Breakthrough: new record bit rate for quantum key distribution

been a milestone for mission critical applications. The next challenge would be to put this level of technology into metropolitan network operation. Our Japan-EU collaboration is going to do this within the next few years.”

Cryptography, the science of information security, is essential to protect electronic business communication and e-commerce, enabling, for example, confidentiality, identification of users and validation of transactions. All of these applications rely upon digital keys, which are shared between the legitimate users, but must be kept secret from everyone else. Maintaining the ability to distribute keys securely is thus one of the most important battlefields in the cryptography arms race. It is essential to be able to distribute keys between users securely. Furthermore, in order to protect the system from crypto-analysis or key theft it is important to change the keys frequently.

Quantum Key Distribution (QKD) is an automated method for distributing secret keys across an optical fiber. A unique feature of QKD is that its security is derived from the fundamental laws of Quantum Physics and does not therefore rely upon assumptions about the computing power of an eavesdropper. An added benefit is that the keys distributed by QKD will be secure in the future as well as today.

QKD is based upon sending encoding single photons (particles of light) along the fiber. The laws of quantum physics dictate that any attempt by an eavesdropper to intercept and measure the photons alters their encoding. This means that eavesdropping on quantum keys can be detected.

The Toshiba QKD system is based on one-way optical propagation and the BB84 protocol using decoy pulses. This protocol has been proven to be unconditionally secure, i.e. satisfying 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 has limited the clock rate of current QKD systems to around 10 MHz and thus the average secure key bit rate to a few kbit/sec for a 50 km fiber.

The Toshiba team has devised a method to detect much weaker avalanches. This strongly reduces the chance for an electron to be trapped, allowing the detector to be operated at much faster rates beyond 2 GHz. As the detector is based on a compact and rugged semiconductor device, it is suitable for real-world applications.

—Read more in A. R. Dixon et al., “Continuous Operation of High Bit Rate Quantum Key Distribution” (forthcoming in Applied Physics Letters)