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EP-4348922-B1 - WEAK PULSE DETECTION FOR DISCRETE-VARIABLE QUANTUM KEY DISTRIBUTION RECEIVER AND METHOD

EP4348922B1EP 4348922 B1EP4348922 B1EP 4348922B1EP-4348922-B1

Inventors

  • BACCO, Davide
  • CATALIOTTI, Francesco Saverio
  • DE NATALE, PAOLO
  • Occhipinti, Tommaso
  • VAGNILUCA, Ilaria
  • ZAVATTA, Alessandro

Dates

Publication Date
20260506
Application Date
20220524

Claims (17)

  1. A receiver for receiving weak pulses of light in a quantum key distribution system with discrete-variable encoding, the receiver comprising: ∘ An optical coupler having a first and a second input and a first and second output, the optical coupler being adapted to be connected to a quantum channel to receive the weak pulses at the first input, each weak pulse having duration T and wavelength λ wp , and local oscillator signals at the second input; ∘ A local oscillator laser adapted to generate the local oscillator signals, each local oscillator signal being an impulse of duration T and wavelength λ LO , wherein the difference between λ LO and λ wp is such that: 1 T < c λ wp − c λ LO ≤ 100 GHz ∘ A laser locking system for locking the difference between the λ LO and λ wp wavelengths to a fixed value, so that the difference remains constant in time; ∘ A synchronizer connected to the local oscillator laser so that the local oscillator laser emits a local oscillator signal at a given time for which the weak pulse and the local oscillator signal reach the first and second input, respectively, at the same time; ∘ A first and a second photodetector, the first and second photodetector being connected to the first and second output, respectively, of the optical coupler and emitting a first and a second electric signal, the first and second electric signal being function of the interference between the weak pulse and the local oscillator signal in the optical coupler; ∘ An electronic circuit configured to obtain a difference signal, the difference signal being function of the difference between the first electric signal emitted by the first photodetector and the second electric signal emitted by the second photodetector; ∘ A filter adapted to filter the difference signal, generating a filtered signal which includes a portion of the difference signal having a frequency in a frequency range around a carrier frequency f, where f = c λ wp − c λ LO ∘ A discriminator, the discriminator being configured to determine whether the filtered signal has a value function of the amplitude above a fixed threshold.
  2. The receiver according to claim 2, wherein the difference between λ LO and λ wp is such that: 1 T < c λ wp − c λ LO ≤ 10 GHz
  3. A receiver for receiving weak pulses in a quantum key distribution system with discrete-variable encoding, the receiver comprising: ∘ An optical coupler having a first and a second input and a first and second output, the optical coupler being adapted to be connected to a quantum channel to receive the weak pulses at the first input, each weak pulse having duration T and wavelength λ wp , and local oscillator signals at the second input; ∘ A local oscillator laser adapted to generate the local oscillator signals, each local oscillator signal being an impulse of duration T and wavelength λ LO identical to λ wp ; ∘ A phase variator, the phase variator being adapted to modulate, with a modulation frequency, the phase of the weak pulse or of the local oscillator signal before they interfere in the optical coupler; ∘ A synchronizer connected to the local oscillator laser so that the local oscillator laser emits a local oscillator signal at a given time for which the weak pulse and the local oscillator signal reach the first and second input, respectively, at the same time; ∘ A first and a second photodetector, the first and second photodetector being connected to the first and second output, respectively, of the optical coupler and emitting a first and a second electric signal, the first and second electric signal being function of the interference between the weak pulse and the local oscillator signal in the optical coupler; ∘ An electronic circuit configured to obtain a difference signal, the difference signal being function of the difference between the first electric signal emitted by the first photodetector and the second electric signal emitted by the second photodetector; ∘ A filter adapted to filter the difference signal, generating a filtered signal that includes a portion of the difference signal having a frequency in a frequency range around the modulation frequency that was previously applied by the phase variator; ∘ A discriminator, the discriminator being configured to determine whether the filtered signal has a value function of the amplitude above a fixed threshold.
