DE102023129994A1 - SECURITY PROOF OF QUANTUM COMMUNICATION PROTOCOLS - Google Patents

Abstract

A method for exchanging information between a sender node and a receiver node according to a quantum communication protocol is disclosed. The method comprises: determining an attack configuration for third-party access to the quantum channel, wherein the attack configuration is defined by at least one quantum circuit system with specific parameters and an attack success limit, the quantum circuit system being configured to intercept a qubit on the quantum channel, determine the state of the intercepted qubit; using the quantum circuit system to measure a first and a second metric by at least intercepting qubits on the quantum channel; determining whether the second metric satisfies a fault tolerance limit and whether the first metric satisfies the attack success limit; terminating the communication protocol if the second metric satisfies the fault tolerance limit and the first metric satisfies the attack success limit.

Description

FIELD OF THE INVENTION

The invention relates to the field of data communication and, in particular, to a method for exchanging information between nodes according to a quantum communication protocol.

BACKGROUND

Quantum communication is a field that utilizes the quantum mechanical properties of a quantum system—properties such as superposition and entanglement—to enable secure and efficient communication protocols. It offers a wide range of applications, such as quantum key distribution (QKD), which allows two parties to securely exchange cryptographic keys between them. Any attempt to intercept the quantum signal by a third party can disrupt the quantum state and alert legitimate users. Another important application of quantum communication is quantum teleportation and quantum cryptography. However, there is a need for improved use of QKD.

SUMMARY

It is an object to provide a method for exchanging information using a data exchange system comprising a sender node and a receiver node using a quantum channel according to a quantum communication protocol. The objects underlying the invention are achieved by the features of the independent claims.

Embodiments provide a method (security proof method) for exchanging information between a sender node and a receiver node according to a quantum communication protocol using a quantum channel, wherein the quantum communication protocol requires or defines a fault tolerance limit for the receiver node, the method comprising: determining an attack configuration for third-party access to the quantum channel, wherein the attack configuration is defined by at least one quantum circuit system with specific parameters and an attack success limit, wherein the quantum circuit system is configured to intercept a qubit on the quantum channel, determine the state of the intercepted qubit, and transmit the qubit to the receiver node via the quantum channel; defining a first metric representing a degree of success of an attack by the third party and a second metric indicating a degree of reception success at the receiver node; Evaluating the second metric using the receiver node and using the quantum circuit system to evaluate the first metric at least by intercepting qubits on the quantum channel; determining whether the first metric satisfies the attack success limit and whether the second metric satisfies the fault tolerance limit; aborting the communication protocol if the second metric satisfies the fault tolerance limit and the first metric satisfies the attack success limit.

Embodiments provide a quantum computing system for enabling the exchange of information between a sender node and a receiver node according to a quantum communication protocol, the quantum computing system comprising a quantum circuit system with specific parameters, the quantum circuit system configured to intercept a qubit on a quantum channel, determine the state of the intercepted qubit, and transmit the qubit to the receiver node via the quantum channel; the quantum computing system configured to evaluate a second metric using the receiver node and use the quantum circuit system to evaluate a first metric at least by intercepting qubits on the quantum channel, the first metric representing a degree of success of an attack using the quantum circuit system and the second metric indicating a degree of reception success at the receiver node; determine whether the first metric satisfies an attack success limit and whether the second metric satisfies a fault tolerance limit; abort the communication protocol if the second metric meets the fault tolerance limit and the first metric meets the attack success limit.

It is understood that one or more of the aforementioned embodiments may be combined, as long as the combined embodiments do not exclude each other.

BRIEF DESCRIPTION OF THE DRAWINGS

• 1 ist ein Blockdiagramm eines Systems gemäß einem Beispiel des vorliegenden Gegenstands.
• 2 ist ein Ablaufdiagramm eines Verfahrens zum Austauschen von Informationen zwischen einem Senderknoten und einem Empfängerknoten gemäß einem Quantenkommunikationsprotokoll.
• 3A ist ein Diagramm, das ein beispielhaftes Quantenkommunikationssystem gemäß einem Beispiel des vorliegenden Gegenstands darstellt.
• 3B stellt schematisch ein beispielhaftes Quantenschaltungssystem für das Quantenrechensystem einer Drittpartei gemäß einem Beispiel des vorliegenden Gegenstands dar.
• 3C stellt schematisch ein beispielhaftes Quantenschaltungssystem für das Quantenrechensystem einer Drittpartei gemäß einem Beispiel des vorliegenden Gegenstands dar.
• 3D stellt schematisch ein beispielhaftes Quantenschaltungssystem für das Quantenrechensystem einer Drittpartei gemäß einem Beispiel des vorliegenden Gegenstands dar.
• 4A ist ein Diagramm, das ein beispielhaftes Quantenkommunikationssystem gemäß einem Beispiel des vorliegenden Gegenstands darstellt.
• 4B stellt schematisch ein beispielhaftes Quantenschaltungssystem für das Quantenrechensystem einer Drittpartei gemäß einem Beispiel des vorliegenden Gegenstands dar.
• 5 ist ein Ablaufdiagramm eines Verfahrens zum Optimieren der Parameter eines Quantenschaltungssystems gemäß einem Beispiel des vorliegenden Gegenstands.

Examples are described in more detail below with reference to the drawings, which show:

• 1 is a block diagram of a system according to an example of the present subject matter.
• 2 is a flowchart of a method for exchanging information between a sender node and a receiver node according to a quantum communication protocol.
• 3A is a diagram illustrating an example quantum communication system according to an example of the present subject matter.
• 3B schematically illustrates an exemplary quantum circuit system for a third party's quantum computing system according to an example of the present subject matter.
• 3C schematically illustrates an exemplary quantum circuit system for a third party's quantum computing system according to an example of the present subject matter.
• 3D schematically illustrates an exemplary quantum circuit system for a third party's quantum computing system according to an example of the present subject matter.
• 4A is a diagram illustrating an example quantum communication system according to an example of the present subject matter.
• 4B schematically illustrates an exemplary quantum circuit system for a third party's quantum computing system according to an example of the present subject matter.
• 5 is a flowchart of a method for optimizing the parameters of a quantum circuit system according to an example of the present subject matter.

DETAILED DESCRIPTION

In the following, similar elements are designated by the same reference numerals.

The present subject matter may enable two or more remote parties to communicate securely with each other. In particular, the present subject matter may provide an accurate assessment of the security level of communication through the quantum channel.

