Patent Application
20250080234
The present disclosure relates to a method and system for matching frequencies of lasers in a quantum communication system, a quantum communication network comprising such a system, and a computer program configured to execute the method.
Quantum information systems are data processing systems that use a quantum system, e.g. a qubit, as an information carrier. In conventional data processing systems the basic unit of information are bits that either have the value ‘0’ or ‘1’. In contrast, the basic unit of information in quantum information systems are qubits, wherein a qubit may be a two-state quantum mechanical system. The special property of a qubit is that it can be in either ‘0’, ‘1’, or a superposition of both states simultaneously. One example of a quantum information system wherein qubits are used is a QKD system.
QKD systems allow two or more users at different locations to securely generate cryptographic keys by at least partly making use of the special property of the qubits. The first proposal for a QKD system (BB84) was done by C. H. Benett and G. Brassard, described in the article “Quantum cryptography: Public key distribution and coin tossing”, Proceedings of IEEE International Conference on Computers, Systems and Signal Processing, volume 175, page 8. New York, 1984. An advantage of using a QKD system is that, at least in theory, the key is even secure in case an eavesdropper is present in the system.
One proposal for QKD is the measurement device independent quantum key distribution (MDI-QKD) protocol. In this protocol, two users, Alice and Bob, send random qubits to a third party, normally referred to as Charlie. Charlie then performs a Bell state measurement (BSM) that projects the state of the two qubits received at Charlie (one from Alice and one from Bob) onto an entangled Bell states. Every time a Bell state is produced, Alice and Bob can compare on a public channel their preparation bases and keep the record of events for which they have picked the same basis. After this comparison on the public channel, Bob post-process his bit values such that they become identical to Alice's. These Bell state measurements are fundamental to many quantum communication applications, with MDI-QKD as one important example.
For the Bell state measurement in an MDI-QKD system to work, it is necessary to generate two indistinguishable photons from two independent laser sources (Alice and Bob) to do a successful BSM. An example of a Bell state measurement in an MDI-QKD system is described in the article by R. Valivarthi et al, “A cost-effective measurement-device-independent quantum key distribution system for quantum networks”, Quantum Science and Technology, 2, 2017.
Often QKD systems are based on lasers and field programmable gate arrays to generate time-bin qubits. Time bin qubits may be formed by a coherent superposition of two independent temporal modes of a single-photon excitation. Time-bin encoding is especially suitable for single-mode optical fiber propagation and compatible with already existing fiber networks. Hence, the formation of time bin qubits in QKD systems is a practical element in the development of practical QDK implementations. QKD protocols such as the above referred BB84 and MDI-QKD protocols, the coherent one-way (COW) QKD protocol and other QKD protocols such as described in the article by Vagniluca et al, Efficient time-bin encoding for practical high-dimensional quantum key distribution, physical review applied 14, 014051 (2020), use a train of phase coherent temporal modes by intensity modulation of the output of a continuous-wave (CW) laser and subsequent attenuation.
CN 112 039 666 A relates to a frequency locking and phase stabilizing method and system based on quantum key distribution, and the system comprises a frequency locking system comprising an ultra-stable laser used for outputting two independent laser sources; a beat frequency module used for acquiring beat frequency signals of the two independent laser sources; a spectrum analyzer used for measuring the frequency difference between the two independent laser sources according to the beat frequency signal; a locking and tuning module connected with a frequency standard instrument; and the frequency standard instrument used for providing a peripheral frequency standard for the locking and tuning module. The locking and tuning module is used for tuning the frequency between the two independent laser sources to be consistent according to the frequency difference based on the frequency standard, and removing the inherent linear drift amount in the output frequency of the ultra-stable laser. According to the invention, the frequency locking between the two independent laser sources is realized in the quantum key distribution process, so that the phases of the reference optical signals and the quantum optical signals of the two independent laser sources are stable.
The requirement of a correctly-implemented Bell state measurement requires that the frequency difference between the frequencies of the light fields that Alice and Bob use to produce time bin qubits should be minimal. Therefore, it is desired to determine the frequency difference and to try to match the frequencies of Alice and Bob based on the determined frequency difference. Furthermore, the matching of the frequencies should be done in an efficient manner.
