A method for real-time state-validation of entanglement in an extendible quantum network

Abstract

This disclosure pertains to a method for real-time state-validation of entanglement between at least two independent distant quantum nodes in an extendible quantum network. Each quantum node comprises at least a communication qubit. The quantum network comprises a midpoint, comprising a measurement apparatus for measuring a herald in a quantum signal, and a digital logic unit for calculating the measurement result. The method comprises the steps of receiving a hybrid signal comprising a quantum signal and a classical signal; measuring, each of the quantum signals to validate entanglement; validating the entanglement based on the outcome of the measurement and on the state-validation information of the classical signal; sending a classical message about the validation of the entanglement attempt.

Description

[0001] TITLE

[0002] A method for real-time state-validation of entanglement in an extendible quantum network

[0003] BACKGROUND

[0004] Quantum network applications are anticipated to have a large impact in different fields. For instance, in the field of secure communication, quantum networks hold the promise to generate and distribute secure keys for encryption of communication. In the field of quantum computers, quantum networks may connect several quantum processors together to obtain greater computation power, and in the field of quantum sensing, quantum networks hold the promise to expand the range of sensor networks to cover larger areas.

[0005] In general, any application of a quantum network requires establishing entanglement between two or more quantum network nodes, wherein each of these quantum network nodes comprises one or more qubits for quantum communication and computation. To establish entanglement, two entanglement generation methods can be distinguished from each other: probabilistic generation and deterministic generation. The latter has a higher efficiency, since no entanglement attempts are wasted, but it comes at an increased complexity and resource intensity. Therefore, the current golden standard is probabilistic entanglement generation, which has a lower entanglement efficiency, but can operate at a much lower operational cost, is a simpler system then a deterministic one, and can achieve higher entanglement attempt rates.

[0006] Because of the probabilistic nature of the generation process, the success of the entanglement must be validated by performing measurements on all the network nodes which were intended to be entangled. Heralded entanglement may be used for confirming whether the entanglement was successful by performing a measurement on the herald instead of directly on the nodes themselves. However, this requires additional information about the state of the quantum node. All this information is communicated from one node to another, causing a high latency, straining the requirements of the quantum memories at the nodes to have long enough lifetimes to ensure the quality of the quantum state. On top of that the nodes need to perform post- processing on the information to confirm the validity, thereby increasing the system costs of the quantum nodes.

[0007] Therefore, it is a goal of the disclosure to provide an easier way for real-time validation of the entanglement of network nodes, thereby potentially easing the requirements of the network nodes and enabling nodes to make more efficient use of any generated entanglement.

[0008] SUMMARY OF THE INVENTION

[0009] The disclosure pertains to a method for real-time state-validation of the entanglement between at least two independent quantum nodes at a midpoint node located between the at least two independent and probabilistic quantum nodes. This method enables upgrading the measurement success message of standard heralded entanglement generation protocols to an entanglement success message, thereby removing the need of post-processing at the quantum nodes.

[0010] To do so, the method for real-time state-validation of entanglement between at least two independent distant quantum nodes in an extendible quantum network, wherein each quantum node comprises at least a communication qubit, and wherein the quantum network comprises a midpoint, comprising a measurement apparatus for measuring a quantum signal, and wherein the quantum network comprises a digital logic unit for calculating the measurement result, comprises the steps of:

[0011] 1) receiving, by the midpoint, from each of the at least two independent distant quantum nodes, a hybrid signal comprising at least one quantum signal and a classical signal, wherein the classical signal comprises state-validation information of said at least one quantum signal;

[0012] 2) measuring, by the measurement apparatus at the midpoint, each of the at least one quantum signal of each independent distant quantum node to herald entanglement between two of the at least two independent distant quantum nodes;

[0013] 3) validating, by the midpoint, the entanglement between the at least two independent distant quantum nodes based on the outcome of the measurement in step 2 and on the state-validation information of the classical signal. 4) sending, by the midpoint, to each of the at least two independent distant quantum nodes, a classical message about the validation of the entanglement attempt based on the information obtained about the entanglement in step 3.

[0014] In prior art entanglement methods, the quantum nodes themselves validate the entanglement generation attempt by checking the outcome of a herald measurement and the state of the quantum node. Subsequently the heralding outcome is combined with the validation outcome at the quantum node, and this combined result information is communicated to the other node(s), which use this information to validate whether the entanglement was successful against their own measurement information.

