Toshiba touts Quantum Key Distribution

Toshiba research scientists have developed a method of distributing quantum keys more efficiently, the company has claimed in a statement:

"[Quantum Key Distribution -- ] QKD -- relies on sending encoded single photons (particles of light) along the fibre. Any attempt by a hacker to intercept these photons scrambles their encoding, alerting the users that their key is not secret. The Toshiba QKD system uses a one-way architecture and the decoy protocol, which has been proven to satisfy ‘unconditional secrecy’, meaning that the security does not rely on assumptions about the technology available to an eavesdropper. This is the most stringent security criterion.

Current QKD systems are limited by the semiconductor devices (avalanche photodiodes) used to detect the single photons. One photon triggers an avalanche of millions of electrons in this semiconductor device which can be sensed by electrical circuitry in the QKD system. The problem in present systems is that some of these avalanche electrons can be trapped in the device and later stimulate a second spurious detection count. As these noise counts cause errors in the key, current detectors must be operated with long dead times to allow the decay of any trapped electrons. This limits the clock rate of current QKD systems to around 10 MHz and thus the secure key bit rate to under 10 kbit/sec for a 20 km fibre.

The Toshiba team has devised a method to detect much weaker electron avalanches. This strongly reduces the chance for an electron to be trapped, allowing the detector to be operated at much faster rates. Using this method Toshiba has increased the clock rate of their QKD system to 1.036 GHz, approaching the value used in conventional optical communications. This allows a raw bit rate of 9 Mbit/sec over 20 km of fibre, which in turn provides 1.02 Mbit/sec of secure key. This is the first time that a secure bit rate exceeding 1 Mbit/sec has been achieved."

Toshiba claimed in the statement that "Cambridge Research Labs’ new breakthrough has increased the bit rate of QKD more than a hundred fold, exceeding 1 Mbit/sec for the first time and allowing ultra-secure networks with thousands of users."

However, one expert said this could not be implememnted in a practical sense for organisations other than intelligence and the military. Cambridge University security expert Richard Clayton, who really knows his stuff when it comes to cryptography, pointed out that "networks with thousands of users" need to be linked by routers, or hubs, or switches. Quantum cryptographic key distribution relies on point to point communications, as any disturbance on a line -- including being passed through a router -- would register as the line being compromised.

The problem with scaling this comes as each one of the thousand people would need to be connected to each other person in the network by a piece of cable or other means of communication, to make the QKD work.

"If you have four people, you need six wires connecting them," said Clayton. "Five people need ten wires to connect them, and so on. This [breakthrough] will be of great interest to people with long pieces of wire."

However, Andrew Shields, the Toshiba group leader for quantum information technology, told me on Tuesday that the Toshiba QKD technology was scalable, and could conceivably cover an entire metropolitan area.

"Our network uses a combination of quantum and classic cryptography," said Shields. "Quantum cryptography ensures secrecy between nodes, classic cryptography within the nodes."

Shields said that much like any communications technology, you can only make links of certain length. The idea with the Toshiba network is that a 'global' key can be sent through the network using a series of hops.

"Each node creates a local key to encrypt and transmit the global key," said Shields.

However, while being strong, the network would not be impenetrable, Shields admitted.

"If an eavesdropper could gain control of a node, [then the network would be compromised]," said Shields. "You have to believe you can trust the equipment at the ends of the fibres."

Quantum cryptography only secures the communications channel -- you also have to guarantee the security of the hardware, said Shields, who added that this was true of point-to-point networks too.

Having demonstrated a network with six nodes running at low bit rate in Vienna last week, Shields said his research team was now concentrating on developing a prototype that would demonstrate a higher bit-rate. Shields estimated that it would take two years to develop the prototype, and three to five years before Toshiba would produce a commercial product.