In a first aspect, the disclosure relates to a method for matching frequencies of lasers in a quantum communication system. The system may comprise sending a first laser signal with a first frequency from a first device at a first location and sending a second laser signal with a second device at a second location to a beam splitter located at a third location such that the two lasers signals interfere at the beam splitter to obtain a beat signal that has a beat frequency, wherein the beat frequency corresponds to the frequency difference between the first and second frequency, measuring the beat frequency of the beat signal by a frequency detection unit that is positioned at the third location, determining if the beat frequency is in a predetermined frequency range, and sending a feedback signal from the third location to at least one of the first or second device, adapting the first or second frequency by sending the feedback signal to at least one of the first or second device, wherein adapting the frequency is executed according to a first algorithm as long as the beat frequency is outside the predetermined frequency range, wherein the first algorithm is configured to bring the beat frequency in the predetermined frequency range, and a second algorithm if the beat frequency is in the predetermined frequency to match the frequencies of the first and second laser signal.
In an embodiment executing the first algorithm comprises keeping the first frequency of the first laser signal constant, changing the second frequency of the second laser signal by sending the feedback signal to the second device, measuring the beat frequency of the beat signal based on the first frequency and the changed second frequency, and repeating the changing of the changed second frequency of the second laser signal if the beat frequency is outside the predetermined frequency range.
In an embodiment changing the second frequency of the second laser signal comprises changing the second frequency in a step-like manner, wherein in a first frequency step a change of the frequency is based on the predetermined frequency range of the frequency detection unit, and wherein for each subsequent frequency step the change of the frequency is increased and a sign of the frequency step is flipped.
In an embodiment the increasing of the change of the frequency in each subsequent frequency step comprises an addition of the change of the frequency of the first frequency step.
In an embodiment the first algorithm further comprises measuring, after each frequency step, the beat frequency of the beat signal, determining if the beat frequency is in the predetermined frequency range, performing, if the beat frequency is in the predetermined frequency range, at least one further frequency step, wherein the at least one further frequency step comprises a change of the frequency that is smaller than the change of the frequency of the first frequency step and wherein the sign of the last frequency step is equal to the preceding frequency step, and repeating, if the beat frequency is outside the predetermined frequency range, the changing of the second frequency of the second laser signal.
In an embodiment the change of the frequency of the at least one further frequency step is half of the change of the frequency of the first frequency step.
In an embodiment the second algorithm comprises keeping the first frequency of the first laser signal constant, determining a gradient of the beat frequency at least partly based on the second frequency and the changed second frequency, changing the second frequency of the second laser signal in the direction of the gradient of the beat frequency, measuring the beat frequency of the beating signal, waiting, if the beat frequency is below a predetermined threshold, for a predetermined time with determining if the beat frequency is in a predetermined frequency range, and repeating, if the beat frequency is above the predetermined threshold, the changing of the second frequency of the second laser signal.
In an embodiment wherein the second algorithm comprises putting a lower bound on the change of the second frequency for preventing an undetectable frequency difference, and putting an upper bound on the change of the second frequency for preventing that the beat signal moves outside of the predetermined frequency range of the frequency detection unit.
In an embodiment when during execution of the first algorithm the changing of the second frequency moves the second frequency outside a valid frequency range of the second laser signal, the first algorithm is performed with the second frequency being kept constant and the first frequency being changed.
In an embodiment the steps of changing the frequency of the second laser are executed by changing a voltage supplied to a laser signal frequency control unit, for example a temperature controller, that is configured to control the frequency of the laser signal output by the second laser.
In a further aspect the disclosure relates to a system for matching frequencies of lasers in a quantum communication system, comprising a first laser unit for emitting a first laser signal with a first frequency that is located at a first location, a second laser unit for emitting a second laser signal with a second frequency that is located at a second location, a beam splitter that is located at a third location, wherein the first and second laser signals are configured to interfere on the beam splitter to obtain a beat signal with a beat frequency, and a frequency detection unit for measuring the beat frequency of the beat signal, wherein the frequency detection unit is located at a third location and comprises a processor that is configured to determine if the beat frequency is in a predetermined frequency range, send a feedback signal to at least one of the first or second laser unit, adapting the first or second frequency by sending the feedback signal to at least one of the first or second device, wherein adapting the frequency is executed according to, a first algorithm as long as the beat frequency is outside the predetermined frequency range, wherein the first algorithm is configured to bring the beat frequency in the predetermined frequency range, and a second algorithm if the beat frequency is in the predetermined frequency range to match the frequencies of the first and second laser signal.
