RU2806904C1 - Device for generating quantum states for quantum communications systems with assessment of quality of preparing states for protocols for quantum key generation on chip - Google Patents

Abstract

FIELD: quantum cryptography; information protection.

SUBSTANCE: invention relates to information protection implemented on a photonic integrated circuit. The device for generating quantum states for quantum communication systems with assessment of the quality of preparing states for quantum key generation protocols on a chip contains a semiconductor laser, an amplitude modulator made in the form of a symmetric Mach-Zehnder interferometer and consisting of a disconnector, a combiner and two phase modulators, narrowband and broadband, additional disconnector installed after the amplitude modulator combiner before the broadband phase modulator, wideband phase modulator, adjustable optical attenuator installed after the broadband phase modulator and made in the form of a disconnector, two narrowband phase modulators and an X-shaped coupler, single-ended Mach-Zehnder interferometer with a delay line in the long arm, a splitter for outputting the optical signal into the quantum channel, and a narrowband phase modulator in the short arm. The arms of an asymmetrical Mach-Zehnder interferometer are connected in a splitter, the beams from which are directed to photodetectors, an additional adjustable optical attenuator installed before entering the quantum channel and made in the form of a splitter, two narrow-band amplitude modulators and a combiner. The semiconductor laser operates in continuous mode.

EFFECT: increase in the stability of a quantum telecommunication system to errors caused by temperature, vibration and other changes in the system, and ability to assess the quality of preparing states for quantum key distribution protocols with phase, phase-time encoding and with trap states.

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Description

Field of technology to which the invention relates

The invention relates to the field of quantum cryptography in the field of information protection, implemented on a photonic integrated circuit.

Glossary

In order to ensure the sufficiency of disclosure of the invention and to ensure the possibility of conducting an information search in relation to the claimed technical solution, as well as to understand the essence of the claimed invention, below is a list of terms used in the description that do not limit the scope of legal protection.

Terms:

A waveguide is a guiding channel in which an electromagnetic wave can propagate.

Interference is a redistribution of the intensity of electromagnetic waves as a result of the superposition (superposition) of several electromagnetic waves.

An interferometer is a measuring device whose operation is based on the phenomenon of interference.

A quantum network is a communication network in which transmitted data is protected by quantum cryptography methods.

Quantum entanglement is a quantum mechanical phenomenon in which the quantum states of two or more objects cannot be described independently. Quantum coherence is a quantum phenomenon consisting in the correlation, or consistency, of the movement of microparticles that form a given physical system. Encryption key - secret information used to encrypt and decrypt messages.

A qubit is the smallest unit of information in a quantum computer (analogous to a bit in a conventional computer).

A laser is a source of electromagnetic radiation with high temporal and spatial coherence.

Modulator is a device for converting electrical signals into the optical domain (electro-optical conversion).

X-shaped coupler is a device that combines the functions of a splitter and a combiner.

Splitter/combiner - a device for dividing/summing input signals. Photodetector is a device for converting optical signals into the electrical domain (optoelectronic conversion). State of the art GENERAL OVERVIEW

The idea of quantum cryptography was first proposed in the 1970s, and active research in this area began after Bennett and Brassard published the first quantum key distribution protocol BB84 in 1984. In cryptography, it is customary to designate the sender of information as Alice, the receiver as Bob, and the interceptor as Eve. The task of quantum key distribution is to transmit a secret key over a quantum communication channel between Alice and Bob in such a way that Eve cannot intercept it. After transmitting the key, the encrypted data can be transmitted over a classic communication channel.

Quantum key distribution protocols can be divided into two groups - protocols with discrete variables and protocols with continuous variables. Discrete variable protocols encode information using single photons, while continuous variable protocols encode information in field quadratures. Both optical fiber and free space can be used to transmit photons between Alice and Bob. Two different wavelengths can be used to transmit photons in optical fiber - the standard telecommunications wavelength of 1550 nm, as well as 850 nm. Losses in the fiber are much lower at a wavelength of 1550 nm and amount to 0.2 dB/km. However, at a wavelength of 850 nm, a single-photon avalanche diode made of silicon can be used to detect single photons. At a wavelength of 1550 nm, it is necessary to use detectors made of germanium or InGaAs alloy, which increases the proportion of bit errors.

