Calibration equipment and method for quantum key distribution

Abstract

The present invention relates to a calibration system for calibrating a QKD transmitter comprising a light source for generating light and an output connector for outputting the generated light, the calibration system comprising a pair of switchable connectors configured to switch between a first position exposed to the outside of the QKD transmitter and a second position internally connected to provide an optical path from the light source to the output connector, and an optical amplifier (5) connected to the connector switched to the first position for calibration treatment.

Description

Technical Field

[0001] The present invention relates to the field of calibration equipment for quantum key distribution, and more particularly, to a device and method for calibrating a quantum key distribution system. Even more particularly, the present invention relates to a device and method for calibrating a transmitter of a quantum key distribution.

Background Art

[0002] In recent years, quantum cryptography has proven to be one of the most reliable technologies in secure information exchange techniques. Among several technologies, quantum key distribution has been shown to be one of the most promising technologies.

[0003] The main purpose of quantum key distribution (QKD) is to be able to share a series of bits between a transmitter and a receiver that can prove privacy with a limited set of assumptions.

[0004] Quantum key distribution (QKD) is a method that enables the delivery of a secret key with provable security between two remote parties, namely a transmitter and a receiver. The two parties encode the key into a basic quantum system such as photons exchanged via a quantum channel such as an optical fiber or a free-space link. The security of this method stems from the well-known fact that the measurement of the quantum state of an unknown quantum system changes the system itself. In other words, a spy eavesdropping on a quantum communication channel cannot obtain information about the key without introducing errors into the key exchanged between the transmitter and the receiver. Similarly, QKD is secure due to the no-cloning theorem of quantum mechanics, which ensures that a spy cannot clone the transmitted quantum system and transfer a perfect copy to the receiver.

[0005] During the manufacturing of a QKD system, the transmitter (commonly called Alice) needs to be calibrated and characterized. One step in this process is to calibrate and characterize the intensity, as well as the time and spectral shape of the optical pulses generated by the laser located inside Alice. In particular, calibration aims to define the parameters of the optical signal output by Alice, such as wavelength, number of photons per pulse μ0, decoy levels μ1 and μ2, and FWHM of the output beam.

[0006] Therefore, a crucial step is to characterize the light source, i.e., the laser's current and temperature, as well as its wavelength, intensity level, FWHM of the signal, phase modulation, etc., which is impossible without amplifying the laser signal in the correct manner. In fact, given the typical losses of a QKD transmitter, even without adding an optical attenuator, the generated signal is too weak to be measured with standard laboratory equipment.

[0007] Those skilled in the art might attempt to perform calibration by plugging standard equipment into Alice's output, but such a weak signal would make it impossible to perform any measurements. Attempting to add a commonly available optical amplifier (with a typical amplification gain of 20-30 dB) to Alice's output would also introduce a significant amount of noise, which would outweigh the signal, making calibration impossible.

[0008] Therefore, it is known in the art to characterize transmitters using a single-photon detector (SPD) that combines a TDC (Time-to-Digital Converter) or counter with other standard optical instruments such as fixed or adjustable optical filters, couplers, or interferometers. However, in addition to the inconvenience that SPDs are non-standard instruments known to only a few experts and therefore not necessarily known to all those skilled in the art, the procedure is time-consuming because it requires integrating the signal over a long period of time to obtain meaningful measurements. Furthermore, the results need to be interpreted and are not in real time. Therefore, it is a time-consuming and impractical testing method.

[0009] Alternatively, the transmitter may not be calibrated at all, meaning only a single component of the transmitter that compels calibration is characterized, rather than the entire system. This is done using a QKD receiver, but the calibration is converted into statistical analysis, which is therefore time-consuming and complex. In addition to the aforementioned statistical analysis problems, if unit testing is performed on a single component rather than the entire Alice assembly, the discovery of a faulty Alice assembly is limited to testing the entire QKD system, not during the manufacturing of the Alice module.

[0010] In light of the above, it can be understood that the problems associated with calibration performed using SPD in combination with TDC are that it is time-consuming, impractical, and only a small number of experts are capable of using this non-standard instrument. Furthermore, the problems associated with using a QKD receiver instead of testing the entire transmitter are that, in practice, the calibration is converted into a statistical analysis, which is therefore time-consuming, and only a small number of components of the transmitter are targeted, making it impossible to measure any faults that may exist in the transmitter system.

[0011] Therefore, solutions are needed to speed up and simplify the calibration of QKD transmitters.