In an embodiment executing the first algorithm comprises keeping the first frequency of the first laser signal constant, changing the second frequency of the second laser signal by sending the feedback signal to the second laser unit, measuring the beat frequency of the beat signal based on the first frequency and the changed second frequency, and repeating the changing of the changed second frequency of the second laser signal if the beat frequency is outside the predetermined frequency range
In an embodiment changing the second frequency of the second laser signal comprises changing the second frequency in a step-like manner, wherein in a first frequency step a change of the frequency is based on the predetermined frequency range of the frequency detection unit, and wherein for each subsequent frequency step the change of the frequency is increased and a sign of the frequency step is flipped.
In an embodiment the increasing of the change of the frequency in each subsequent frequency step comprises an addition of the change of the frequency of the first frequency step.
In an embodiment the first algorithm further comprises measuring, after each frequency step, the beat frequency of the beat signal, determining if the beat frequency is in the predetermined frequency range, performing, if the beat frequency is in the predetermined frequency range, at least one further frequency step, wherein the at least one further frequency step comprises a change of the frequency that is smaller than the change of the frequency of the first frequency step and wherein the sign of the last frequency step is equal to the preceding frequency step, and repeating, if the beat frequency is outside the predetermined frequency range, the changing of the second frequency of the second laser signal.
In an embodiment the change of the frequency of the at least one further frequency step is half of the change of the frequency of the first frequency step.
In an embodiment the second algorithm comprises keeping the first frequency of the first laser signal constant, determining a gradient of the beat frequency at least partly based on the second frequency and the changed second frequency, changing the second frequency of the second laser signal in the direction of the gradient of the beat frequency, measuring the beat frequency of the beating signal, waiting, if the beat frequency is below a predetermined threshold, for a predetermined time with determining if the beat frequency is in a predetermined frequency range, and repeating, if the beat frequency is above the predetermined threshold, the changing of the second frequency of the second laser signal.
In an embodiment wherein the second algorithm comprises putting a lower bound on the change of the second frequency for preventing an undetectable frequency difference, and putting an upper bound on the change of the second frequency for preventing that the beat signal moves outside of the predetermined frequency range of the frequency detection unit.
In an embodiment when during execution of the first algorithm the changing of the second frequency moves the second frequency outside a valid frequency range of the second laser signal, the first algorithm is performed with the second frequency being kept constant and the first frequency being changed.
In an embodiment the system further comprises a first and second frequency control unit, for example temperature controllers, that are respectively operatively connected to the first and second laser unit, wherein the frequency control units are configured to change the frequency of the lasers by a change of voltage that is supplied to the frequency control units.
In a further aspect the disclosure relates to a quantum communication network comprising a Bell state measurement system and a system according to any one foregoing embodiments.
In a further aspect the disclosure relates to a computer program configured to execute the method steps according to any one of the foregoing embodiments.
Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. Exemplary embodiments are described below with reference to the figures.
It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present disclosure will be determined by the appended claims.
Furthermore, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Still, certain elements are defined below for the sake of clarity and ease of reference.
Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is encompassed within the disclosure. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges and are also encompassed within the disclosure, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the disclosure.
It is noted that, as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for use of such exclusive terminology as “solely.” “only” and the like in connection with the recitation of claim elements, or use of a “negative” limitation.
As will be apparent to those of skill in the art upon reading this disclosure, each of the individual exemplifying embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several exemplifying embodiments. Any recited method can be carried out in the order of events recited or in any other order which is logically possible.
The qubit that is sent over optical fiber 108 is received at Bell state measurement (BSM) device 138, wherein a Bell state measurement can be performed on the qubit. This qubit information corresponding to the Bell state measurement is then sent to a local computer COMP 129 for the creation of a secret key.
Dots 254 are measurements of a beat frequency that is measured by frequency counter 244 at a specific voltage supplied to frequency control unit 250. Line 256 through dots 254 is the relation between the change of the voltage supplied to frequency control unit 250 and the beat frequency measured by frequency counter 244, under the condition that the voltage of the feedback signal supplied to frequency control unit 252 is held constant. In area 258 the frequency counter 244 does not measure a beat frequency. In predetermined frequency range F the frequency counter 244 measures a beat frequency. In the present example, the predetermined frequency range F is between 1.200 V and 1.204 V. It is clear for the skilled person that the predetermined frequency range is dependent on the specific set up of the system. The skilled person would understand that different lasers result in a different range of voltage for the predetermined frequency range F. Line 256 has a shape comprising a valley 260. At point 262 of valley 260 the beat frequency is substantially zero Hz, meaning that at a lowest point 262 the difference between the frequencies of laser 214a and laser 214b is substantially zero. It is an object for the present invention to determine the voltage of the feedback signal wherein the beat frequency is substantially zero, such that the frequencies of laser 214a and laser 214b are substantially the same. In area 258 a first algorithm may be executed. In predetermined frequency range F a second algorithm may be executed. In the context of the present disclosure, the first algorithm may be characterized as a broad search algorithm. The first and second algorithm may be executed by controller 246 of central node 210.