Currently, the implementation of quantum key distribution using a class of devices called “photonic integrated circuits” is particularly developed. Photonic integrated circuits are a multi-component optoelectronic device (monolithic or hybrid integrated circuit), manufactured on a single substrate and capable of performing the functions of generating, detecting and processing optical signals, and gradually occupying a place as the basis of the range of components of all optical systems.

Quantum key distribution devices place more stringent requirements on the optical components used than standard telecommunications systems.

Photonic integrated circuits provide a platform that allows the monolithic integration of multiple optical components performing different functions on a single chip. The FIS platform allows, using well-developed and high-precision lithography technology, to produce chips that meet the requirements of devices for quantum key distribution. Photonic integrated circuits can be fabricated on multiple material platforms, each with advantages and disadvantages.

In modern and future systems based on photonics, where many different elements are used, the logical solution is their integration on a chip. This is what integrated photonics does - a relatively new and actively developing global industry. The known advantages of photonic integrated circuits (PICs), such as compactness, speed, and energy efficiency, allow them to complement or replace both discrete photonics and microelectronics in many applications.

While quantum communication systems based on traditional optical components use weak coherent pulses that are vulnerable to photon splitting attacks, the FIS platform allows the fabrication of other light sources for use in quantum communications - single photon generators and entangled photon pair generators.

At the moment, on the FIS platform, not only individual components for the implementation of quantum communication systems have been obtained, but also the implementation of quantum key distribution protocols using FIS both as a transmitter and as a receiver has been successfully demonstrated.

One of the main problems in the field of quantum telecommunications is the miniaturization of dimensions and reduction in cost of quantum communication systems. At the moment, the solutions and technologies being developed make it possible to create stationary samples of quantum communication systems, the dimensions of which are significant and cannot be used for mobile and, especially, personal use. For example, a prototype of a sender module in free space has dimensions of the order of 250x250x200 mm3 and a weight of several kg. In the fiber version, the sender module can be implemented in dimensions of the order of 200x150x30 mm3, which is also significant, and the weight reaches ~1 kg. At the same time, it is already obvious that in the future such systems will need to be integrated not only into stationary backbone systems, but also into compact mobile devices, so such large dimensions are not acceptable. It is also worth noting that pilot projects are underway around the world to create such miniature systems on a chip, but they are still at the development stage and require additional research.

In addition, a significant reduction in the cost of technology and obtaining small dimensions of quantum communication systems will significantly expand their areas of application and move from niche applications in quantum communication systems of backbone and city point-to-point lines to use in large-scale user markets of the Internet of Things with quantum protection protected by personal data. mobile communications, 5G and later 6G.

Such penetration of quantum-protected systems is only possible if technologies are available that allow the creation of compact systems with low cost, weight and power consumption that can be easily integrated into user devices. With the help of photonic integrated circuits, it is possible to replace bulk discrete components with a single chip, the dimensions of which will not exceed 20x20x1 mm3 and weight 10 grams.

As a technical solution that made it possible to increase the reliability of a protected communication line using a miniature device, a “Device for generating quantum states for quantum communication systems with assessment of the quality of preparing states for quantum key generation protocols on a chip” is proposed. The proposed device allows you to choose two types of encoding for the formation of quantum states: phase encoding, in which information is recorded in the phase difference of attenuated laser pulses, and time encoding, in which the value of the qubit is determined by the time the pulse arrives at the detector.

Sometimes these encoding methods are combined into one - the so-called phase-time or simply time encoding. Indeed, from a physical point of view, both coding methods are associated with changes in the timing characteristics of signals. The only difference is that with phase encoding the carrier of the optical signal is shifted in time, while with time encoding the pulse envelope is shifted in time. From a mathematical point of view, phase-time coding is isomorphic to polarization coding, often used in quantum key distribution systems.

From the point of view of practical implementation, both encoding methods (polarization and phase-time) are largely equivalent, however, in the context of the formation of quantum states on a chip (using integrated optics), phase encoding is preferable, since it is easier to manipulate the phase of the optical signal in the waveguide than to change its polarization.

