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Quantum cryptography, more aptly named quantum key distribution (QKD), has emerged as a new paradigm for high-speed delivery of encryption key material between two remote parties. Typically, the security integrity of key exchange protocols is rooted in either a trusted third party, such as a trusted courier for symmetric encryption protocols, or the hypothesized computational complexity of one-way mathematical functions, such as the RSA encryption protocol.

QKD derives its security from the fundamental physical laws of quantum mechanics, affording the capability to remove from security proofs many of the assumptions about the capabilities of eavesdroppers in a public channel. In 2003, as part of the DARPA QuIST program, BBN Technologies deployed the world's first quantum network in metropolitan Boston and demonstrated how quantum cryptography can be used as an important tool in securing the world's most critical information-carrying networks.

The QKD Protocol
QKD uses a single quantum particle as the physical medium on which to encode a single bit of key material. A quantum particle encoded with information is referred to as a quantum bit, or qubit. The quantum mechanical nature of these particles exhibit two uniquely quantum physical characteristics which make the encoded information robust against interception by eavesdroppers:

  • Quantum particles are indivisible units of energy, so they cannot be divided by an eavesdropper for passive monitoring.
  • Quantum particles are subject to the Heisenberg uncertainty principle, so measurement of a quantum particle by an eavesdropper irreversibly alters the state of the particle, yielding an effect that is noticeable to the two communicating parties.
While there is a broad spectrum of implementation techniques for performing practical QKD, there are overarching commonalities to all the protocols and techniques. Figure 1 shows a system-level schematic. A designated sender and receiver have distinct roles in the protocol.

To begin the negotiation of a secret key, the sender prepares a single photon for transmission to the receiver by generating a bright laser pulse and attenuating the pulse to an intensity much less than one photon per pulse, ensuring that very rarely a data pulse exits the transmitter that has two photons that would provide an eavesdropper with excess information. Next, the transmitter randomly encodes two bits of information on the photon from a set labeled ΦS, and the encoded photon is directed into the transmission channel. The information can be encoded in any measurable quantity of the photon such as electric field polarization or optical phase.

The transmission channel can consist of any transparent medium, whether it is free-space or fiber-optics. For long-distance, high-data-rate communications, telecommunications-band optical fiber is often the channel of choice. As photons enter the receiver from the channel, the receiver randomly chooses a measurement basis, from one of two choices ΦR, in which to measure the photon, and then performs photon counting with two single photon detectors (SPDs).

The sender and receiver repeatedly execute this protocol and monitor the error rate of the resulting bit streams. Since any interrogation of the photon in the channel by an eavesdropper alters the state of the quantum bit, the presence of an eavesdropper can be detected as an increase in the error rate of the communications, as tested through error detection routines for the protocol utilizing an unsecured classical communications channel.

The DARPA Quantum Network
In 2003, in collaboration with Boston University and Harvard University, Raytheon BBN Technologies deployed the world's first quantum key distribution network in the metropolitan Boston area1. A multidisciplinary team of physicists, software and hardware engineers, and network architects designed and built the quantum network. QKD nodes at each university were connected to BBN via dedicated optical fiber channels and networked through an optical switch located in the laboratories at BBN. In addition, several variant QKD systems were integrated into the network, including free-space and quantum-entanglement-based links.

The system was engineered to operate without manual intervention, continuously generating key material shared between pairs of locations. A critical component to the project focused on integrating QKD with the security protocols for network communications that are currently used. BBN developed a suite of protocols for key negotiation, as well as the integration of key material into protocols such as IPSec, commonly used for secure communications on the Internet.

The Future of Quantum Networks
Since the deployment of BBN's quantum network, several other demonstrations have emerged around the world. Perhaps the most recent is the deployment of the European SECOQC network2 in Vienna, integrating several QKD technologies into a ring topology network. The European network has addressed the important issue of network scalability by forming a trust model between intermediate nodes in the network through which key material flows. Ultimately, for quantum networks to scale without such a constrained trust model, it requires the integration of quantum entanglement sources and quantum memories to construct quantum repeater stations at intermediate nodes between users, and Raytheon BBN Technologies is pursuing these technologies.

QKD has been demonstrated as a practical and useful tool in securing critical communication networks. Important challenges lie ahead, including increasing key exchange throughput, and extending reach and compatibility with currently installed fiber networks that are not optically transparent from user to user. Continued research on quantum-based sources, detectors and processing subsystems is aimed at addressing these challenges.

Jonathan L. Habif

1C. Elliott, D. Pearson and G. Troxel, "Current status of the DARPA quantum network," Computer Communication Review, v. 33, n. 4, p. 227-238.