Quantum computation can enable secure access to information. Quantum channels can be used for this purpose. The quantum channel can be a communication channel capable of transmitting quantum information. The quantum channel can further be used to transmit classical information. An example of quantum information can be information specifying the state of a qubit. The state of the qubit can refer to the computational basis state of a given basis. The qubit can be physically implemented using a quantum system that has two states, where the quantum system can be, for example, a photon or an ion. The quantum channel can be implemented using, for example, optical fibers or free space. For example, the vertical and horizontal polarization of the photon can be used as a qubit state. The two energy levels of an ion can be used as a qubit state. For example, in the case of a qubit represented by a photon, the computational basis states can be horizontal and vertical polarization states, or they can be diagonal and antidiagonal polarization states.

The quantum channel can be used to exchange or deliver information. For example, a system (referred to as a data exchange system or quantum communication system) is provided that includes transmitter and receiver nodes. The transmitter node can, for example, include a transmitter quantum computing system, and the receiver node can, for example, include a receiver quantum computing system. The terms "receiver node" and "receiver quantum computing system" are used interchangeably. The data exchange system can include the transmitter quantum computing system connected to the receiver quantum computing system via the quantum channel. For example, the transmitter quantum computing system can use the quantum channel to exchange information with the receiver quantum computing system, or vice versa. The exchanged information can, for example, be encoded in states of qubits that travel through the quantum channel from the transmitter quantum computing system to the receiver quantum computing system.

The exchange of information between the sender quantum computing system and the receiver quantum computing system can be carried out according to the quantum communication protocol. The quantum communication protocol can be a rule system that enables two or more entities of a communication system to exchange (e.g., transmit) information. The quantum communication protocol can, for example, define rules, syntax, semantics, and synchronization of the communication and possible error recovery procedures. In one example, the quantum communication protocol can be used by the sender quantum computing system and the receiver quantum computing system to obtain two identical copies of a sequence of bits, where the exchanged information can comprise at least the sequence of bits. The sequence of bits can, for example, be random and secret. The sender quantum computing system can, for example, prepare qubits in states that each represent the sequence of bits.

The quantum communication protocol can, for example, be a single-qubit-based protocol or an entangled-state-based protocol. The quantum communication protocol can control the security of exchanging information using security boundaries. The security boundaries can be defined, for example, by using a theoretical method through theoretical analysis, a simulation method through building models and performing simulations on the modeled systems, or a statistical method through analyzing data and using statistical techniques to identify patterns.

A security boundary can be defined for the reception of qubits at the receiving node. For example, this security boundary can be named or referred to as the fault tolerance boundary. The fault tolerance boundary can refer to a value or a range of values. The fault tolerance boundary can refer to a value or a range of values of an error rate, reception success, or fidelity. The fault tolerance boundary can, for example, represent a maximum error rate in the transmission of quantum information or a minimum degree of success in the reception of qubits that should not be exceeded for the secure sharing of the information; otherwise, the sharing of information can be classified as unsafe. A low error rate can indicate a high degree of accuracy in the transmitted qubits, while a high reception success indicates the ability to reliably detect and measure these qubits. For example, a higher reception success probability can indicate a lower probability of errors occurring during the transmission and measurement of qubits. Thus, a higher reception success rate may be associated with a lower error rate. In one example, the error tolerance limit may represent an error caused by a maximum level of eavesdropping.

The present subject matter may further employ another security boundary representing eavesdropping. This security boundary may be referred to as an attack success boundary. The attack success boundary may be used to evaluate the security of the quantum communication protocol. The attack success boundary may indicate the degree of success of a (simulated) attack or eavesdropping by the third party. The attack success boundary may refer to a value or a range of values. The attack success boundary may refer to a value or a range of values of a fidelity or degree of success of the eavesdropping.

The present subject matter may enable two or more remote parties to communicate securely with each other. In particular, the present subject matter may provide an accurate assessment of the security level of communication through the quantum channel. To this end, a third-party quantum computing system (also referred to as the third party) may be provided. The third-party quantum computing system may be configured to connect to the quantum channel between the sender quantum computing system and the receiver quantum computing system. The sender quantum computing system may prepare qubits in respective basis states and send them to the receiver quantum computing system via the quantum channel. The receiver quantum computing system may be configured to receive the prepared qubits via the quantum channel and measure their states. The third-party quantum computing system may be configured to intercept the prepared qubits on the quantum channel, determine or estimate their states, and send (retransmit) the intercepted qubits to the receiver node via the quantum channel. In one example, the third-party quantum computing system may be configured to intercept the prepared qubits on the quantum channel and determine their states without destroying or altering the states of the intercepted qubits, and send the qubits to the receiving node. In one example, the third-party quantum computing system may be configured to intercept the prepared qubits on the quantum channel, create approximate copies of the intercepted qubits, send the approximate copies to the receiving node via the quantum channel, and use the approximate copy of the intercepted qubit to estimate the state of the intercepted qubit. The third-party quantum computing system may enable attacks or eavesdropping on the quantum channel to be simulated. The third-party quantum computing system may be used to evaluate the degree of success of an attack (e.g., the degree of success of eavesdropping on the quantum channel).

The degree of success of the attack can be assessed using the states of intercepted qubits determined by a third-party quantum computing system. The states determined by the third-party quantum computing system can be compared with the prepared states of the intercepted qubits to assess the degree of success of the attack. Once they have arrived at the receiving quantum computing system, the states of the intercepted qubits can be read and stored on the receiving quantum computing system. measured, and the measured states can be compared with the prepared states of the intercepted qubits to evaluate the degree of reception success at the receiving quantum computing system. The degree of success of the attack can be represented by the first metric. The degree of reception success can be represented by the second metric. For example, a third-party quantum computing system can be used to evaluate the first metric. The first metric can be, for example, a distance measure such as fidelity. For example, a third-party quantum computing system can be used to determine elements of the density matrices that can be used to calculate the fidelity between the state of the intercepted qubit and a state of the prepared qubit as prepared by the sender node. The receiving quantum computing system can be used to evaluate the degree of reception success. For example, the receiving quantum computing system can be used to evaluate the second metric representing the degree of reception success. The second metric can, for example, be a distance measure such as fidelity. For example, the receiving quantum computing system can be used to determine elements of the density matrices that can be used to calculate the fidelity between the state of the received qubit and a state of the prepared qubit, as prepared by the sender node.

Each quantum computing system of the transmitter quantum computing system, the receiver quantum computing system, and the third-party quantum computing system may, for example, comprise a quantum circuit system. The quantum circuit system may use the principles of quantum mechanics to perform quantum computations by operating one or more qubits. The quantum circuit system may further use the principles of quantum mechanics to perform quantum communication to transmit or receive quantum information. The quantum circuit system may, for example, be a quantum computer. For example, each quantum computing system may be a photonic quantum computer that uses photons to represent the qubits, where the quantum circuit system may, for example, comprise components such as: a polarization controller, a pulse pattern generator, a light source, a planar lightwave circuit, or any other component that may enable the quantum circuit system to perform quantum computation and/or quantum communication. For example, any quantum computing system can be configured to perform classical computations using a classical computer or by simulating classical computations. The classical computer may or may not be part of any quantum computing system. The classical computer may include hardware, such as a processor and memory, and software, such as an operating system.