The use of the “Device for generating quantum states for quantum communication systems with assessment of the quality of preparing states for protocols for quantum key generation on a chip” makes it possible to ensure the security of a secure quantum communication line, as well as to provide the ability to quickly inform about an attempt to compromise a secure communication line using a device with minimal dimensions and energy consumption, as well as reduce the percentage of errors by compensating for various changes, including temperature, typical for miniature devices.

During the patent search, documents were discovered that define the state of the art and are not considered particularly relevant to the claimed invention, namely: “High-speed auto-compensation circuit for quantum key distribution” (patent No. RU 2 671 620 C1)

“Transmitting device, receiving device, and quantum key distribution system”, patent No. US11387992B2

The above technical solutions represent various options for devices for quantum key distribution based on photonic integrated circuits. As an analogue of the claimed invention, one can consider the technical solution disclosed in the patent “High-speed auto-compensation circuit for quantum key distribution” (patent No. RU 2 671 620 C1, priority date 2016.12.29). The solution relates to quantum cryptography, which lies in the field of information security. The technical result is to increase the maximum repetition rate of laser pulses at a fixed value of their width, which allows the use of an auto-compensation circuit at a frequency whose period is equal to the laser pulse width, which is the maximum possible result. The communication system for transmitting a cryptographic key between the ends of the channel includes a transmitting unit containing a beam splitter, an electro-optical attenuator, an amplitude modulator, a phase modulator, an accumulation line, a Faraday mirror, and a synchronization detector; a receiving unit containing a laser, avalanche photodiodes, a beam splitter, a circulator, a delay line, a phase modulator, a polarization beam splitter, and a Mach-Zehnder interferometer; as well as a quantum channel for connecting these nodes. In this case, the storage line is placed between the sender's electro-optical phase modulator and the Faraday mirror.

The difference between this technical solution is a fundamentally different sequence of modulations of the initial signal, including several delay lines and a polarization divider.

As a prototype, we can highlight the technical solution “Transmitting device, receiving device, and quantum key distribution system,” patent No. US11387992B2, priority date 2018-01-18. According to the description, this solution is a transmitting device for a quantum key distribution system and includes a light source, a beam splitter, an encoder and a beam combinator. The light source is configured to generate an optical pulse. The beam splitter is configured to split the optical pulse into a signal pulse traveling along a first path and a polarization control pulse traveling along a second path, the second path having a different optical path length from the first path. An encoder is installed on the first path and configured to encode information in the signal pulse. The beam combiner is configured to combine the signal pulse passing through the encoder and the polarization control pulse.

The difference between this technical solution is a fundamentally different sequence of modulations of the initial signal, in which the initial beam from the light source goes along two routes in which the signal is modulated, then the separated beams are connected and modulated again. In this case, the signal modulation occurs in parallel, not sequentially.

The above technical solutions, like the claimed invention, are to one degree or another intended for quantum cryptography, however, the sequence of modulations of the initial signal of the above technical solutions differs significantly from that proposed in the present invention.

In particular, the key difference between the proposed method and the prototype is the absence of delay lines on the path of the beams directly encoding the quantum key. The technical problem to be solved by the present invention is to increase the speed of generating a secret cryptographic key, the ability to assess the quality of preparation of states for quantum key generation protocols with phase, phase-time encoding and with trap states.

The technical result of the solution under study is to increase the stability of a quantum telecommunication system to errors caused by temperature, vibration and other changes in the system, and the ability to assess the quality of preparing states for quantum key distribution protocols with phase, phase-time encoding and with trap states.

Disclosure of the Invention

To solve the problem and achieve the above technical result, a “Device for generating quantum states for quantum communications systems with assessment of the quality of preparing states for protocols for quantum key generation on a chip” is proposed.

The device for implementing this method consists of the following elements:

1. Semiconductor laser operating in continuous mode 1;

2. Amplitude modulator 3, made in the form of a symmetrical Mach-Zehnder interferometer and consisting of a disconnector 2 and a combiner 6 and two phase modulators: narrowband 4 and broadband 5;

3. Additional disconnector 7, installed after the combiner 6 of the amplitude modulator 3 in front of the broadband phase modulator 9, necessary to control the operating point of the amplitude modulator 8.

