Optics 101

How Are Lasers Used for Quantum Cryptography?

During World War II, computational cryptography used hidden messages, ciphering codes and encryption tactics all based in machinery.

Today, cryptography is more widely-used than ever, however, the ciphering codes have developed into complex mathematical algorithms, and ‘cryptography keys’ are produced by merging algorithms with random number generators. This form of cryptography can be seen everywhere from the internet to energy infrastructure to satellite systems.

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These widely-used systems are increasingly under the looming threat of quantum computer technology. These next-generation computers could crack today's most protected systems in mere seconds. To combat this looming threat, agencies around the world are investigating next-generation encryption technologies like quantum key distribution (QKD).

Founded on fundamental physics, QKD allows for two parties to share an encryption key in a manner that is impossible to hack without detection. This feature significantly augments the current methods of key distribution.

QKD involves passing random numbers between two points, through 'entangled' photons, which are then used to produce the encryption key. Hacking this type of communication requires measuring the photons. Measurement permanently alters the quantum qualities of these particles, a shift that would be detected by the parties trying to communicate.

QKD technology is limited when used over standard optical fibre, which confines the distance data can be sent to a few hundred kilometres. Scientific studies have revealed that laser photons can address this issue, as they can be received and sent over long distances, including from satellite to satellite, satellite to ground and ground to satellite. Therefore, a satellite sending a key can pass it to a receiver anywhere on Earth.

Creating entangled pairs of photons

One of the primary aspects of emerging quantum cryptography systems is the use of photon entanglement. Entangled pairs of photons must be produced at the exact same time with identical qualities. This is typically performed through the use of frequency combs.

A frequency comb changes a single wavelength into several wavelengths, producing tens to hundreds of lasers from a single beam. State-of-the-art frequency comb systems are the size of a human hair and use one-thousand times less power to operate than previous iterations. This size and efficiency does allow for mobile applications.

The initial step in creating a frequency comb occurs when the principal laser produces a secondary pair of wavelengths. Due to energy conservation, one wavelength must have more energy and one wavelength must have less energy. These energies must add up to the same energy as the first laser, and the two new wavelengths must be produced at the identical time. Strictly speaking, frequency comb generators can be considered entangled photon generators.

While manageable size and efficiency were major technical developments, there are still numerous integration and production difficulties that must be addressed before the creation of a portable quantum cryptography platform.

Passing a critical test

The quantum cryptography depends on individual photons to transport quantum data. However, even the best optical fibres can carry photons around 200 kilometres. Hence, quantum cryptography has not been possible over long distances, until last year.

In January 2018, China demonstrated the first intercontinental quantum cryptography service. Scientists successfully tested the system by staging up a secure videoconference between Europe and China.

This was feat accomplished using China’s Micius satellite. The orbiting satellite distributed a secure quantum key from using a laser. Micius was in a sun-synchronous orbit over the poles, which meant it moved over every location at approximately the same local time each day.

When the satellite was over the Chinese ground station, it sent the key to the ground using a laser and well-established protocol. As soon as the ground station in Austria came into view, Micius sent the same key using the same system. The two locations then had the same key, allowing them to initiate totally secure communication over a standard connection.

While there are imperfections in the system, the work pointed towards a competent solution for a long-distance global quantum network, laying the foundation for a future quantum Internet.

References:

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Brett Smith

Written by

Brett Smith

Brett Smith is an American freelance writer with a bachelor’s degree in journalism from Buffalo State College and has 8 years of experience working in a professional laboratory.

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