[0015] The at least one communication qubit may be solid state qubits such as color centers in diamond, trapped ion qubits, neutral atoms, superconducting qubits, photonic qubits or quantum dots. Additional memory qubit or data qubits, besides the communication qubit (which has some limited memory capability) may be present in each distant quantum node. Note that, a communication qubit can always act as a memory qubit, but may have a shorter memory lifetime than a qubit that is only a memory qubit. However, only memory qubits enable complex quantum protocols, such as error correction, which require multiple steps.

[0016] Since this communication process requires time, because of the measuring by the quantum nodes themselves and the long communication lines, a lot of strain is put on the lifetime of the qubits of the quantum node. Namely, a quantum node may actually be in the correct quantum state and may even be entangled to another quantum node, however due to too high latency (too slow) communication and cross- validation, the respective quantum states may decohere, thereby making their use in subsequent quantum network operations or local quantum application execution false or incorrect. This would, for instance, lead to an incorrect assignment of the state of the system, which could potentially not be true, thereby causing errors and uncertainty in any future quantum operation performed with said pair of quantum nodes. Alternatively, this would lead to an incorrect assignment of the state of the system, within a network of quantum nodes, wherein actually two different quantum nodes are entangled. Namely, prior art methods do not provide adequate verification of the quantum node.

[0017] By means of the method according to the disclosure the validity of the entanglement attempt can be determined at a midpoint between the at least two quantum nodes and the decision result can subsequently be communicated by the midpoint to the respective independent quantum nodes. This removes the need for post-processing at the quantum nodes and speeds up the entanglement process drastically because of shortened communication lines. Therefore, less strain is put on the technical requirements of the quantum nodes, in particular the lifetime of the quantum state.

[0018] An additional benefit to the configuration of the method allows for quantum nodes to truly operate independently from each other. Therefore, additional nodes can be added to the quantum network without needing to adjust the network in its entirety. These truly independent distant quantum nodes may thus be stand-alone nodes, which can be added to and / or removed from the network at will.

[0019] Yet another benefit is that no a-priori communication needs to be established between the quantum nodes to provide readiness of said node.

[0020] The method relies on receiving a hybrid signal comprising a quantum signal and a classical signal, wherein the classical signal comprises state-validation information of said quantum signal. By logic and computational operations at the midpoint on the state-validation information combined with the herald measurement outcome, the entanglement of at least two quantum nodes can be validated. One can imagine the quantum signal of two nodes being received by the midpoint and subsequently being checked to give a first indication whether the nodes may be entangled or not, for instance by measuring the timing of the signals’ arrival and / or the interference of the quantum signals via optical components and photon detectors. Independently, the classical signal of those at least two quantum nodes is being received by the midpoint and is checked to verify that both quantum nodes were in the correct quantum state, giving a second indication that the entanglement was successful. Utilizing both indications (the state-validation information and the herald measurement), the midpoint may send back a (classical) message to the respective independent nodes indicating whether the attempt was successful or not.

[0021] In the simplest case, the (classical) response signal could be a single bit encoding of a yes / no outcome. This could be a (classical) photonic encoding, a voltage pulse, an ethernet packet, etc. In the more complex case, e.g. when the midpoint may instruct one or more nodes to pause entanglement generation, the new response message would necessarily differ from prior art in that the response message may be several bits or bytes and possibly include fields such as the next timestamp at which to send photons. The encodings suitable for this message are the same, but if the message becomes large an ethernet packet may be preferred. The message according to the disclosure allows to include more information, even in the simpler case. E.g., in the simpler case the response message could be 2- bits, one for heralded success and one for validation success.

[0022] There are various methods for measuring the quantum signals / heralds that exist in the field, the expert would understand that a method such as a measurement done on the quantum signals / herald including a so-called interference measurement, wherein the two quantum signals are sent to at least one optical component at which they interact, typically a 50 / 50 beam splitter, may be preferable. The output of the optical component at which the signals interact must then be sent to signal detectors such as photon detectors, which are connected to a digital logic device which computes the outcome of the measurement. Such a measurement may also be performed at the same time with a photon arrival measurement, but does not necessarily have to be.

[0023] Furthermore, the wording “at least one quantum signal” in each hybrid signal should be understood that there may be as many quantum signals emitted as entanglement attempts. This is achievable, since the method according to the disclosure does not rely on receiving the quantum signal and the classical signal at the same time. A classical signal might thus be transmitted once per “n” entanglement attempts, where “n” is a number that comes from an optimal trade-off between fidelity and the emission rate.