The third-party quantum computing system may be configured to provide a third-party quantum system. The third-party quantum computing system may be defined according to an attack configuration of a simulated attack. For example, the third-party quantum system may be defined by the prepared qubit intercepted on the quantum channel and a set of one or more additional qubits. For example, the third-party quantum computing system may comprise the quantum circuit system comprising the intercepted qubit and the set of additional qubits. The quantum circuit system may be further configured to perform a set of one or more quantum operations with specific parameters. The quantum operation may be a unitary operation. Applying the set of one or more quantum operations may enable the state of the intercepted qubit to be determined or predicted by measuring one or more additional qubits of the quantum circuit system after applying the set of one or more quantum operations. After applying the set of one or more quantum operations, the intercepted qubit can remain unchanged and be returned to the receiving node by the quantum circuit system via the quantum channel. The quantum operation can, for example, be an operation or a sequence of operations of a defined set of universal quantum gates, including, for example, the rotation operators and the controlled-not-not (CNOT) operator.

For example, the set of one or more quantum operations may be a controlled operation that uses the intercepted qubit as a control qubit. The controlled operation may, for example, be a controlled rotation applied to the additional qubit and using the intercepted qubit as a control qubit. The controlled operation may, for example, be a CNOT operation acting on two qubits to perform the NOT operation on the additional qubit based on the state of the intercepted qubit. After applying the controlled operation, the intercepted qubit may remain unchanged and be returned by the quantum circuit system to the receiver node via the quantum channel. After applying the controlled operation, the state of the additional qubit participating in the controlled operation involved, specify the state of the intercepted qubit. In another example, the set of quantum operations may include the controlled operation in addition to additional quantum operations. The additional quantum operations may be applied to at least a portion of the set of additional qubits. After applying the set of quantum operations, at least one of the set of additional qubits may be measured, and the result of the measurement may be used to determine or estimate the state of the intercepted qubit.

For example, the third-party quantum computing system may comprise control means that control the states of the qubits of the quantum circuit system, e.g., controlling the quantum circuit system to perform the set of quantum operations. The quantum operation may be applied to at least a portion of the qubits of the quantum circuit system. The quantum operation may be a controlled quantum operation, with the intercepted qubit acting as a controller for the quantum operation. The specific parameters of the quantum circuit system may, for example, be parameters of the quantum operations, such as rotation angles of rotation operations. For example, the state of the qubit may be transformed by rotating it around the x, y, and z axes in the Bloch sphere.

The value of the first metric can be compared to the attack success threshold (first comparison) to determine whether the value of the first metric satisfies the attack success threshold. The value of the first metric that satisfies the attack success threshold may mean that the attack is successful. For example, the success degree of the attack that satisfies the attack success threshold may mean that the success degree of the attack is higher than the attack success threshold. For example, a fidelity metric (representing the success degree of the attack) may be higher than 80%, where 80% may be the attack success threshold. The value of the second metric can be compared to the fault tolerance threshold (second comparison) to determine whether the value of the second metric satisfies the fault tolerance threshold. The value of the second metric that satisfies the fault tolerance threshold may mean that the reception of information at the receiving node is successful (e.g., secure). For example, the reception success rate at the receiver node meeting the fault tolerance limit may mean that the reception success rate at the receiver node is higher than the fault tolerance limit, where the fault tolerance limit may be a minimum reception success rate. In another example, if the fault tolerance limit is a maximum error rate, the reception error rate at the receiver node meets this limit if it is less than the fault tolerance limit. For example, the error rate at the receiver node may be less than 10%, where 10% may be the fault tolerance limit.

The present subject matter may not only use the second comparison results to decide on the security of the quantum communication protocol, but may use the results of both the first and second comparisons to decide on the security of the quantum communication protocol. For example, the quantum communication protocol may be aborted if the second metric satisfies the fault tolerance limit and the first metric satisfies the attack success limit. In another example, the quantum communication protocol may be aborted if the second metric does not satisfy the fault tolerance limit and the first metric satisfies the attack success limit. The quantum communication protocol may not be aborted if the second metric satisfies the fault tolerance limit and the first metric does not satisfy the attack success limit.

The present subject matter can thus enable two or more remote parties to communicate securely with each other. For example, the data exchange system can be a real system (i.e., not a simulated system) used to genuinely exchange information over the quantum channel. This communication can be useful in a wide range of applications where security may be critical, such as financial transactions, government communications, and military operations.

For example, aborting the quantum communication protocol may involve providing warning or error signals to the sender quantum computing system and the receiver quantum computing system. The signal may indicate that the communication is not secure and that the shared information may be accessed by an eavesdropper.

In one example, upon receiving the signal, the sender quantum computing system and the receiver quantum computing system may exchange the information by adjusting the quantum communication protocol, or may repeat the exchange of information at a different time, or may stop the exchange of information. In one example, in response to the quantum communication protocol being discontinued, the information may be exchanged again (e.g., at a later time) between the sender quantum computing system and the receiver quantum computing system. The first and second metrics may can be evaluated, and a further check can be performed to determine whether or not to abort the quantum communication protocol. This can be advantageous if the security issue is transient, e.g., if it only occurs due to temporary misconfigurations. In one example, in response to the aborting of the quantum communication protocol, the quantum communication protocol can be adapted, and the security proof procedure can be repeated using the adapted quantum communication protocol to exchange information between the sender quantum computing system and the receiver quantum computing system.

In one example, the information may be exchanged between the sender quantum computing system and the receiver quantum computing system by at least the following: sending qubits by the sender quantum computing system to the receiver quantum computing system. The sender quantum computing system may prepare each qubit in a respective computational basis state before sending the prepared qubit to the receiver quantum computing system via the quantum channel. The states of the prepared qubits provide the information exchanged between the sender quantum computing system and the receiver quantum computing system. This example may be advantageous because it may enable seamless integration of the present subject matter with existing communication protocols, e.g., the quantum communication protocol may be the BB84 protocol. For example, the sender quantum computing system may prepare the qubit using predefined bases. For example, to encode or prepare a qubit, the sender quantum computing system may select one of the predefined bases and prepare the qubit in a computational basis state using the selected basis. The receiver quantum computing system may measure the prepared qubit once it has been received over the quantum channel by selecting a basis from the predefined bases and using the selected basis to measure the prepared qubit received at the receiver quantum computing system. This example may enable the exchange of information according to the single-qubit-based protocol.

