In the future, our society and our lives will become even more reliant on networks, so it will be vital that information is transmitted quickly and safely. However, the rise of quantum computers, with their overwhelming computational capabilities, will pose a threat to the security of cryptographic communications. Hopes are high for quantum key distribution as a method of secure encrypted communication. According to information theory, quantum key distribution, which uses the principles of quantum mechanics, is an unbreakable form of encryption. Toshiba is leading the world in developing unique technologies for accelerating and stabilizing quantum key distribution. 

In this running feature, we'll explain this technology over the course of several articles. 

In part 1, we explained the principles of quantum key distribution technology and the BB84 protocol. In part 2, we'll learn about the technologies that are used to produce the higher speeds and greater stability required for quantum key distribution.

Digitalization has proceeded at a rapid pace throughout society. In business and academic research settings, tremendous amounts of data are relayed through information and communication networks (hereafter referred to simply as "networks") when conducting high resolution video conferences, performing genome analysis, or the like. In people's homes, as well, products such as automobiles and appliances are increasingly becoming connected to networks, which is making these networks even more important.

As we explained in part 1, the valuable information passed through these networks is protected by encryption. Currently, quantum cryptographic communication is considered the safest form of cryptographic communication. In quantum cryptographic communication, cryptographic keys generated through quantum key distribution are used to communicate securely.

For this highly secure quantum cryptographic communication to be used in various applications, it is vital that the quantum key distribution system that generates these cryptographic keys is made faster and more stable. It must be made faster because the amounts of highly confidential data that are transmitted are sometimes extremely large, so numerous cryptographic keys are used. It must be made more stable because it is essential that the weak photons that carry cryptographic key information reach their destination. At Toshiba, we consider the speed and stability of quantum key distribution systems to be important parts of the practical deployment of quantum cryptographic communications. That's why we've been at the forefront of technical development in these areas.

Toshiba's quantum key distribution systems generate cryptographic keys at some of the highest speeds in the world. The quantum key distribution system we developed in 2017 achieved a key distribution speed of 13.7 Mbps in an optical fiber environment built to simulate the transmission of quantum key distribution over a distance of 10 kilometers. This was the first system in the world to break the 10 Mbps barrier.

* Mbps: Unit used to indicate transmission speed. A transmission speed of 13.7 Mbps means roughly 13.7 megabits are transmitted per second.

Even in real-world environments, in which the quantum key distribution system is significantly affected by various factors, including the optical fiber used to transmit the photons, the system is capable of stably transmitting cryptographic keys. In 2018, a quantum key distribution system transmitter was connected via optical fiber to a receiver located roughly 7 km away. Over the course of a month, the system successfully generated cryptographic keys at an average speed in excess of 10 Mbps. This was a world first.

Achieving these high speeds and stability required the development of technologies for rapid and precise photon detection, elemental technologies used in every step of the cryptographic key generation process, and technologies for integrating the detection and generation technologies in a quantum key distribution system.

Quantum key distribution systems are made up of a transmitter and a receiver for sending and receiving photons, together with control servers for performing the processing performed when generating cryptographic keys. Cryptographic keys are created in a four step process (Fig. 1).

In the first step, the transmitter sends photons containing cryptographic key information (bit information) and basis information, as defined in the BB84 protocol. These photons are detected by a receiver (Fig. 1 - (1) Photon transmission and detection).

* The BB84 protocol is explained in depth in part 1 of this series.

Next, the basis information for the transmitted photons is shared between the transmitter and the receiver. Based on this information, the bit information that can be used as a cryptographic key is selected (Fig. 1 - (2) Sifting).

There may be errors in the photon data sent by the transmitter due to noise while the photon was being transmitted. In the third step, errors in the data received by the receiver are corrected to prevent them from affecting later processing (Fig. 1 - (3) Error correction).

In the last step, after correcting errors for the received data, data is compressed to a size whose security can be guaranteed (Fig. 1 - (4) Privacy amplification)

The data compressed by privacy amplification is provided as a cryptographic key to the cryptographic communication server that performs encryption. This enables highly secure cryptographic communication.

Now let's look at the Toshiba technologies used to accelerate each of these four cryptographic key generation steps.

First, we'll look at how to speed up step number one, "photon detection" (Fig. 1 - (1)).

The receiver, which detects photons, must be a highly sensitive detector in order to detect the extremely weak photons that carry the cryptographic key information. However, the more sensitive a detector is, the more susceptible it is to ambient noise, which prevents it from detecting photons that get drowned out by the noise.

Toshiba has tackled this problem by using Avalanche Photo Diodes (APDs), photo diodes with improved detection efficiency, as detectors. It has also developed self-differencing circuit technologies with embedded delay and difference circuits to eliminate periodic noise (*1) (Fig. 2).

These technologies improve photon detection efficiency, increasing the speed with which keys can be generated.

To continue sending cryptographic keys correctly, it is important that the transmission of photons is itself stable.

The signals sent in quantum key distribution are carried by photons, which are extremely weak. This makes it difficult to stably transmit photons when transmission is affected by outside factors.

In real-world environments, in particular, when photons are passed down optical fibers, they are easily affected by factors such as the ambient temperature or the vibration of the optical fibers themselves. These factors can change the phase or polarization of the light. This causes the bit error rate to increase and prevents the sending of accurate bit information.

To address this problem, Toshiba has developed stabilization control technology. This technology periodically sends stabilization pulses along the same optical fibers where the photons for keys pass through. The stabilization pulses is used to detect and eliminate any deviations during standard operation. This stabilization control technology lowers bit error rates and continuously stabilizes the distribution of keys (Fig. 3).

• Quantum Key Distribution