  4. The receiver according to claim 3, wherein the phase variator is adapted to vary the phase of the local oscillator signal.
  5. The receiver according to one or more of the preceding claims, comprising an amplifier positioned at the output of the electronic circuit adapted to amplify the difference signal.
  6. The receiver according to one or more of the preceding claims, wherein the first and second photodetector are a P-I-N photodetectors.
  7. The receiver according to one or more of the preceding claims, wherein the filter comprises a pass band filter.
  8. The receiver according to one or more of the preceding claims, wherein the optical coupler is an 50% optical coupler.
  9. A method to detect the presence or the absence of weak pulses in a quantum key distribution system with discrete-variable encoding, the method comprising: ∘ Receiving a weak pulse from a quantum channel having duration T and wavelength λ wp , ∘ Generating a local oscillator signal having duration T and wavelength λ LO , wherein the difference between λ LO and λ wp is such that: 1 T < c λ wp − c λ LO ≤ 100 GHz ; ∘ Keeping the difference between λ LO and λ wp constant in time; ∘ Inputting to a first and second input of an optical coupler at the same time the weak pulse and the local oscillator signal; ∘ Creating an interference signal between the weak pulse and the local oscillator signal using the optical coupler; ∘ Detecting the interference signal at a first and at a second output of the optical coupler; ∘ Emitting a first and a second electric signal function of the detected interference signal; ∘ Subtracting the first and the second electric signal obtaining a difference signal; ∘ Filtering the difference signal generating a filtered signal which includes a portion of the difference signal having a frequency in a frequency range around a carrier frequency f = c λ wp − c λ LO ∘ Comparing a value function of the amplitude of the filtered signal with a threshold; ∘ Determining that a weak pulse has been received if the value function of the amplitude of the filtered signal is above the threshold.
  10. The method according to claim 9, wherein the difference between λ LO and λ wp is such that: 1 T < c λ wp − c λ LO ≤ 10 GHz .
  11. A method to detect the presence or the absence of weak pulses in a quantum key distribution system with discrete-variable encoding, the method comprising: ∘ Receiving a weak pulse from a quantum channel having duration T and wavelength λ wp , ∘ Generating a local oscillator signal having duration T and wavelength λ LO identical to λ wp ; ∘ Inputting to a first and second input of an optical coupler at the same time the weak pulse and the local oscillator signal; ∘ Creating an interference signal between the weak pulse and the local oscillator signal using the optical coupler; ∘ modulating the phase of the weak pulse or of the local oscillator signal before interference in the optical coupler, resulting in a modulated interference signal; ∘ Detecting the interference signal at a first and at a second output of the optical coupler; ∘ Emitting a first and a second electric signal function of the detected interference signal; ∘ Subtracting the first and the second electric signal obtaining a difference signal; ∘ Filtering the difference signal generating a filtered signal which includes a portion of the difference signal having a frequency in a frequency range around the modulation frequency; ∘ Comparing a value function of the amplitude of the filtered signal with a threshold; ∘ Determining that a weak pulse has been received if the value function of the amplitude of the filtered signal is above the threshold.
  12. The method according to claim 9, 10 or 11, wherein the duration T is comprised between 100 picoseconds and 10 nanoseconds.
  13. The method according to one or more of claims 9 - 12, wherein the carrier frequency or the modulation frequency is bigger than 1/T.
  14. The method according to one or more of claims 9 - 13, wherein the wavelength of the weak pulse λ cs is comprised between 800 nm and 1625 nm.
  15. The method according to one or more of claims 9 - 14, wherein the frequency range is comprised in 0.1 KHz and 100 GHz, preferably the frequency range is comprised in 0.1 KHz and 10 GHz.
  16. The method according to one or more of claims 9 - 15, wherein the carrier frequency or the modulation frequency f is comprised between 1 GHz and 100 GHz, preferably the carrier frequency or the modulation frequency f is comprised between 1 GHz and 10 GHz.
  17. The method according to one or more of claims 9 - 16, wherein the optical coupler is an 50% optical coupler.