In one example, the information may be exchanged between the sender quantum computing system and the receiver quantum computing system using entanglement. To do this, a two-qubit system may be defined for each qubit pair of a set of qubit pairs, where one qubit of the two-qubit system may be part of the sender quantum computing system and the other qubit may be part of the receiver quantum computing system. The state of the two-qubit system may be an entangled state. For example, the entangled state of the two-qubit system may be any of the Bell states; e.g., the sender quantum computing system and the receiver quantum computing system may each receive a photon of a polarization-entangled pair.

The present subject matter can use the entangled state of the two-qubit system to transmit or exchange information between the sender quantum computing system and the receiver quantum computing system. The information can be teleported from the sender's qubit to the receiver's qubit using entanglement. For example, the two qubits can be entangled such that when a particular property is measured in one qubit, the opposite state can be immediately observed on the entangled qubit.

The entangled state may be provided as a distributed entangled state such that the entangled qubit pair is located at different locations in the respective sender quantum computing system and the receiver quantum computing system. The distributed entangled state between the qubit pair may be formed, for example, by first forming the entangled state of the qubit pair locally at the sender quantum computing system and then sending one of the qubit pairs by the sender quantum computing system to the receiver quantum computing system. The entangled state may be formed, for example, by applying a CNOT operation to the qubit pair. Thus, for exchanging information between the sender quantum computing system and the receiver quantum computing system, multiple distributed entangled states may be created, each using the qubit pairs. Using entanglement may further secure the exchange of information according to the present subject matter. In one example, the quantum communication protocol may be an entangled QKD protocol, e.g., an entangled BB84 variant.

In one example, the sender quantum computing system and the receiver quantum computing system may be further configured to communicate over a classical public channel, e.g., to exchange public data, such as the bases used to prepare or measure the qubits. The third-party quantum computing system may be configured to to intercept and receive data communicated over the classical public channel. A third-party quantum computing system can use this intercepted information, for example, to determine or improve the attack configuration and to determine the initial metric.

The present subject matter may utilize advantageous techniques to provide different (simulated) attack configurations of a third-party quantum computing system, which may enable a reliable security proof of the quantum communication protocol using the quantum channel. To this end, one or more attack configurations may be defined using offline analysis and a quantum simulator. The offline analysis may be referred to as an attack simulation method. The quantum simulator may, for example, be configured to simulate the quantum communication protocol using a model of a real communication system that enables the communication of quantum information. For example, the quantum simulator may be configured to control parameters for individual components and subprotocols in the system including the quantum channel and communicating parties. The quantum simulator may, for example, be implemented as a software application.

In a first optimization example, the attack simulation method may use the quantum simulator to optimize the specific parameters of a third-party quantum computing system to improve the attack success rate. The attack simulation method may comprise defining a quantum circuit system comprising at least the intercepted qubit and one or more additional qubits, and performing one or more quantum operations with the specific parameters, wherein the quantum operation(s) enable the state of the intercepted qubit to be determined, e.g., without affecting the prepared state of the intercepted qubit. The attack simulation method may further comprise evaluating the first metric using the determined/estimated states of the intercepted qubits. The attack simulation method may be repeated multiple times, using different values of the specific parameters in each iteration. The repetition may be performed until a desired target of the first metric is reached, with the values of the parameters that provide the desired target being provided as optimal parameters. Alternatively, the repetition may be performed a predefined number of times, the resulting first metric values may be compared, and the values of the parameters that provided the best first metric may be provided as optimal parameters. Thus, the attack simulation method may enable optimized values of the specific parameters to be obtained. These optimized values (and the associated quantum circuit system) can be used by the present security proof method to prove the security of exchanging information between the sender quantum computing system and the receiving quantum computing system.

In a second optimization example, the attack simulation method may use the quantum simulator to optimize the specific parameters of a third-party quantum computing system to improve attack success using machine learning. For this purpose, an objective function may be defined. The attack simulation method may be repeated multiple times, with each iteration defining different values of the specific parameters based on the evaluated objective function. The iteration may be performed until the objective function meets a convergence criterion. This example may enable a precise determination of the optimal parameters. This may further improve the security proof according to the present subject matter. The objective function may be defined based on the type of first metric used.

Dies kann es ermöglichen, die Auswirkungen des Abhörens auf Qubits in Bezug auf die Fidelität der Qubits, die am Senderknoten vorbereitet werden, und der Qubits, die am Empfängerknoten erhalten werden, zu vergleichen.In one example, the second metric may be a fidelity between the state of the prepared qubit, as prepared by the sender node, and the measured state of the qubit received at and measured by the receiver quantum computing system. The state of the prepared qubit may be represented by a density matrix ρ ₁ , and the state measured by the receiver quantum computing system may be represented by a density matrix ρ ₂ . The second metric may thus be defined as follows: $F_1\left({\rho }_1,{\rho }_2\right)={\left(\text{tr}\sqrt{\sqrt{{\rho }_1}{\rho }_2\sqrt{{\rho }_2}}\right)}^2.$

This may make it possible to compare the effects of eavesdropping on qubits in terms of the fidelity of the qubits prepared at the sender node and the qubits received at the receiver node.

$$

Die Referenzen Charles H. Bennett, F. Bessette, G. Brassard, L. Salvail und J. Smolin, Experimental quantum cryptography, Journal of cryptology, 5:3-28 und Umesh Vazirani and Thomas Videck, Fully device-independent quantum key distribution, Physical Review Letters, 113(14), Sep 2014 stellen beispielhafte Implementierungen für die Bewertung der ersten Metrik und der zweiten Metrik bereit.In one example, the first metric (F ₂ ) may be the fidelity between the state of the intercepted qubit, as determined or estimated by the third party, and a state of the prepared qubit, as prepared by the sender. The first metric may thus be defined as follows: $F_2\left({\rho }_1,{\rho }_2\right)={\left(\text{tr}\sqrt{\sqrt{{\rho }_1}{\rho }_2\sqrt{{\rho }_2}}\right)}^2.$

$$

The references Charles H. Bennett, F. Bessette, G. Brassard, L. Salvail and J. Smolin, Experimental quantum cryptography phy, Journal of cryptology, 5:3-28 and Umesh Vazirani and Thomas Videck, Fully device-independent quantum key distribution, Physical Review Letters, 113(14), Sep 2014 provide example implementations for evaluating the first metric and the second metric.

For example, the first metric may have the value 1 if the states are identical and the value 0 if both are orthogonal to each other. The third-party quantum computing system may deploy a quantum circuit system to keep its fidelity and the fidelity at the receiver as close to 1 as possible. In this case, the attack can only be detected with a low probability. For example, if the quantum communication protocol is the BB84 protocol, four different states can be selected with equal probability. In this case, the average fidelity (e.g., the fidelity is measured for all different states and averaged over four different states) can be used as the first metric and the second metric for the third-party quantum computing system and the receiver quantum computing system, respectively.