4. Wideband phase modulator 9, necessary to change the phase of laser pulses;

5. Adjustable optical attenuator 11, installed after the broadband phase modulator 9 and made in the form of a disconnector 10, two narrowband phase modulators 12 13, and an X-shaped coupler 14;

6. Asymmetrical Mach-Zehnder interferometer 15 with a 165 ps delay line 16 in the long arm, a splitter 17 in the short arm for outputting the optical signal to the quantum channel 26 and a narrow-band phase modulator 18, while the interferometer arms are connected in an X-shaped coupler 19, beams from which are sent to photodetectors 20;

7. Additional adjustable optical attenuator 22, installed before entering the quantum channel 26 and made in the form of a separator 21, two narrowband amplitude modulators 23, 24 and a combiner 25; necessary to achieve the required level of attenuation of laser pulses.

The operation of the device is described as follows:

1. A beam of light is directed from laser source 1 along a waveguide to amplitude modulator 3 to separator 2, enters phase modulators 4 and 5, and is connected in combiner 6.

2. Next, the single beam goes to the separator 7, from where it enters the control of the operating point 8, which adjusts the voltage on the narrow-band phase modulator 4 according to the optical signal (for example, when temperature, voltage, etc. change).

3. Through another channel, the beam is sent to a broadband phase modulator 9, which introduces an information load to the signal, where the phase difference is set.

4. The beam is directed to the separator 10, then enters the attenuator (slow amplitude modulator) 11, where the beams are given a certain phase difference on the phase modulators 12 and 13.

5. Next, the beams from the phase modulators 12 and 13 enter the X-shaped coupler 14, which, depending on the phase difference, directs the beams in different proportions into two channels into the interferometer 15, in which operation is possible in two modes

6. In mode No. 1, test, in order to understand what phase difference is set by the phase modulator 9, the beam enters the interferometer 15, where two successive pulses are interfered with. Narrowband phase modulator 4 is configured for a shift of 0 or π/2. One beam goes along the delay line 16, the second goes to the splitter 17, from where half of the beams from it go to the narrow-band phase modulator 18, then two beams (from the delay line 16 or the narrow-band phase modulator 18) go to the X-shaped coupler 19, from where depending on the phase difference, they arrive in different proportions at photodetectors 20.

7. In mode No. 2, operating, most of the radiation follows from the attenuator 11 through the X-shaped coupler 14 to the delay line 16 in the interferometer 15, from where it is scattered. A smaller part of the radiation enters the splitter 17, from where half goes to the attenuator 22 through the splitter 21. From the attenuator 22, the beams interfere with the combiner 25, while most of it is scattered, and the smaller part enters the quantum channel 26.

Amplitude modulator 3 is necessary for the formation of optical pulses with a given repetition rate, which are “cut” with its help from a continuous laser beam. The frequency of preparation of quantum states corresponds to the repetition rate of pulse pairs, and the time delay between pulses in a pair encoding a quantum state is fixed and must correspond to the delay line of the asymmetric Mach-Zehnder interferometer 15 (165 ps).

In supported quantum key distribution protocols, the qubit is encoded in the phase difference between pulses. To set the phase difference between the pulses in a pair, a wideband phase modulator 9 is used, following the amplitude modulator 3. The depth of the phase modulation must be at least 3π/2 in order to be able to encode at least four phase difference values from the list: 0, π /2, π, 3π/2.

An adjustable optical attenuator 11 is used to attenuate the prepared coherent states in the circuit of “a device for generating quantum states for quantum communications systems with assessing the quality of preparing states for on-chip quantum key generation protocols.”

An additional adjustable optical attenuator 22 in the optical circuit “Device for the formation of quantum states for quantum communications systems with assessment of the quality of preparation of states for quantum key generation protocols on a chip”, located before the exit to the quantum channel 26, is used to achieve the required level of attenuation of laser pulses. Using adjustable optical attenuators 11 and 22, weak coherent states are created and sent to the quantum channel.

The declared “Device for generating quantum states for quantum communication systems with assessment of the quality of preparing states for on-chip quantum key generation protocols” makes it possible to implement at least the following known protocols:

1. 4-state protocol with phase coding (BB84).

