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The problem connected with the personal information protection transferred on public telecommunication channels today represents great interest. Possibilities of the high-speed computers and quantum computers which are intensively developing lately have considerably simplified process of interpretation of information. For example, the RSA Pentium 4 processor was controlled with cracking of 1024-bit enciphering a little more than in 104 hours.
Regardless of specific implementation, safety of information is ensured due to excess of average time of interpretation over time of relevance of data today. This problem can be solved with the help of the "absolutely resistant cipher" developed by K. Shannon. However in this case before the user there will be task to exchange unique keys before each communication session that increases risk of their interception and the subsequent compromise of data.
And now imagine such communication line to which it is impossible to listen in any ways as it contradicts laws of physics. Whatever the malefactor tried to undertake, at him it will not turn out to intercept the transmitted data. Such devices for data transmission using the principles of quantum cryptography are created in JSC Kvantovye kommunikatsii – the small innovation enterprise at ITMO University.
We have achieved considerable success in development of the system of quantum communication for optical fibers – it possesses parameters close and even exceeding the record results received in laboratory systems. If to compare to the systems of quantum communication presented on the market, we have learned to transmit quantum signals and, as a result, to implement the protected data transmission at the speeds, by 10 times exceeding the best world analogs. As for range, we can transmit signal 3 times farther.
Background of the project
The project began as NIR which was carried out at university and had especially fundamental character. During work it was offered to generate information mediums – single photons – not radiation source, and as a result of phase shift keying of classical impulses. Then the task to find out has been set: whether it is possible to transmit quantum signals to long distance on the optical cable in this way, whether perhaps it physically?
During tests it has been established that such modes are implemented and really allow to reach the advantages described above in the speed and range. After carrying out series of laboratory researches the model which has been sent to exhibition to Lenexpo has been created and tested by potential audience. The experimental sample is already developed and the first model of section of the quantum network based on use of subcarrier (side) frequencies, connecting the body of university is created. It is the Russia's first pilot section of "civil" quantum network.
The systems of quantum cryptography based on use of subcarrier frequencies
In systems of mailing of quantum key as carriers of information signal single photons which are in most cases generated by limit weakening of laser radiation are used. Alternative approach is offered in the systems of quantum cryptography based on use of side (subcarriers) chastotv result of phase frequency shift keying quantum signals are taken out on the next frequency components (figure 1).
Thus the spectral interval between bearing and side frequencies is defined by parameters of modulating signal and makes about 10-20 pm. Systems at side frequencies are characterized by higher generation rate of keys and low probability of emergence of errors. However their main advantage is opportunity to generate at once some side frequencies in neighborhood of one central that allows to place to 10 independent quantum channels in one window of the multiplexer.
In combination with WDM this technology allows to increase spectral efficiency of systems of quantum cryptography on the optical networks from today's 4% to 40% and more, having implemented quantum networks at data transmission rate about 400 Mbps. All this does technology cost-efficient – now the speed of world-best quantum systems of only 1-2 Mbps (50 km), while the flow capacity of the channel on the Ethernet network – about 1 Gbit / to page.
Fig. 1. Range of optical signal after modulation in the block of the sender (and) i the receiver in case of constructive (b) and destructive (c) interferences
(Fig. 2) have given the simplified diagram of our system below. The junction laser generates radiation with narrow range on the wavelength of 1550 nanometers. After that radiation comes to the phase modulator FM1 managed by electronic control unit (electronic control unit). As a result of phase shift keying in radiation there are two side frequencies differing from bearing 4,4 GHz on the value of the modulating radio signal.
Power of side frequencies is controlled by change of amplitude of modulating signal. The modulated signal arrives on AT attenuator on which output signal power at side frequencies corresponds to median number of photons on impulse (unit order). Each bit of the transmitted signal is coded by means of phase shift ΦА, added to modulating signal. Phase shift is controlled by electronic control unit and on each bit is selected accidentally from four values: 0; π/2; π and 3π/2.