The quantum communication protocol can be used by the sender quantum computing system and the receiver quantum computing system to obtain two identical copies of a sequence of bits, where the exchanged information can be the sequence of bits. The sequence of bits can, for example, be random and secret. A third-party quantum computing system can attempt to learn original bits. During this process of exchanging information, each of the three parties can receive a string of bits. The three strings can be interpreted as binary random variables. The degree of dependence between two random variables can be measured in terms of mutual information. In one example, the first metric can alternatively comprise the mutual information (transinformation) between binary random variables of the sender node and the third-party.

wobei X die Zufallsvariable sein kann, die Bits in dem Sender-Quantenrechensystem zugeordnet ist, und Y die Zufallsvariable sein kann, die Bits in dem Quantensystem einer Drittpartei zugeordnet ist, wobei p(x, y) die gemeinsame Wahrscheinlichkeit ist und p(x) und p(y) die einzelnen Wahrscheinlichkeiten sind.Mutual information can be defined, for example, as follows: $I\left(X,Y\right)={\displaystyle {\sum }_x{\displaystyle {\sum }_{y}p\left(x,y\right)}{\text{log}}_2}\left(\frac{p\left(x,\text{ y}\right)}{p\left(x\right)p\left(y\right)}\right),$

where X can be the random variable associated with bits in the sender quantum computing system, and Y can be the random variable associated with bits in the third-party quantum system, where p(x, y) is the joint probability and p(x) and p(y) are the individual probabilities.

For example, two random bits b1, b2 ∈ [0, 1] are input to the quantum channel, and the output can be the XOR value b1⊕b2. The corresponding probabilities are shown in the following table: input 00 01 10 11 Issue 0 ¼ 0 0 ¼ Issue 1 0 ¼ 1/4 0

For example, input 00 occurs together with output 0 with probability 1/4. Thus, the value of the transinformation can be 1; e.g., out of two possible bits of information, 1 bit is transmitted through the channel. However, no information about the individual bits may have been received, because even with knowledge of b1⊕b2, the value of b1 and b2 is still equally distributed. Nevertheless, the information about the value of b1⊕b2 can be valuable to an attacker (or eavesdropper), as it can allow the search space in a brute-force search to be narrowed from 4 possibilities to 2.

For example, the fidelity for the receiving quantum computing system can be denoted as F ₁ and the fidelity for the third-party quantum computing system can be denoted as F ₂ . The objective function of the second optimization example can be defined, for example, as: -F ₂ + 10 × (F ₁ - target) ² , where target is a predefined target value for the fidelity F _1. The mutual information for the third-party quantum computing system can be denoted as I ₂ . In this case, the objective function can be defined, for example, as: I ₂ + 10 × (F ₁ - target) ² .

The first and second metrics can be evaluated using a metric determination system. The metric determination system may or may not be part of a third-party quantum computing system. The metric determination system may be a classical computing system or a quantum computing system. For example, the metric determination system may use quantum state tomography to construct the density matrices ρ ₁ and ρ ₂ , e.g., with a series of measurements performed in different bases, and to estimate the fidelity.

In one example, the quantum communication protocol may be the Quantum Key Distribution (QKD) protocol, where the shared information comprises a key. The QKD protocol may be, for example, the BB84 protocol or the Eckert protocol E91. The transmitted qubits may be photons prepared in computational basis states, where the computational basis states have horizontal and vertical polarization states. states or diagonal polarization states.

The present subject matter can further improve the security proof of the quantum communication protocol using the quantum channel. Various structures of the quantum circuit system of a third-party quantum computing system can be used for this purpose.

In one example, various quantum circuit systems may be defined, and for each quantum circuit system, the quantum circuit system may be used to intercept the qubit on the quantum channel and evaluate the first metric. The quantum circuit system having the best value of the first metric may be selected and used to perform the security proof method according to the present subject matter. Each quantum circuit system may include the intercepted qubit, a defined number of additional qubits, and perform a set of quantum operations that includes one controlled operation and zero or more additional quantum operations. The parameters (e.g., rotation angles) of the set of quantum operations may be the specific parameters of the quantum circuit system. The number of additional qubits and additional quantum operations of each quantum circuit system may, for example, be randomly selected or user-defined.

In a first circuit example, the quantum circuit system comprises the intercepted qubit and one or more qubits. The quantum circuit system is configured to perform a sequence of one or more quantum operations on the qubits to determine the state of the intercepted qubit without destroying the state of the intercepted qubit.

In a second circuit example, the quantum circuit system comprises the captured qubit and a target qubit. The target qubit is prepared in an initial basis state. Additionally, the sequence of one or more quantum operations is a single quantum operation that transforms the initial basis state of the target qubit depending on the state of the captured qubit without destroying the state of the captured qubit. The state of the captured qubit is determined or estimated using a state of the target qubit measured after applying the transformation.

In a third circuit example, the quantum communication protocol can be a QKD protocol such as the BB84 protocol. According to this quantum communication protocol, the sender quantum computing system can send the bases used to prepare the intercepted qubits to the receiver quantum computing system via a classical public channel. The third-party quantum computing system can be configured to intercept data communicated via the classical public channel and have access to the used bases. The third-party quantum computing system can further be configured to store the intercepted qubits, e.g., for a later operation. The attack simulation can make use of this to control the quantum circuit system to perform a basis-dependent operation using the stored qubit and information obtained from the bases. To this end, the quantum circuit system comprises the intercepted qubit and a target qubit. The target qubit is prepared in an initial basis state. The sequence of one or more quantum operations includes a controlled quantum operation that transforms the initial basis state of the target qubit depending on the state of the captured qubit, and another quantum operation that transforms the state of the transformed target qubit considering the basis used to prepare the captured qubit. The state of the captured qubit can be determined or estimated using a state of the target qubit measured after applying the two transformations, without destroying the state of the captured qubit.

In a fourth circuit example, the quantum circuit system comprises the intercepted qubit, a first target qubit, and a second target qubit. The sequence of quantum operations includes a controlled quantum operation that allows the state of the first target qubit to be determined depending on the state of the intercepted qubit, and a quantum operation that allows the state of the second target qubit to be determined in a specific computational basis. The quantum operations can be applied by the quantum circuit system without destroying the state of the intercepted qubit.

