JP-2026075823-A - Quantum key distribution system

JP2026075823AJP 2026075823 AJP2026075823 AJP 2026075823AJP-2026075823-A

Abstract

[Problem] To provide a quantum key distribution system that can detect eavesdropping without using bit error rate. [Solution] In the quantum key distribution system comprising a transmitting node and a receiving node of the present disclosure, the transmitting node transmits a pulse train in which decoy pulses with an average number of photons greater than the average number of photons of each pulse in the coherent pulse train are randomly inserted into an ultra-weak coherent pulse train in which the phase of each pulse is 0 or π. The receiving node inputs the received pulse train into a delayed Mach-Zehnder interferometer (MZI) with a delay phase difference of 0, detects the photons of the pulses output from the MZI, and the transmitting node and the receiving node each generate secret key bits from photon detection events from two consecutive pulses in the ultra-weak coherent pulse train. The receiving node is configured to detect eavesdropping using photon detection events in a time slot adjacent to the time slot in which the decoy pulse was received. [Selection Diagram] Figure 3

Inventors

本庄利守
武居弘樹
生田拓也
井上恭

Assignees

ＮＴＴ株式会社
国立大学法人大阪大学

Dates

Publication Date: 20260511
Application Date: 20241023

Claims (3)

A quantum key distribution system that supplies secret keys for symmetric-key cryptography, It comprises a transmitting node and a receiving node, The transmitting node is configured to transmit a pulse train in which pulses with an average number of photons greater than the average number of photons of each pulse in the coherent pulse train are randomly inserted into an ultra-weak coherent pulse train in which the phase of each pulse is 0 or π. The receiving node has a delayed Mach-Zehnder interferometer with a delay phase difference of 0, and is configured to input the pulse train sent from the transmitting node to the delayed Mach-Zehnder interferometer and to detect the photons of the pulses output from the delayed Mach-Zehnder interferometer. The transmitting node and the receiving node are each configured to generate a secret key bit from photon detection events from two consecutive pulses in an ultra-weak coherent pulse train. A quantum key distribution system in which the receiving node is configured to detect eavesdropping using photon detection events in time slots adjacent to the time slot in which a pulse with a large average number of photons was received.
The quantum key distribution system according to claim 1, wherein the receiving node is configured to detect eavesdropping by comparing a photon detection event in a time slot adjacent to the time slot in which a pulse with a large average number of photons was received with a predetermined photon detection event.
The transmitting node is configured to notify the receiving node of a time slot in which a pulse with a large average number of photons is inserted. The quantum key distribution system according to claim 1, wherein the receiving node is configured to detect eavesdropping based on a notification from the transmitting node, using a photon detection event in a time slot adjacent to the time slot in which the pulse with a large average number of photons was received.

Description

This invention relates to a quantum key distribution system. Conventionally, research and development of quantum cryptography or quantum key distribution (QKD) has been progressing as a system for securely supplying a common secret key for encrypting and decrypting communication data to two parties engaged in encrypted communication. One of the QKD systems is called differential phase shift (DPS) QKD (DPS-QKD) (see Non-Patent Literature 1). The configuration of a conventional DPS-QKD system will be explained with reference to Figure 1. The DPS-QKD system 100 shown in Figure 1 has a transmitting node 120 and a receiving node 140. The transmitting node 120 and the receiving node 140 are connected via a transmission line 130. The transmitting node 120 comprises a coherent pulse light source 122, a phase modulator 124, and an attenuator 126. The transmitting node 120 corresponds to the node of the sender, one of the two parties engaging in encrypted communication. The transmitting node 120 is configured to generate and transmit a weak coherent pulse train in which the phase of each pulse is 0 or π. The coherent pulse light source 122 is a light source that generates and transmits a train of coherent pulses. The phase modulator 124 is a modulator that imparts a phase of 0 or π to each pulse in the coherent pulse train from the coherent pulse light source 122. The attenuator 126 is an attenuator that applies attenuation to the phase-modulated pulses from the phase modulator 124. The attenuator 126 is configured to attenuate the average optical energy per pulse in the pulse train to less than one photon energy (e.g., 0.1 photon energy). That is, the average number of photons per pulse in the pulse train emitted from the attenuator 126 is minuscule (e.g., 0.1 photons). At the transmitting node 120, the coherent pulse train emitted from the coherent pulse light source 122 is given phase by the phase modulator 124, and further attenuated by the attenuator 126, resulting in a coherent pulse train 132 consisting of weak pulses with a phase of 0 or π, which is then sent to the transmission line 130. The receiving node 140 has a delayed Mach-Zehnder (MZI) 150 and photon detectors D0 162 and D1 164 positioned at the two outputs of the MZI 150. The receiving node 140 corresponds to the receiver node, which is the other party in the encrypted communication. The receiving node 140 is configured to detect photons from adjacent pulses of a weakly coherent pulse train whose phase from the transmitting node 120 is 0 or π. The delay MZI 150 comprises two splitters 152 and 154, and two mirrors 156 and 158. The two mirrors 156 and 158 are positioned in one of the two optical paths formed between splitter 152 and splitter 154. Nothing is positioned in the other optical path between splitter 152 and splitter 154. The optical path with the two mirrors 156 and 158 is referred to as the long path, and the optical path with nothing positioned is referred to as the short path. The two mirrors 156 and 158 are positioned in the delay MZI 150 such that the delay time imparted to the light propagating along the long path, due to the difference between the long and short paths, is equal to the time interval between two pulses in the pulse train. Furthermore, the two splitters 152 and 154, and the two mirrors 156 and 158 are configured such that the propagation phase difference between the light propagating along the long path and the light propagating along the short path is an integer multiple of 2π. The operation of the delay MZI 150 will be explained with reference to Figure 2. Two adjacent pulses in the coherent pulse train 132 received from the transmitting node 120 are input to the delay MZI 150, split into two by the splitter 152, and propagate along a long path and a short path. The two pulses propagating along the long path are delayed by an interval equal to the time interval between the two pulses. Therefore, the second pulse of the two pulses propagating along the short path is incident on the splitter 154 simultaneously with the first pulse of the two pulses propagating along the long path, and they are combined again. In the delayed MZI 150 with the configuration described above, two adjacent pulses in the coherent pulse train 132 received from the transmitting node 120 overlap and interfere with each other at the splitter 154, resulting in the output. As a result of the interference, if the phase difference between two adjacent pulses is 0, a photon is detected at the photon detector D0 162, and if the phase difference is π, a photon is detected at the photon detector D1 164. However, because the average number of photons is very small, the detection of photons is rare and random. Using the DPS-QKD system 100 configured as described above, the transmitting node 120 and the receiving node 140 generate secret key bits according to the following procedure. (1) After the transmitting node 120 and the receiving node 140 have sent and