Raytheon BBN Technologies

Anyone who is not shocked by quantum theory has not understood it.

-Niels Bohr

Quantum theory remains as shocking today as it was when Bohr first proposed the quantum theory for the atom early in the last century. The notion that very small things, such as atoms and molecules, do not behave in the same way as macroscopic matter was then, and remains today, nearly incomprehensible to the human mind. We rely on experience and observation to develop our intuition, and most of us have never observed the behavior of individual atomic or subatomic particles.

But a few have explored the world of the very small. And among those few, a handful of visionaries have been able to fathom ways to use the discontinuous (quantum) behavior of these small particles to our advantage. Quantum cryptography is one example of applying a deep understanding of quantum physics to create a novel technology of potentially enormous significance.

The DARPA Quantum Network - World's First Quantum Cryptographic Network

Under DARPA sponsorship, and together with our academic colleagues Harvard University and Boston University, BBN Technologies has recently built and begun to operate the world's first Quantum Key Distribution (QKD) network. The DARPA Quantum Network employs 24x7 quantum cryptography to provide unprecedented levels of security for standard Internet traffic flows such as web-browsing, e-commerce, and streaming video.

The DARPA Quantum Network became fully operational on October 23, 2003 in BBN's laboratories, and has run continuously since. It currently consists of two BBN-built, interoperable weak-coherent QKD systems running at a 5 MHz pulse rate (0.1 mean photons per pulse) through telecommunications fiber, and inter-connected via a photonic switch, together with a full suite of production-quality QKD protocols. In the near future, we plan to roll out this network into dark fiber between our campuses through the Cambridge, Massachusetts metropolitan area, introduce a series of new quantum cryptographic links based on a variety of physical phenomena, and start testing the resulting network against sophisticated attacks.

The principles underlying a quantum cryptographic network have already been proven on a limited scale. Using lasers and photo detectors, light is sent, in a manner in which eavesdropping is always detectable, through either fiber optic cable or the atmosphere to distribute cryptographic keys that are used to scramble (encrypt) and de-scramble (decrypt) a message. The DARPA Quantum Network has improved upon these techniques to create an extremely secure, highly robust network protected by quantum cryptography, This secure network technology is 100% compatible with conventional Internet technology.

Do We Need More Security?

BBN's quantum cryptography systems are now fully operational, with prototypes being deployed across metropolitan areas to protect Internet traffic. But what are they? And what do they offer? Precisely because these systems are so novel, quantum cryptography is best understood as the third and final insight that transformed cryptography in the 20th century.

In the 1940s, Claude Shannon provided the information-theoretic basis for secrecy, establishing that the amount of uncertainty that can be introduced into an encoded message cannot be greater than that of the cryptographic key used to encode it. To achieve perfect secrecy, the key must be at least as long as the message and never reused -- i.e. Vernam ciphers must be employed. Unfortunately, it has proved very difficult in practice to distribute the completely secret, completely random, one-time pads needed for Vernam ciphers, so they have not to date been widely adopted.

Several decades later, a number of researchers invented cryptographic techniques based on computational complexity. These "public key" techniques are now ubiquitous; the best known are probably Diffie-Hellman and RSA. Unlike Shannon, who posited an adversary with unlimited mathematical abilities, these techniques assume that certain mathematical functions are "one way," that is, easy to do in one direction, but too hard for an adversary to undo in a reasonable time. (These assumptions are as yet unproven.) For example, RSA assumes that it is relatively easy to multiply two large prime numbers to get their product, but quite hard to factor that product into the two original primes.

Quantum cryptography, invented by Charles Bennet and Giles Brassard in 1984, begins with a radically different premise -- one can base security on known physical laws rather than mathematical complexity. Physical devices, with specialized cryptographic protocols, can conjure up an ever-flowing stream of random bits whose values are unknown to any third party. When these bits are used as key material for Vernam ciphers, Shannon's ideal of perfect secrecy can be achieved cheaply and easily. By contrast with the unproven foundations of public key techniques, quantum cryptography promises information-theoretic secrecy firmly based on the laws of physics.

BBN's Role

Since developing the ARPANET over thirty years ago, BBN Technologies has been a pioneer in networking technologies. In creating the DARPA Quantum Network, we are applying this deep expertise to integrate new quantum optical sources and detectors with novel networking protocols. Our work is focused on three crucial areas:

• Building a network based on the fundamental principles of quantum physics that is fully compatible with the current and future arena of most message traffic, the Internet. This has required the design and development of new hardware, software, and network protocols, and careful thought about the foundations of quantum cryptography.
• Dramatically increasing both the speed and security of quantum cryptography by creating high-speed detectors and cryptographic systems based on entangled photons.
• Identifying the potential problems and vulnerabilities posed by the most sophisticated "quantum hacking" techniques and integrating safeguards into the Quantum Network design.