In a fifth circuit example, the quantum circuit system may be configured to create one or more approximate state copies of the intercepted qubit. This quantum circuit system may allow preparing a state that has as high a fidelity as possible, while also preserving the fidelity for the receiver as best as possible. The quantum circuit system may include the intercepted qubit having a state ρ and an additional qubit that can be prepared in a basis state, e.g., |0 >. The quantum circuit system can be configured to perform a set of one or more quantum operations on the input state ρ⊕|0 >< 0| to obtain a state ρ̃⊕ρ̃, where ρ̃ is the approximate state copy of the state ρ of the intercepted qubit. The approximate copy ρ̃ can be received at the receiving quantum computing system, but because it is an approximate copy, the fidelity at the receiving node can still be high. The approximate copies can be used to estimate the first metric and the second metric. The approximate copy ρ̃ can be compared to the original state ρ to estimate the first metric. The approximate copy ρ̃ received at the receiving quantum computing system can be compared with the original state ρ to estimate the second metric.

In one example, each quantum circuit system of the above circuit examples may be further used to determine the first metric and configured to send the intercepted qubit to the receiver node.

In one example, the present security proof method may be implemented in a quantum repeater. This may enable seamless integration of the present subject matter into existing systems. The present quantum computing system for enabling the exchange of information may, for example, be contained in the quantum repeater.

1 is a diagram of a system 100 according to an example of the present subject matter. The system 100 includes a quantum communication system 110 and a metric determination system 105. The quantum communication system 110 includes a transmitter quantum computing system 101 and a receiver quantum computing system 102 configured to exchange or share information via a quantum channel 104. The quantum communication system 110 further includes a third-party quantum computing system 103 configured to intercept qubits on the quantum channel 104 and estimate the states of the intercepted qubits, e.g., without destroying the states of the intercepted qubits and sending the qubits to the receiver node 102. To this end, the third-party quantum computing system 103 may include a quantum circuit system that is an experimental implementation of the schematically illustrated circuit of the 3B-3D and 4B The third-party quantum computing system 103 may be located at any point on the quantum channel 104. For example, the third-party quantum computing system 103 may be located near the receiving node 102, e.g., the third-party quantum computing system 103 may or may not be part of the receiving node 102.

The metric determination system 105 may include a classical computer or quantum computer and may communicate with components of the quantum communication system 110. The metric determination system 105 may be configured to control the operation of the third-party quantum computing system 103. The metric determination system 105 may cause the quantum circuit system of the third-party quantum computing system 103 to be used to intercept one or more qubits transmitted on the quantum channel 104 and perform respective measurements to determine output information that enables the metric, such as the first metric, to be determined. The metric determination system 105 may be configured to control the operation of the receiver node 102 to evaluate the second metric.

2 is a flowchart of an exemplary method for exchanging information between a sender node and a receiver node according to a quantum communication protocol. For purposes of explanation, the 2 described method using the method described in 1 The system illustrated may be implemented, but is not limited to this implementation. For example, the sender node may be the sender quantum computing system 101, and the receiver node may be the receiver quantum computing system 102.

In step 201, an attack configuration for the third party's access to the quantum channel can be determined. The attack configuration is defined by at least one quantum circuit system with specific parameters and an attack success threshold. The quantum circuit system is configured to intercept a qubit on the quantum channel, determine the state of the intercepted qubit, and send the qubit (or an approximate copy thereof) to the receiving node. 3B-3D provide examples of attack configurations.

In step 203, a first metric representing a degree of success of an attack by the third party and a second metric indicating a degree of reception success at the receiver node may be defined. For example, the first and second metrics may be user-defined metrics, e.g., in step 203, an input may be received that provides the definition of the first and second metrics. In another example, the The first and second metrics are defined by selecting them (e.g. randomly) from a list of predefined metrics.

In step 205, the quantum circuit system may be used to evaluate the first metric at least by intercepting qubits on the quantum channel. In step 205, the second metric may be evaluated using the receiver node.

In step 207, it may be determined whether the second metric satisfies the fault tolerance limit and whether the first metric satisfies the attack success limit.

In step 209, the quantum communication protocol may be aborted if the second metric satisfies the fault tolerance limit and the first metric satisfies the attack success limit. In one example, if the first metric satisfies the attack success limit and the second metric does not satisfy the fault tolerance limit, the quantum communication protocol may be aborted.

In one example, if the first metric does not meet the attack success limit and the second metric meets the fault tolerance limit, the shared information can be classified as secure and can thus be used by the sender and the receiver.

3A is a diagram illustrating an exemplary quantum communication system according to an example of the present subject matter. In particular, the quantum communication system is represented by a quantum circuit 300. The quantum circuit 300 represents a qubit q0, which is the qubit transmitted from the transmitter (Alice) 301 to the receiver (Bob) 302. The qubit q0 can be intercepted by the third-party quantum computing system (Eve) 303 and used as a control qubit for another qubit q1 of the quantum circuit 300. The quantum circuit 300 further shows measuring devices for measuring the qubits q0 and q1, wherein the qubit q0 is measured at the receiver 302 and the qubit q1 is measured at the third-party quantum computing system 303. The third-party quantum computing system 303 may be a quantum circuit system as the schematically illustrated circuit of 3B , 3C or 3D The quantum circuit systems of 3B and 3C can allow intercepting the qubit and estimating its state without destroying or changing its state, i.e., the receiver can receive the qubit as prepared by the transmitter.

3B schematically illustrates an exemplary quantum circuit system 310 for a third-party quantum computing system according to an example of the present subject matter. The quantum circuit system 310 may consist of a controlled rotation on Eve's system, while Alice's and Bob's systems serve as the control system. The quantum circuit system 310 may be configured to perform a rotation along the y-axis on the qubit q1 using the angle α, where the angle α is the parameter of the quantum circuit system 310. The rotation may use the qubit q1 as the target and the intercepted qubit q0 as the control qubit.

3C schematically illustrates an exemplary quantum circuit system 311 for a third-party quantum computing system according to an example of the present subject matter. The quantum circuit system 311 is an extension of the controlled rotation of the quantum circuit system 310 on the Eve system by a different, but uncontrolled, operation on the Eve system. The quantum circuit system 311 may be configured to perform a first rotation on the qubit q1 along the y-axis using angle a and using the qubit q0 as the control qubit, followed by the uncontrolled rotation along the y-axis using angle b, where angles a and b are the parameters of the quantum circuit system 311.

