What is quantum cryptography?

The main feature of quantum cryptography is that information is transmitted in such a way that it is impossible to tap it. Moreover, complicated data encryption algorithms are not used. Hacking of a quantum based system contradicts laws of physics.

Unlike traditional ways of signal transmitting our system transmits a signal using single photons. A photon cannot be separated or estimated. It is also impossible to make its copy because a photon will be ruined. The main problem is how to make effective transmission process because our system is not ideal. Passing form a sender to a recipient a photon is affected by different things that can ruin it. We aim to develop high bit rate system, which transmits information as far as it possible.


How do quantum communications work?

There are several operational concepts. A lot of companies use quantum cryptography systems. ITMO University researchers invented a new technology. We use side frequencies in order to transmit signals. Such systems are much faster than traditional.


To realize advantages of your method it is necessary to compare it with classical systems. Please, describe it in details.

To encode quantum information one has to make signal modulation i.e.  shift a phase. Then we estimate position of phases. If waves are out of phase it means that phase shifts are the same and signal power is equal to zero. To enhance a field waves should be in phase. Thanks to interference we receive a resultant wave. To make it clear I will use the following example: if two men march in step the phases are equal if they break step it means that phases are different. To be out of phase first man has to start walking when the second one has made a half of his step.

The main target is to estimate a photon phase correctly in order to get enhanced signal. Interferometers providing estimations in classical systems are not perfect. Under the influence of heat, cold or vibrations a signal becomes worse. Thus disturbances appear.


Please tell us advantages of side frequencies.

We use spectrum based technology. A special laser generates   fixed-frequency pulses. Then it generates a pulse, which has lower frequency. Using modulators we encode signals estimating a phase position of additional sinusoidal function instead of main function.

This kind of encoding is very effective because a spectrum is transmitted by only one pulse. So if something affects a channel the pulse changes entirely. Moreover effectiveness of this method is about 40% as opposed to 4% set by classical ways.

Yuri Mazurenko, senior researcher of ITMO University Center of Optical Information Technologies, invented this method. Nowadays side frequency based encoding of quantum information is developed by two groups of researchers from France and Spain. However Russian scientists provide the main research project.


How does this method work?

All these complicated processes lead to secure key generation, which is a random sequence. We mix this sequence with a message. Thus this information cannot be captured. Information security systems work on the same principle as VPN devices.


Please explain us encoding process.

Thanks to a random number generator each device generates photons quantum state. According to quantum communication transmitter and receiver modules are usually called Alice and Bob respectively. For example, Alice and Bob determine a quantum state, which is equal to zero. Thus optical radiation phases coincide with each other and a signal is high. If Alice chooses zero while Bob chooses one a detector doesn`t work because the phases are different. Then the receiver determines those transmissions where phases are equal. Thus the key uses only “successful” transmissions. The most important thing is that only Alice and Bob know what kind of information they exchange, I mean zero or one.


Why this system cannot be hacked?

A photon is indivisible. If it is retrieved from a channel Bob will receive nothing. Thus photon detector will fail and transmitter and receiver won`t use this data bit as a key element. A hacker has an opportunity to capture this photon but it is useless because a bit, which is encoded in this photon, is not used in transmission process. It is also impossible to copy it because a photon is crashed by copying.  


Aleksander Pushkash

News Portal Editorial Office