University, Boston University and BBN since June 2004. The DARPA Quantum Network is the world's first quantum cryptography network, and perhaps also the first QKD system providing continuous operation across a metropolitan area.
NIST performs core research on the creation, transmission, processing and measurement of optical qbits. It demonstrated high-speed QKD systems that generate secure keys for encryption and decryption of information using a one-time pad cipher, and extended them into a three-node quantum communications network.
Toshiba's Quantum Key Distribution System delivers digital keys for cryptographic applications on fiber-optic-based computer networks based on quantum cryptography. In particular, it allows key distribution over standard telecom fiber links exceeding 100km in length and bit rates sufficient to generate 1 megabit per second of key material over a distance of 50km—sufficiently long for metropolitan coverage ().
The current status of quantum cryptography in Japan includes an inter-city QKD testbed based on DPS-QKD, a field test of a one-way BB84 system over 97km with noise-free WDM clock synchronization, and so on ("Toward New Generation Quantum Cryptography—Japanese Strategy" by Nukuikita, Koganei).
The 973 Program and 863 program of China have funded support to the QKD research (Post-Quantum Cryptography: Third International Workshop, Pqcrypto 2010, Darmstadt, Germany, May 25–28, 2010, Proceedings, 1st ed.).
In Europe, the SEcure COmmunication based on Quantum Cryptography (SECOQC, 2004–2008) project was funded for the same reason (http://vcq.quantum.at/publications/all-publications/details/643.html).
In 2004, ID Quantique was the first in the world to bring a quantum key distribution system to a commercial market. ID Quantique's QKD product was used in conjunction with layer 2 Ethernet encryption to secure elections in Geneva. Other companies, like MagicQ, QinetiQ and NEC, also are working in this field. Companies claim to offer or to be developing QKD products, but limited information is publicly available. However, it's likely that the situation will evolve in the near future.
W. Chen, H.-W. Li, S. Wang, Z.-Q. Yin, Z. Zhou, Y.-H. Li, Z.-F. Han and G.C. Guo (2012). "Quantum Cryptography", Applied Cryptography and Network Security, Dr. Jaydip Sen (Ed.), ISBN: 978-953-51-0218-2, InTech, available from http://www.intechopen.com/books/applied-cryptography-and-network-security/quantum-cryptography
"Quantum Cryptography Hits the Fast Lane" by Adrian Cho: http://news.sciencemag.org/sciencenow/2010/04/quantum-cryptography-hits-the-fa.html
"Do we need quantum cryptography?" by Peter Rohde: http://www.peterrohde.org/2012/06/29/do-we-need-quantum-cryptography
"A Little (q)bit of Quantum Computing" by Douglas Eadline: http://www.linux-mag.com/id/8753
"What is a quantum computer?" by Dr Boaz Tamir: http://thefutureofthings.com/column/5/what-is-a-quantum-computer.html
Quantum Computation and Quantum Information by Michael A. Nielsen and Isaac L. Chuang, Cambridge University Press, 2011.
"Quantum Communication": http://w3.antd.nist.gov/qin/index.shtml
Subhendu Bera is from West Bengal (India). He completed his Master of Science degree in Computer Science from Banaras Hindu University and his Bachelor of Science degree in Computer Science from University of Calcutta.