3D schematically illustrates an exemplary quantum circuit system 312 for a third-party quantum computing system for the BB84 protocol, according to an example of the present subject matter. The quantum circuit system 312 may be configured to perform a rotation along the y-axis on the qubit q1 using the angle π/2, followed by a CNOT operation using the qubit q1 as the control qubit and q0 as the target. The rotation may use the qubit q1 as the target and the intercepted qubit q0 as the control qubit. The quantum circuit system 312 may be configured to create approximate copies of the intercepted qubit. This quantum circuit system 312 may allow for preparing a state that has as high a state quality as possible with the transmitted state, while simultaneously preserving the state quality for the receiver as well. The quantum circuit system may comprise the trapped qubit having a state ρ and an additional qubit that can be prepared in a basis state |0>. The quantum circuit system may be configured to perform a set of one or more quantum operations on the input state ρ⊗|0 >< 0| to obtain a state ρ̃⊗ρ̃, where ρ̃ is the approximate state copy of the state ρ of the trapped qubit.

$$

sein. Jeder der kopierten Zustände kann mit dem jeweiligen ursprünglichen Zustand verglichen werden. Die erste und die zweite Metrik können durch Verwenden einer teilweisen Verfolgung der ungefähren Zustände bewertet werden, um den Vergleich des ungefähren Zustands am Empfängerknoten und an der Drittpartei mit dem ursprünglichen Zustand des übertragenen Qubits zu ermöglichen. Zusätzlich können die ungefähren Zustände, die in der spezifischen Basis definiert sind, nicht orthogonal zueinander sein und können nicht ohne Fehler voneinander unterschieden werden. Die Pretty Good Measurement (PGM) kann zum Beispiel zum Unterscheiden der ungefähren Zustände verwendet werden. Dazu können die Dichtematrizen ρ_i mit den zugehörigen Wahrscheinlichkeiten p_i zu einer Matrix G addiert werden, und dann wird die Wurzel des Pseudoinversen von G berechnet. Dies kann die Operatoren ${\mathrm{\Pi }}_i=p_i\sqrt{G^{-1}}{\rho }_i\sqrt{G^{-1}}$

eines Positive-Operator-Valued-Maßes (POVM) auf dem Raum bereitstellen, der durch die Zustände überspannt wird. Wenn der simulierte Angriff die Fähigkeit hat, die ungefähre Kopie des Zustands zu speichern, bis Alice und Bob Informationen über die gewählte Basis austauschen, kann die Drittpartei Operationen am gespeicherten Zustand durchführen, die von der Basis abhängen. Da Bob und Eve den gleichen Zustand empfangen können, wird erwartet, dass Bob und Eve auch die gleiche Erfolgswahrscheinlichkeit beim Identifizieren der übertragenen Zustände haben können. In diesem Fall kann die Messung in einer der zwei Basen $B_1=\left\{\sqrt{\frac{1}{2}}\left(\begin{array}{c}1\\ 1\end{array}\right),\sqrt{\frac{1}{2}}\left(\begin{array}{c}1\\ -1\end{array}\right)\right\}\text{ oder }{B}_2=\left\{\sqrt{\frac{1}{2}}\left(\begin{array}{c}1\\ i\end{array}\right),\sqrt{\frac{1}{2}}\left(\begin{array}{c}1\\ -i\end{array}\right)\right\}$

durchgeführt werden.The copied states can be defined in a specific basis in which the original states of the transferred qubit are defined according to the BB84 protocol. The basis states can, for example, $\sqrt{\frac{1}{2}}\left(\begin{array}{c}1\\ 1\end{array}\right),\sqrt{\frac{1}{2}}\left(\begin{array}{c}1\\ i\end{array}\right),\sqrt{\frac{1}{2}}\left(\begin{array}{c}1\\ -1\end{array}\right),\sqrt{\frac{1}{2}}\left(\begin{array}{c}1\\ -i\end{array}\right)$

$$

Each of the copied states can be compared with the respective original state. The first and second metrics can be evaluated by using a partial tracking of the approximate states to enable the comparison of the approximate state at the receiving node and at the third party with the original state of the transmitted qubit. In addition, the approximate states defined in the specific basis may not be orthogonal to each other and cannot be distinguished from each other without error. Pretty Good Measurement (PGM), for example, can be used to distinguish the approximate states. To do this, the density matrices ρ _i with the associated probabilities p _i can be added to a matrix G, and then the root of the pseudoinverse of G is calculated. This can use the operators ${\mathrm{\Pi }}_i=p_i\sqrt{G^{-1}}{\rho }_i\sqrt{G^{-1}}$

a positive operator-valued measure (POVM) on the space spanned by the states. If the simulated attack has the ability to store the approximate copy of the state until Alice and Bob exchange information about the chosen basis, the third party can perform operations on the stored state that depend on the basis. Since Bob and Eve can receive the same state, it is expected that Bob and Eve can also have the same probability of success in identifying the transmitted states. In this case, the measurement can be in either of the two bases. $B_1=\left\{\sqrt{\frac{1}{2}}\left(\begin{array}{c}1\\ 1\end{array}\right),\sqrt{\frac{1}{2}}\left(\begin{array}{c}1\\ -1\end{array}\right)\right\}\text{ oder }{B}_2=\left\{\sqrt{\frac{1}{2}}\left(\begin{array}{c}1\\ i\end{array}\right),\sqrt{\frac{1}{2}}\left(\begin{array}{c}1\\ -i\end{array}\right)\right\}$

be performed.

In one example, if the third party is configured to store the intercepted qubit, the simulated attack can be divided into two parts: The first part is the interaction with Alice and Bob's system and the third party's system, which can be stored. After revealing the selected bases, the third party's system can perform a basis-dependent operation on the stored qubit before the qubit is measured. This can be done, for example, using the circuit of 4A be implemented.

4A is a diagram illustrating a quantum communication system according to an example of the present subject matter. In particular, the quantum communication system is represented by a quantum circuit 400. The quantum circuit 400 represents a qubit q0, which is the qubit transmitted from the transmitter (Alice) 401 to the receiver (Bob) 402. The qubit q0 can be intercepted by the third-party quantum computing system (Eve) 403 and used as a control qubit for another qubit q1. The quantum circuit 400 further shows measuring devices for measuring the qubits q0 and q1, wherein the qubit q0 is measured at the receiver 402 and the qubit q1 is measured at the third-party quantum computing system 403. The third-party quantum computing system 403 can include a first quantum circuit system, as described with reference to 3B or 3C In addition, the third-party quantum computing system 403 can have two 4B shown second quantum circuit systems, each associated with a specific basis (for example, if the qubits are photons, the basis may comprise horizontal and vertical polarization states).

In this example, the intercepted qubit may be stored or secured by a third-party quantum computing system. Additionally, the third-party quantum computing system may have access to the basis used by the sender to prepare the state of the intercepted qubit, which may be the case, for example, with the BB84 protocol, where the bases are sent over a classical public channel and intercepted by a third-party quantum computing system.

As in 4A As stated, the quantum circuit 400 may enable a sequence of at least two operations (rotations around the y-axis) to be performed on the qubit q1, e.g., the operation implemented by the third-party quantum computing system 403 and the operation 406. The qubit q1 is prepared in an initial basis state. The sequence of quantum operations includes a quantum operation performed by the first quantum circuit system for transforming the initial basis state of the qubit q1 depending on the state of the intercepted qubit q0. The sequence of quantum operations further includes another quantum operation that transforms the state of the transformed qubit q1 taking into account the basis used to prepare the intercepted qubit q0. This other quantum operation may be performed by one of the second quantum circuit systems associated with the basis used to prepare the intercepted qubit q0. This other quantum operation can be called a basis-dependent operation. As in 4A As stated, this basis-dependent operation 406 can be performed even after Bob's system has measured the qubit q0. The state of the intercepted qubit q0 can be determined by a third-party quantum computing system using a state of the qubit q1 measured after applying the sequence of operations.