[0024] In an example of the method, the quantum signal part of the hybrid signal from each of the at least two independent distant quantum nodes according to step 1) is received within a predefined heralding time window at the midpoint, such that the quantum signal parts are received (close to) concurrently.

[0025] A herald in entanglement generation is known to serve as an indicator signaling the presence of an entangled pair. These heralds may comprise: a photon, an electron, an atom, or an ion, or the like.

[0026] A heralding time window may be understood as a time frame during which the heralds in the quantum signal part of the respective entangled quantum nodes can be interfered as in the above-described interference measurement and detected confidently to assert the existence of an entangled pair of quantum nodes. A wider window may increase the probability of detecting the herald, but also increases the false-positive rate. A narrower window thus leads to higher confidence, because of a lower false-positive rate, but on the other hand reduces the detection occurrences.

[0027] Therefore, the heralding time windows is a crucial parameter and the inventors have found that the exact heralding time window size is dependent on the exact implementation of the herald (i.e., photon, electron, etc.), influencing both the detection occurrence and the confidence.

[0028] In another example, receiving the classical signal part of the hybrid signal from each of the at least two independent distant quantum nodes by the midpoint according to step 1) is limited to a predefined time period.

[0029] The method utilizes both the quantum signal and the classical signal part of a hybrid message to validate the entanglement of said respective quantum node with respect to another quantum node at a midpoint. Therefore, it is important that the timing of receiving both parts of the message falls within a predefined time period. Otherwise, the risk of decoherence at the quantum node may increase.

[0030] The method of the disclosure beneficially does not require the simultaneous arrival of the quantum signal and the classical signal part. They only need to be linked together to a corresponding entanglement attempt. This is beneficial since no extra requirements are needed for the quantum and the classical communication line between the distant quantum node and the midpoint to tailor them for simultaneous arrival, thereby easing the complexity of the quantum network, yet providing all the information for the midpoint to validate the entanglement attempt.

[0031] In yet another example, the midpoint comprises a quantum memory. This quantum memory may be adapted to store the quantum signal part of the hybrid signal of any one or more of the at least two independent quantum nodes. Such a quantum memory may comprise at least one memory qubit, might include atomic gas memories, solid state memories or optical components such as delay lines.

[0032] It should be noted that only in an example of the method, the midpoint comprises a quantum memory, meaning that it is not necessary for the midpoint to comprise the quantum memory. For instance, the method can also function with a herald in the quantum signal part, which is measured and processed directly upon receiving by the midpoint. However, the possibility of storing the herald or any other information of the quantum part of the hybrid signal allows for a more optimized process at the midpoint. For instance, the midpoint may be receiving multiple hybrid signals of a plurality of independent nodes, wherein two are intended to be entangled. In that case, the midpoint could store the other quantum signal part(s) of the other hybrid message(s) into the quantum memory.

[0033] Having such storing capabilities would relax the requirements on the quantum signals parts sent by the independent quantum nodes to successfully arrive at the midpoint at the same attempt, e.g. one could receive a signal first from “Node A”, then switch to receive a signal from “Node B” at a later stage. This could speed up the entanglement generation process (depending on technical parameters such as loss and switching speed). In such case, it would be clear for the expert in the field that the quantum memory stores the entire received quantum signal. Furthermore, a quantum memory may be understood as one or more memory qubits.

[0034] Alternatively, having such storing capabilities allows the midpoint to more efficiently allocate its quantum computing power. For instance, it may store an early received quantum signal part of a hybrid message, awaiting the classical signal part of the hybrid message. Only upon receiving of said classical signal part, it may initiate the logic and computation operation on the quantum signal part, stored in the quantum memory of the midpoint.

[0035] In another example, the midpoint further comprises a clock signal generator to distribute a centralized timing signal in the quantum network.

[0036] By means of a centralized timing signal the midpoint indicates to the independent distant quantum nodes when they are allowed / intended to initiate an entanglement process. In this configuration, where the midpoint validates the success of the entanglement of two independent distant quantum nodes, the midpoint must act as a master and the quantum nodes must act as slaves. Otherwise a cacophony of (hybrid) signals would arrive at the midpoint, making it impossible to distinguish what from whom.

[0037] When the midpoint acts as a master dictating a centralized timing by means of a wall clock time and / or a heartbeat, the distant quantum nodes receive cues on when to initiate entanglement attempts. The generation of an entanglement attempt at the quantum nodes occurs probabilistically. Therefore, such timing cues help towards entanglement generation at the correct timing, thereby increasing the probability of entanglement. Otherwise, without centralized timing the likelihood of entanglement generation would be nihil, since the likelihood of generating the correct quantum state at the correct timing for two independent quantum nodes would likely never overlap.