Figure 2 – Schematic circuit of system of the quantum cryptography based on use of subcarrier frequencies
Electronic control units in the transferring and accepting modules are synchronized by means of signal of special form: sinusoids with frequency of 10 MHz and gating pulse lasting 10 nanoseconds. The starting strobe signal initiates generation of key, and the subsequent – synchronize record of quantum counting in buffer memory of the transferring and accepting modules. Modulation generators of the transferring and accepting modules are synchronized by sine signal. Synchronization signals are transferred on separate optical fiber.
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Figure 3 – Range and the oscillogram (and) weakening and the strengthening interference
The cryptographic key is generated under the BB84 protocol with strong reference pulse. The quantum signal in the accepting module arises at the time of passing by the laser of the polarizing controler, the phase modulator FM2 and the spectral SF filter connected consistently.
The spectral filter selects signal of side frequencies which is traced by means of the detector (counter) of photons of DOF. At this stage the signal is exposed to repeated phase shift keying. The phase modulator FM2 is controlled electronic control unit, and the bit sequence is coded in the same way as in the transferring module.
Frequency of change of phase is equal 100 MHz. The phase modulator enters to modulating signal phase displacement ΦB; each bit shift is selected in a random way from four possible values. The resulting power of subcarrier wave depends on phase shifts ΦA and ΦB. If ΦA = ΦB, there is strengthening interference (fig. 3b), and the power of optical signal is other than zero. If ΦA – ΦB = π, the weakening interference (fig. 3a) is shown, and the power of optical signal is comparable to noise of dark current of the detector of photons. Statuses with phase difference π/2 are discarded.
Information exchange, necessary for processing of results of measurement, is executed on the open channel. Thus the "crude" key is generated at the same time in the transferring and accepting modules. After that for "crude" key the error rate (QBER) by which legitimate users can determine is calculated, whether attempt of the wiretap has been made. If the wiretap was not, errors are adjusted, and in the transferring and accepting modules the secret cryptographic key is generated.
We have made series of experiments in vitro (fig. 4). Such parameters of system as generation rate of key and frequency of quantum errors (QBER) at distances to 250 km have been measured in optical corning SMF-28 ULL fiber.
Figure 4 – Experimental installation with the superconductor one-photon detector on the basis of ultrathin film (SNSPD)
The value QBER is defined as the relation of number of wrong bits to total number of the received bits:
where Rsift (length of the "sifted" key) equals to number of coincidence of bases of Alice and Bob (the receiver and the transmitter) that in turn equals to half of length of "crude" key:
where frep – repetition rate of impulses, μ – median number of photons for impulse, tlink – transmission ratio, that is probability of photon to reach the Bean detector, η – probability of detection of photon, that is quantum efficiency of the detector. Q factor<=1 введен для систем с фазовым кодированием, чтобы учесть не интерферирующие фотоны.
Results are collected in the table below and presented on diagrams.
If the distance of transfer equally 200 m, generation rate of the "sifted" key makes 200 bps, at QBER equal only 1%. That is remarkable, such results have been received on sinkhrochastota in 100 MHz that considerably simplifies electronic subsystem and leaves backlog for further increase in speed (in record systems with comparable parameters the sinkhrochastota about 1 GHz is used).
It has become possible thanks to high visibility of the interference figure (V> 98,9%) caused by exact control of relative phase shifts, to the low level of the dark account and one-orientation of the channel. For the maximum distance at which experiment was made, the speed of mailing of the "sifted" key has made 28 bps, at QBER of equal 9,3% that still allows safe key generation.
Implementation of system allows to create networks of new type which give new quality of ideally safe world: they can be built in the Internet of things, network concepts. It is platform which can be used for development of end-to-end systems of safety and communication and also as basis of safe data transmission for other solutions. For example, for protection of control signals of robot planes.
If to approach question from the user party, the system can be applied in the software implementing the data exchange modes between clients. The ideology is comparable with Android or iOS OS – the user has platform, and he solves the specific objectives with its help. They have to be optional connected with space and military technologies – the system is suitable for solution of questions, the most approximate to the user. We will tell about these questions and other subjects connected with perspectives of use of quantum networks in our following materials.
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