4B schematically illustrates an example of two quantum circuit systems 410 and 411 for a third-party quantum computing system according to an example of the present subject matter. Each of the two quantum circuit systems is in 3C shown. The two quantum circuit systems can be used depending on the basis used by Alice's system to prepare the intercepted qubit. Since two bases can be used by Alice's system, these two quantum circuit systems 410 and 411 are provided to be used for the two bases, respectively. To this end, each of the two quantum circuit systems is configured to perform a controlled operation and a subsequent uncontrolled operation on the qubit q1. The controlled operation in the two circuits uses the same angle a, while the uncontrolled operation uses two different angles b and c in the two quantum circuit systems.

5 is a flowchart of a method for optimizing the parameters of the quantum circuit system according to an example of the present subject matter. In step 501, the parameters may be set to current values. In step 503, the quantum circuit system may be used to measure the first metric. In step 505, it may be determined whether the first metric satisfies a convergence criterion. If the first metric satisfies the convergence criterion, the parameter values may be provided in step 507, for example, in the method of 2 to be used. If the first metric does not meet the convergence criterion, the method returns to step 501 to adjust the parameters to other values. For example, the convergence criterion may require that the first metric be within a target range of values.

QUOTES CONTAINED IN THE DESCRIPTION

This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• Charles H. Bennett, F. Bessette, G. Brassard, L. Salvail and J. Smolin, Experimental quantum cryptography, Journal of cryptology, 5:3-28 [0036]
• Umesh Vazirani and Thomas Videck, Fully device-independent quantum key distribution, Physical Review Letters, 113(14), Sep 2014 [0036]

Claims (15)

A method for exchanging information between a sender node (101) and a receiver node (102) using a quantum channel (104) according to a quantum communication protocol, wherein the quantum communication protocol requires a fault tolerance limit for the receiver node, the method comprising: determining (201) an attack configuration for third-party access to the quantum channel, wherein the attack configuration is defined by at least one quantum circuit system (310, 311, 312) with specific parameters and an attack success limit, wherein the quantum circuit system is configured to intercept a qubit on the quantum channel, determine the state of the intercepted qubit, and transmit the qubit to the receiver node; defining (203) a first metric representing a degree of success of an attack by the third party and a second metric indicating a degree of reception success at the receiver node; Evaluating the second metric using the receiver node and using (205) the quantum circuit system to evaluate the first metric at least by intercepting qubits on the quantum channel; Determining (207) whether the second metric satisfies the fault tolerance limit and whether the first metric satisfies the attack success limit; Aborting (209) the communication protocol if the second metric satisfies the fault tolerance limit and the first metric satisfies the attack success limit. Procedure according to Claim 1 , comprising: repeatedly: preparing a qubit in a respective basis state by the sender node; sending the prepared qubit by the sender node via the quantum channel to the receiver node to enable the exchange of information; wherein the intercepted qubits are the prepared qubits. Procedure according to Claim 1 , comprising: repeatedly forming an entanglement state between a transmitter qubit in the transmitter node and a receiver qubit; sending the receiver qubit by the transmitter node to the receiver node via the quantum channel; wherein the intercepted qubits are the receiver qubits. The method of any preceding claim, further comprising: Using a quantum simulator to optimize parameters of the quantum circuit system, wherein the optimization comprises repeatedly changing values of the parameters of the quantum circuit system and evaluating the first metric, wherein the repetition is performed to obtain a target value of the first metric; Providing the specific parameters of the quantum circuit system as optimized parameters associated with the target value of the first metric. A method according to any one of the preceding claims, wherein the quantum circuit system comprises the intercepted qubit and one or more qubits, the quantum circuit system being configured to perform a sequence of one or more quantum operations on the qubits to determine the state of the intercepted qubit. Procedure according to Claim 5 , wherein the quantum circuit system comprises the intercepted qubit and a target qubit prepared in an initial basis state, wherein the sequence of one or more quantum operations is a quantum operation that transforms the initial basis state of the target qubit depending on the state of the intercepted qubit, wherein the determination of the state of the intercepted qubit is performed using a state of the target qubit measured after application of the transformation. Procedure according to Claim 5 , wherein the quantum circuit system comprises the intercepted qubit and a target qubit prepared in an initial basis state, wherein the sequence of one or more quantum operations comprises a quantum operation that transforms the initial basis state of the target qubit depending on the state of the intercepted qubit, and another quantum operation that transforms the state of the transformed target qubit taking into account the basis used to prepare the intercepted qubit, wherein the determination of the state of the intercepted qubit is performed using a state of the target qubit measured after applying the two transformations. Procedure according to Claim 5 , wherein the quantum circuit system comprises the intercepted qubit and a first target qubit and a second target qubit, wherein the sequence of quantum operations comprises a quantum operation that enables the determination of the state of the first target qubit depending on the state of the intercepted qubit, and a quantum operation that allows the determination of the state of the second target qubit in a specific computational basis. Method according to one of the preceding Claims 5 until 8 , where the quantum operation is a rotation operation around a specific axis. A method according to any one of the preceding claims, wherein the second metric is a fidelity between the state of the qubit received at the receiver node and a state of the prepared qubit. A method according to any one of the preceding claims, wherein the first metric is a fidelity between the state of the intercepted qubit and a state of the prepared qubit or a mutual information between the sender node and the third party. Method according to one of the preceding claims, wherein the quantum communication protocol is a quantum key distributed protocol. A method according to any one of the preceding claims, wherein the shared information comprises an encryption key, a token or a secret. Method according to one of the preceding claims, which is implemented in a quantum repeater. A quantum computing system (100) for enabling the exchange of information between a sender node (101) and a receiver node (102) according to a quantum communication protocol, wherein the quantum computing system comprises a quantum circuit system (310, 311, 312) with specific parameters, wherein the quantum circuit system is configured to intercept a qubit on a quantum channel (104), determine the state of the intercepted qubit, and send the qubit to the receiver node; wherein the quantum computing system is configured to evaluate a second metric using the receiver node and to use the quantum circuit system to evaluate a first metric at least by intercepting qubits on the quantum channel; to determine whether the second metric satisfies a fault tolerance limit and whether the first metric satisfies an attack success limit; abort the communication protocol if the second metric meets the fault tolerance limit and the first metric meets the attack success limit.

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