Description

The present invention relates to a quantum key distribution receiver for detecting the presence or the absence of a quantum state, the quantum state belonging to a cryptographic key created using a quantum communication protocol with N-dimensional quantum states. Quantum communications, by exploiting single photons or weak pulses of light, give rise to novel types of applications, not possible with standard optical communication. For example, the possibility of distributing cryptographic keys in an unconditionally secure way is currently the most relevant application from a commercial point of view. Quantum cryptography offers indeed the possibility of distributing cryptographic keys between two or more users, by exploiting the laws of quantum mechanics. These keys allow secure communication of data in real time by using encryption algorithms. Quantum states can be created using different degrees of freedom, for example polarization, time, space, phase, frequency and others or a combination of them. One of the most effective and suitable system for today's telecommunication networks is time encoding. In time encoding, different time-slots are defined and the different quantum states can be distinguished according to the time of arrival of the photon (or the weak light pulse). The basis of quantum states associated to the encoded key bits is called computational basis. In addition to the computational basis, it is also necessary to prepare a second basis of quantum states, the so-called superposition basis, in order to certify the security of the quantum protocol. Therefore, the coding of information is performed on very attenuated optical signals, with a light mean intensity even below that of individual photons. These very weak signals are called qubits (or quantum bits) and their use allows exploiting the interesting properties of quantum mechanics, as happens for example for the secure transmission of private information through an untrusted channel. To decode this information, the receiver must make a measurement of the quantum state carried by each qubit. In particular, in discrete-variable quantum communication systems, information is usually encoded in the state of polarization, in the time of arrival (time-bin) or in the relative phase of strongly attenuated laser signals. In order to detect these qubits, the receiver generally uses measuring equipment containing one or more single-photon detectors, or photo-detection devices capable of signaling the arrival of very weak light signals. The most used wavelengths are those typical of optical telecommunications, such as windows at 1310 nm and 1550 nm used for signal propagation in optical fibers, or at 800/1550 nm for signal propagation in the atmosphere. Single-photon detectors based on semiconductors (SPAD) include avalanche photodiodes operating in the breakdown regime. Another type of single-photon detectors is based on superconducting materials (SNSPD), in which the arrival of a photon is signaled by the momentary interruption of superconductivity, caused by the absorbed photon. SNSPD devices generally work better than SPADs, but need to be cooled to cryogenic temperatures < 3°K (while for SPADs a thermoelectric cooling is sufficient at about 183°K-253°K). In general, however, all these devices have considerable dimensions, high energy consumption and a high price, especially for infrared wavelengths and superconducting detectors. This constitutes a serious limitation to the development and diffusion of quantum communication technologies, in particular for discrete-variable encoding. Furthermore, these limitations also hinder their potential integration into existing and currently used telecommunications fiber optic infrastructures and networks. In addition, both SPADs and SNSPDs are highly susceptible to reading errors caused by noise signals. The noise can originate both from the intrinsic material impurities and from surrounding environment (such as sunlight, black body radiation and scattered light). The latter case is common when the same fiber is used simultaneously for the transport of classical and quantum signals. Examples of a key recovery apparatus and method in the field of quantum communication are disclosed in the patent application EP 3413503 A1. An high-speed pulsed homodyne detector in optical communication wavelength band is described in EP 2365647 A1. There is therefore a need for a detector or a method to detect weak pulses of light that it is accurate and at the same time relatively inexpensive. There is also a need for a detector or a method to detect weak pulses of light that it is accurate and may work at room temperature. There is also a need for a detector or a method to detect weak pulses of light that it is accurate and compatible with existing telecommunication systems and devices. The invention may satisfy one or more of the above needs. According to an aspect, the invention relates to a receiver for receiving weak pu