[0012] In this regard, the main objective of the present invention is to provide a device and method for easily, quickly, and reliably calibrating a quantum key distribution transmitter. [Overview of the Initiative]

[0013] The above problem is solved by the present invention, which includes an optical amplifier used in combination with a photodiode placed at the output of Alice. This calibration equipment is then preferably connected to standard telecommunications characterization equipment consisting of either an oscilloscope (real-time or sampling) for performing timing and intensity measurements or an optical spectral analyzer for performing wavelength measurements. Such calibration equipment is faster because it provides a real-time response of the output of Alice and therefore eliminates the need for time for integration. Furthermore, such equipment is easy to implement.

[0014] In this regard, a first aspect of the present invention is a calibration system for calibrating a QKD transmitter comprising a light source for generating light, an output connector for outputting the generated light, and at least one, preferably a pair of switchable connectors, wherein the switchable connectors are configured to be switchable between a first position exposed to the outside of the QKD transmitter and a second position internally connected to provide an optical path from the light source to the output connector, the system further comprising an optical amplifier and standard telecommunications characterization equipment, wherein the optical amplifier is connected to the connector in the first position, and the standard telecommunications characterization equipment is located at the output connector.

[0015] Preferably, the optical amplifier is an erbium-doped fiber amplifier (EDFA).

[0016] Furthermore, a standard telecommunications characterization system includes a photodiode and an acquisition system capable of reading the signal from the photodiode.

[0017] Preferably, the acquisition system is an oscilloscope for measuring timing and intensity, or an optical spectrum analyzer for wavelength measurement.

[0018] Conveniently, the photodiode is an avalanche photodiode.

[0019] According to a preferred embodiment of the present invention, the avalanche photodiode is a high-speed photodiode having a bandwidth higher than 10 GHz.

[0020] In a preferred manner, the QKD transmitter further comprises an encoder stage including a modulation optical system, passive optical components, and an attenuator.

[0021] Conveniently, the modulation optical system comprises at least one active or passive modulation optical system such as an intensity modulator and / or a phase modulator.

[0022] Optionally, the optical attenuator is one of a variable attenuator and / or a fixed attenuator.

[0023] According to a preferred embodiment of the present invention, at least one switchable connector is arranged between the light source and the encoder stage.

[0024] Further specific advantages and features of the present invention will become more apparent from the following non-limiting description of at least one embodiment of the present invention referring to the accompanying drawings.

Brief Description of the Drawings

[0025] [Figure 1] Shows a conventional QKD transmitter. [Figure 2] Shows calibration equipment for a standard QKD transmitter. [Figure 3A] Shows the QKD transmitter of the present invention. [Figure 3B] Shows the QKD transmitter of the present invention. [Figure 4] Shows calibration equipment for the QKD transmitter of the present invention.

Best Mode for Carrying Out the Invention

[0026] This detailed description is intended to describe the present invention in a non-limiting manner since any feature of one embodiment can be combined in a convenient way with any other feature of another embodiment.

[0027] FIG. 1 shows a conventional QKD transmitter comprising a laser source 1, an encoder stage 2, and an output connector 3, where the encoder stage 2 preferably comprises at least one of a modulation optical system and an attenuator, among other things, so as to be able to execute a QKD protocol. In addition, FIG. 2 shows calibration equipment for a conventional QKD transmitter that uses a single photon detector (SPD) in combination with a time-digital converter (TDC) connected to a computer.

[0028] As can be seen, a standard QKD transmitter does not provide internal access to the optical path and needs to be calibrated with a highly sensitive external device (SPD + TDC) that causes the drawbacks described above.

[0029] To solve these problems, the present invention also comprises a laser source 1, an encoder stage 2, and an output connector 3, and further shows in FIGS. 3A, 3B, and 4 a schematic view of a QKD transmitter in which at least one, preferably an additional pair of switchable connectors 4, is provided between the laser source 1 and the encoder stage 2.

[0030] The encoder stage can comprise either an active or a passive modulation optical system, such as an intensity modulator or a phase modulator. Furthermore, it may comprise passive optical components such as optical filters, interferometers, or couplers. The encoder stage can also include variable and / or fixed attenuators that function to attenuate the signal to the single photon level. The above enumeration is not limiting, and the enumeration and order of these components vary according to specific embodiments and protocols.