[0038] Furthermore, with the midpoint acting as a master dictating a centralized timing, quantum nodes can be added and removed from the quantum network truly independently without requiring a-priori communication establishment or changing the communication channels of other quantum nodes in the quantum network.

[0039] The centralized timing may comprise a wall clock time and / or a heartbeat. The exact configuration depends on the exact implementation of the method. For instance, a wall clock time may be a clock ran on every quantum node and on the midpoint themselves, but with predetermined windows of when an action is allowed. Since clocks of different nodes typically are not entirely synchronized, a heartbeat may be broadcast by the midpoint. This heartbeat is timed to the wall clock time of the midpoint and upon arrival at a respective quantum node it can be used to adjust its internal clock.

[0040] The centralized timing to dictate when entanglement attempts are to be initiated, ensures that hybrid signals of different quantum nodes arrive at the midpoint approximately at the same time. Preferably, the quantum parts of those hybrid signals arrive within a heralding time window, but the classical signal parts of the hybrid signals do not have to arrive at the same time or within the heralding time window for the method to still properly function. It is desired, however, that the classical signal parts arrive close to the arrival of the quantum signal parts.

[0041] The definition of a centralized clock can thus be understood as: the midpoint requiring and distributing a master reference clock. This clock is known as the central clock. The controller of the midpoint has access to the central clock and any timebased actions taken by the midpoint are based on this clock.

[0042] In the example where receiving of the classical signal is limited to a pre-defined time period, this pre-defined time period is defined with respect to the central clock. It is the midpoint, with access to the central clock, which would enforce disregarding or invalidating any signals that arrive outside the pre-defined time period.

[0043] Another example occurs in discussing the validation at the midpoint. It is said that "it is important that the timing of receiving both parts of the message fall within a predefined time period". Here this pre-defined time period is according to the central clock, and enforcement of this time period is handled by the controller of the midpoint.

[0044] In yet another example, the quantum network operates with at least one distributed clock.

[0045] In this example, the network may comprise at the quantum node level and / or at the midpoint level at least one distributed clock. The midpoint sends a timing signal, derived from this at least one distributed clock to each node independently. This at least one distributed clock may be comprised by the midpoint and / or the quantum nodes and preferably is a plurality of distributed clocks. That way, there is no single point of failure, and the correct timing is derived from said plurality of distributed clocks.

[0046] Via a timing protocol (such as the white rabbit precision timing protocol) and use of the distributed clock, the local clock (a distributed clock at a quantum node) of each node may be synchronized to each other. In could then also be the case that the midpoint also comprises a distributed clock. One of the nodes in the quantum network, preferably being the midpoint is then chosen as a reference distributed clock for the other clocks to adjust to. The synchronization of the local clocks with the reference distributed clock can be achieved with sub-nanosecond accuracy.

[0047] In such case, at each node, all time-based events are based on the local clock. Examples include triggering entanglement generation or time-tagging an event.

[0048] The distributed clock and the timing protocol may be used at each node to derive important quantities such as a heartbeat and an offset. The heartbeat can be equal to the distributed clock, or it can be a lower frequency clock derived from the distributed clock in such a way that it is in phase with the distributed clock (For example every 1000-th tick of the distributed clock could exactly coincide with every tick of a derived heartbeat).

[0049] A node may program an offset equal to it's one way communication time over the communication interface, such as an optical fiber link, between the node and the midpoint. In timing protocols such as White Rabbit, this communication time is known as a consequence of the implementation of the timing protocol. Moreover, any drifts that occur in this communication time are also known. The node may then trigger entanglement generation based on ticks of its local clock and the offset. In another example, measuring, by the measurement apparatus at the midpoint, of step 2) is performed through a projective measurement of the herald in the quantum signal, preferably a Bell-state measurement.

[0050] A projective measurement is a type of measurement that collapses the herald into one of its eigenstates, thereby making the herald measurable to determine its quantum state and thus the quantum state of the quantum node it came from. In the preferable example, a Bell-state measurement is performed as such projective measurement. Bell state measurements are known in the art as joint quantummechanical measurements that determine which of the four Bell states the two qubits are in.