Each state is a pair of optical pulses separated by a time delay ΔT=165 ps. The period of repetition of quantum states, indicated in Fig. 2 as _Тр , can vary within wide limits: the minimum value is _Тр =330 ps, which corresponds to the frequency of preparation of quantum states of 3.03(03) GHz, and the maximum value, generally speaking, can be made as large as desired. The prepared pulses pass through attenuators 3 and 22, configured to be attenuated so that each pair of pulses produces an average of 0.1-0.3 photons. This means that 70-90% of the prepared states will be “empty”, i.e. they will not contain light quanta. Some of the “non-empty” parcels can be represented as a photon, as if “spread” over two time modes corresponding to a pair of laser pulses. The qubits are encoded in the phase difference between the pulses in a pair. If the phase difference is 0 or π, then the state is said to be prepared in the X basis; if the phase difference is equal to π/2 or 3π/2, then the state is said to be prepared in the Y basis. Phase differences 0 and π/2 can be assigned a logical zero, '0', and phase differences π and 3π/2 - a logical one, '1'. Taking into account this agreement, pulses with phase differences 0 and π can be called states X ₀ and x ₁ , respectively, and pulses with phase differences π/2 or 3π/2 states Y ₀ and Y ₁ , respectively.

To decode quantum states on the receiver side, you can use an asymmetric Mach-Zehnder interferometer, the delay line of which is equal to ΔT, i.e. coincides with the time delay between pulses in a pair.

In FIG. Figure 3 shows the results of pulse interference in an asymmetric receiver interferometer. It can be seen that when choosing the X basis on the receiver side (ϕ_B=0), the result of interference will be deterministic in the case when quantum states are also prepared in the X basis, i.e. when ϕ_A=0,π. In this case, the phase difference ϕ_A-ϕ_B will be equal to 0 or π, which will give constructive or destructive interference, respectively. In terms of quasi-single-photon pulses, this means that either the DO detector will work (if ϕ_A-ϕ_B=0) or the D1 detector (if ϕ_A-ϕ_B=\π), so that it will be possible to unambiguously decode the measured qubit. The same is true for the case when the Y basis is selected on the transmitter and receiver sides. If the bases of the transmitter and receiver do not coincide, for example, pulses with a phase difference ϕ__A=0 were prepared at the UVKS, and the phase ϕ_B=π/2 was set on the phase modulator of the receiver, then ϕ_A-ϕ_B=-π/2=3π/2, and with some probability, both detector D0 and detector D1 may work, so it will not be possible to unambiguously decode the measured qubit. The scheme for selecting states and bases in the BB84 protocol for the case of phase coding is shown in Fig. 4.

2. Protocol on an increased number of states (T12)

In order to enhance the secrecy of the quantum key distribution system, you can use additional states that are non-orthogonal to the states X ₁ , X ₂ , Y ₁ and Y ₂ . Pulses schematically shown in Fig. 1 can be used as such states. 5, in relation to which they are said to be prepared in a Z-basis. In this case, they also talk about phase-time encoding, since qubits in the X- and Y-bases are encoded in the phase difference between pulses, and in the Z-basis - in the time of arrival of a pulse at the detector. The time frame allocated for the qubit (the frames are outlined by dotted rectangles in Fig. 1.5) is divided into two time slots, t ₁ , t ₂ , and if the laser pulse is generated in the time slot t ₁ , then this corresponds to a logical '0', and if in slot t ₂ - logical '1'. It is worth noting here that the energy in a pulse encoding a state in the Z basis must be equal to the total energy in a pair of pulses encoding a state in the X basis. Although this condition is not mandatory, it allows you to obtain more key in post-processing.

To decode the quantum states shown in FIG. 5, you can use an asymmetric Mach-Zehnder interferometer with a delay line equal to Δt, as well as a single photon detector with pulse arrival time resolution. If a qubit passes through the splitter and enters the interferometer, this means that the X- or Y-basis will be selected, and the choice between these bases must be carried out actively, by changing the phase on the broadband phase modulator in the interferometer. If the qubit is deflected and does not enter the interferometer, this means that the Z basis has been selected. If at the same time a state in the Z basis was prepared on the device, then when the second detector is triggered in time slot t ₁ , the measured state is decoded as '0', but if detector D2 is triggered in slot t ₂ - then as '1'. 3. Protocol with decoy states.