The voltage difference added during the first step is at least partly determined by the predetermined frequency range F. In an embodiment, the first step may comprise a voltage difference that is less than half of the predetermined frequency range F. In an embodiment, the first step comprises a voltage difference that is less than half of the predetermined frequency range F and more than one quarter of the predetermined frequency range F.
In
In an embodiment, the second step comprises a voltage difference that is two times the voltage difference of the first step. In an embodiment, the sign of the voltage difference from the first step and the second step is different. In the present example, the sign of the first step is positive, while the sign of the second step is negative.
In
In an embodiment, the third step comprises a voltage difference that is three times the voltage difference of the first step. In an embodiment, the sign of the voltage difference from the second step and the third step is different. In the present example, the sign of the second step is negative, while the sign of the third step is positive.
In
In an embodiment, the fourth step comprises a voltage difference that is half of the voltage difference of the first step. Controller 246 may determine to execute a fourth step wherein the voltage difference is half of the first step in case the preceding step brings the voltage in the predetermined frequency range F. In an embodiment, the sign of the voltage difference from the third step and the fourth step is the same. In the present example, the sign of the third step is positive, while the sign of the fourth step is positive.
After the fourth step, controller 246 may execute the second algorithm. In the context of the present disclosure, the second algorithm may be characterized as a gradient descent algorithm. The second algorithm is used to find a minimum of the beat frequency in the predetermined frequency range F.
If the beat frequency is outside the predetermined frequency range F, controller 246 may change the second frequency in a second frequency step in step 620. The voltage change of the second frequency step is in one embodiment twice as much as the voltage change in the first frequency step. The sign of the voltage change of the second frequency step is in one embodiment different than the sign of the voltage change of the first frequency step. For example, the first frequency step may comprise a voltage change of +2 mV. while the second frequency step may comprise a voltage change of −4 mV. After the second frequency is changed by the feedback signal in the second frequency step, the beat frequency of the first frequency and the changed second frequency may be determined by photodiode 242 and frequency counter 244. In step 622 controller 246 may determine if the beat frequency of the first frequency and the changed second frequency is in the predetermined frequency range F. If the beat frequency is inside the predetermined frequency range F, controller 246 may perform one further frequency step in step 618.
If the beat frequency is outside the predetermined frequency range F, controller 246 may change the second frequency in a third frequency step in step 624. The voltage change of the third frequency step in one embodiment has the voltage change of the first frequency step added to the voltage change in the second frequency step. In other words, the voltage change of the third frequency step is three times the voltage change of the first frequency step. The sign of the voltage change of the third frequency step is in one embodiment different than the sign of the voltage change of the second frequency step. For example, the second frequency step may comprise a voltage change of −4 mV, while the third frequency step may comprise a voltage change of +6 mV.
In step 618, the one further frequency step may comprise a voltage change that is half of the voltage change of the first frequency step. The sign of the voltage change of the one further frequency step is in one embodiment equal to the sign of the voltage change of the preceding frequency step. For example, if the preceding frequency step was a third frequency step with a voltage change of +6 mV, the further frequency step may comprise a voltage change of +1 mV. After performing one further frequency step the controller 246 may execute the second algorithm in step 626.
In an embodiment, when a frequency step is taken outside a valid frequency range of second laser unit 214b, controller 246 may change the feedback signals through communication channels 249 and 248 by keeping the feedback signal to second laser unit 214b constant while changing the feedback signal to first laser unit 214a. A valid frequency range in the present disclosure denotes the workable range of frequencies of laser units 214a, 214b. In practice, the valid frequency range amounts to the frequencies between the minimum and maximum frequencies of laser units 214a, 214b.
The present disclosure is by no means limited to the above described preferred embodiments thereof. The rights sought are defined by the following claims within the scope of which many modifications can be envisaged.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2030077 | Dec 2021 | NL | national |
This Application is a Section 371 National Stage Application of International Application No. PCT/NL2022/050710, filed Dec. 8, 2022 and published as WO 2023/106923 on Jun. 15, 2023, in English.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/NL2022/050710 | 12/8/2022 | WO |