[0031] This pair of switchable connectors 4 are configured to switch between two positions, as shown in Figures 3A and 3B, respectively. Figure 3A shows the transmitter 9 in a first position where connector 4 is exposed to the outside of the QKD transmitter for calibration, and Figure 3B shows the transmitter 9 in a second position where connector 4 is connected internally for normal operation.

[0032] Although not shown in the diagram, please note that an operating system can be provided to automate the switching of connectors.

[0033] Thanks to connector 4, the QKD transmitter 9 incorporates an optical amplifier 6 between the laser source 1 and the encoder stage 2 during calibration, allowing the optical output of transmitter 9 to be boosted to measure the time or spectral shape of the optical signal coming from transmitter 9 using a standard telecommunications characterization setup consisting of a photodiode and an oscilloscope, as shown in Figure 4, which represents the calibration setup for the QKD transmitter according to the present invention.

[0034] Given the low light levels during operation, in some specific cases, the photodiode can be an amplifying photodiode that can further boost the output signal. This is only necessary depending on the intrinsic losses of the QKD transmitter.

[0035] Preferably, the bandwidth of the amplifying photodiode is higher than 10 GHz. This is because typical signals to be measured in a QKD system may be shorter than a few hundred picoseconds, and such a bandwidth is preferable for accurately measuring them, thus improving the system's performance. On the other hand, slower QKD systems do not require such bandwidth.

[0036] This calibration equipment makes it possible to read signals from the photodiode and provide instruments (such as an oscilloscope or optical spectrum analyzer) that can be used to perform calibration procedures. A typical calibration procedure for a QKD transmitter involves measuring the time and / or spectral shape of the light emitted from the transmitter to measure various parameters such as the FWHM of the pulse, the shape of the pulse, the intensity of the pulse over time, the distance between pulses, and the emission spectrum of the pulse, and then calibrating the transmitter according to the results.

[0037] While embodiments have been described in relation to several embodiments, it goes without saying that many alternative, modified, and variant forms are considered, or are obvious to those skilled in the art. Therefore, this disclosure is intended to encompass all such alternative, modified, equivalent, and variant forms that fall within the scope of this disclosure. This is particularly true, for example, with respect to different available devices. [Explanation of Symbols]

[0038] 9 QKD transmitter 1. Laser source 2 encoder stages 3 Output Connectors 4 Calibration Connectors 5 Amplifier 6 Photodiodes 7. Oscilloscope 9' Conventional QKD transmitter 6' SPD 7' TDC 8' Computer

Claims

1. A calibration system for calibrating a QKD transmitter (9), comprising a QKD transmitter (9) having a light source (1) for generating light and an output connector (3) for outputting the generated light, and telecommunications characterization equipment (6, 7) arranged on the output connector (3), At least one switchable connector (4) is configured to switch between a first position exposed to the outside of the QKD transmitter (9) and a second position internally connected to provide an optical path from the light source (1) to the output connector (3), The present invention further comprises an optical amplifier (5) connected to the switchable connector (4) which has been switched to the first position for calibration purposes, Calibration system.

2. The optical amplifier (5) is characterized in that it is an erbium-doped fiber amplifier (EDFA). The calibration system according to claim 1.

3. The standard telecommunications characterization equipment is characterized by comprising a photodiode (6) and an acquisition system (7) capable of reading the signal from the photodiode (6). The calibration system according to claim 1 or 2.

4. The acquisition system is characterized in that it is an oscilloscope (7). The calibration system according to claim 3.

5. The acquisition system is characterized in that it is an optical spectrum analyzer (7). The calibration system according to claim 3.

6. The photodiode (6) is characterized in that it is an amplifying photodiode. The calibration system according to claim 3, 4, or 5.

7. The amplification photodiode (6) is characterized in that it is a high-speed photodiode having a bandwidth higher than 10 GHz. The calibration system according to claim 6.

8. The QKD transmitter (7) is characterized by further comprising an encoder stage (2), The calibration system according to any one of claims 1 to 7.

9. The encoder stage (2) is characterized by comprising at least one active or passive modulation optical system, such as an intensity modulator and / or a phase modulator. The calibration system according to any one of claims 1 to 8.

10. The encoder stage (2) is characterized by comprising at least a fixed and / or variable optical attenuator. The calibration system according to any one of claims 1 to 8.

11. The pair of switchable connectors (4) are characterized in that they are positioned between the light source (1) and the encoder stage (2). The calibration system according to any one of claims 1 to 10.

12. The at least one switchable connector is characterized by comprising a pair of switchable connectors (4), The calibration system according to any one of claims 1 to 11.

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