If Alice uses attenuated coherent states instead of single photons, then Eve has the additional opportunity to eavesdrop on the key without being noticed. It can carry out what is called a photon splitting attack. To counter this attack, a technique was developed for decoy states - additional coherent states that Alice sends to Bob mixed with information states, and in which the average number of photons differs from the average number of photons in information pulses. Since Eve does not know which impulses are informational, she will act on all impulses. By then revealing which states were traps, Alice and Bob will be able to understand whether the attack was carried out to split the number of photons, i.e. will be able to notice Eve's presence.

At the same time, a protocol with trap states is, in essence, an add-on to other protocols and is implemented by adding states with other intensities to the “pure” (without traps) protocol. In the general case, the number of trap states is not limited, but, as a rule, 1-3 values of additional signal amplitudes are used. For example, in the claimed device it is proposed to use two trap states, as shown in Fig. 6, and it is proposed to use the vacuum state indicated in FIG. as one of the trap states. 6 as |vac〉. The notations |E〉 and |L〉, in turn, correspond to coherent states in time slots t ₁ and t ₂ , respectively (see Fig. 5), and the parameter s is selected in the range from 0 to 1.

To summarize, the description of the device can be presented as follows: A device for the formation of quantum states for quantum communications systems with assessment of the quality of preparation of states for quantum key generation protocols on a chip, including a semiconductor laser operating in continuous mode, an amplitude modulator made in the form of a symmetric Mach-Zehnder interferometer and consisting of a disconnector and a combiner and two phase modulators - a narrowband low-frequency and a broadband high-frequency, an additional disconnector installed after the amplitude modulator combiner in front of the broadband phase modulator, necessary to control the operating point of the amplitude modulator, a wideband phase modulator, necessary to change the phase of laser pulses, adjustable an optical attenuator installed after the broadband phase modulator and made in the form of a disconnector, two narrowband phase modulators, and an X-shaped coupler, an asymmetrical Mach-Zehnder interferometer with a delay line in the long arm, a phase modulator in the short arm, a coupler in the short arm for optical output signal into the quantum channel, while the arms of the interferometer are connected in an X-shaped coupler, the beams from which are directed to external photodetectors located outside the chip, an additional adjustable optical attenuator installed before entering the quantum channel and designed as a splitter, two narrow-band amplitude modulators and unifier; necessary to achieve the required level of attenuation of laser pulses.

The invention is disclosed and illustrated in the following drawings:

Fig. 1 - Schematic diagram of the device. The above figure schematically shows all the components of the device, showing the sequence of their arrangement. In this case, except for the sequence of arrangement of elements, there are no other requirements for the arrangement of elements.

Fig. 2 - States prepared on the Device side in accordance with the BB84 protocol.

Fig. 3 - Results of pulse interference in an asymmetric receiver interferometer.

Fig. 4 - Schemes for selecting states and bases in the BB84 protocol.

Fig. 5 - States prepared on the UFKS side in accordance with the T12 protocol.

Fig. 6 - Pulses in a protocol with trap states.

Claims (1)

A device for generating quantum states for quantum communications systems with assessment of the quality of preparing states for protocols for quantum key generation on a chip, including a semiconductor laser operating in continuous mode, an amplitude modulator made in the form of a symmetric Mach-Zehnder interferometer and consisting of a disconnector and a combiner and two phase modulators - narrowband low-frequency and broadband high-frequency, an additional disconnector installed after the amplitude modulator combiner before the broadband phase modulator, necessary to control the operating point of the amplitude modulator, a broadband phase modulator necessary to change the phase of laser pulses, an adjustable optical attenuator installed after the wideband phase modulator and made in the form of a disconnector, two narrow-band phase modulators, and an X-shaped coupler, an asymmetrical Mach-Zehnder interferometer with a delay line in the long arm, a phase modulator in the short arm, a coupler in the short arm for outputting the optical signal into the quantum channel, and the arms of the interferometer are connected in an X-shaped coupler, the beams from which are directed to external photodetectors located outside the chip, an additional adjustable optical attenuator installed before exiting the quantum channel and made in the form of a splitter, two narrowband amplitude modulators and a combiner; necessary to achieve the required level of attenuation of laser pulses.