[0051] The fundamental feature common to any implementation of the method according to the disclosure is that the two quantum signals must have some type of interaction, preferably being interference via an optical device, to measure the quantum state. In case a quantum memory is used by the midpoint to store one of the quantum signals, the interference may occur after the state is retrieved from said quantum memory.

[0052] In a further example of the method, validating the entanglement between two of the at least two independent distant quantum nodes of step 3) is performed through logic operations and computation on the output of the measurements of the measurement apparatus and on the state-validation information in the classical signal of the hybrid signal.

[0053] As previously discussed, two types of information are used by the midpoint to validate whether the entanglement between at least two quantum nodes is successful or not. In this example of the method, the outputs of the measurement performed by the midpoint on the received quantum signal parts are used in combination with the state-validation information of the classical signal part through logic and computation operations. Such operations may include AND, OR, XOR, NOT, NOR, NAND, etc. For instance, in the simplest example, three AND operators are used. In another example, the first logic operation (between the two quantum signals) depends on the chosen encoding of the quantum state. If the time-bin encoding is chosen, it can be seen as an AND gate between the signal in the early and late time-bin exclusive-or the detectors (early_det_1 XOR early_det_2) AND (late_det_1 XOR late_det_2). In the example of the photon-number encoding, the first logic operation is an OR gate between the signal of the two detectors between a single heralding time window (det_1 XOR det_2).

[0054] Another example of the method further comprises the step of correcting, by the midpoint, the at least one quantum signal in real-time based on the classical signal,

[0055] Such corrections allow for real-time adjustment based on supplied information in the classical signal, e.g. delay, rotation or other signal mutation may be known for a given communication channel, which can be used to correct the obtained measurement outcome of the quantum signal.

[0056] Yet another example of the method further comprises the step of obtaining, by the midpoint, information in real-time on which of the at least two independent distant quantum nodes in the quantum network to entangle.

[0057] This information may be encoded or comprised in the classical signal of the hybrid message and is beneficial for switching and routing of communication lines. In particular in more extended networks. Furthermore, it allows for efficient resource allocation at the midpoint.

[0058] In another example of the method, the midpoint is co-located with one of the at least two independent distant quantum nodes. The midpoint may still act as an individual actor within the quantum network, but its physical location can be co-located with one of the distant quantum nodes. The method does not require any distance to be between independent quantum nodes and the midpoint, so the placement of the midpoint "between" the nodes doesn't matter. It only needs to be in communication with the two or more independent quantum nodes.

[0059] In another aspect, the disclosure pertains to a method for real-time statevalidation of entanglement between a midpoint and at least two independent distant quantum nodes in an extendible quantum network, comprising the steps of: i) sending, by one of the at least two independent distant quantum nodes, to the midpoint, a hybrid signal comprising a quantum signal and a classical signal, wherein the classical signal comprises state-validation information of said quantum signal; ii) receiving, by one of the at least two independent distant quantum nodes, to the midpoint, a classical message about the validation of the entanglement attempt based on the state-validation measurement; iii) determining, based on the information in the classical message about the validation of the entanglement attempt whether or not to continue quantum operations with said quantum node.

[0060] The above-described method is directed to the functioning of an independent distant quantum node in an extendible quantum network. The distant quantum node is configured to send a hybrid signal to the midpoint containing two pieces of information. The quantum signal contains information about the quantum state of the quantum node, and the classical signal part contains information about the state-validation of said quantum node. With these two pieces of information the midpoint can validate the entanglement attempt with respect to another quantum node in the network.

[0061] In a further example thereof, the sending of the hybrid signal by one of the at least two independent distant quantum nodes of step i) is performed probabilistically.

[0062] And in yet another example of said method for real-time state-validation of entanglement between a midpoint and at least two independent distant quantum nodes in an extendible quantum network, the sending of the hybrid signal by one of the at least two independent distant quantum nodes of step i) is performed based on a centralized timing signal received from the midpoint.

[0063] Low-cost entanglement generation can be performed probabilistically as laid out in the background section. These systems typically are much simpler and require less technical requirements of the nodes. However, since the entanglement attempt process is probabilistic, the emission of the hybrid signal is also probabilistic. Without proper instructions for the quantum nodes of when to perform entanglement attempts, the communication lines would be cluttered with signals. Furthermore because of the probabilistic nature, the chances of achieving entanglement would become close to zero. Therefore, a centralized timing is broadcast in the quantum network to indicate to the distant quantum nodes when to perform an entanglement attempt. This way the communication lines are structured and chances of entanglement are improved.

[0064] In a further example, each of the at least two quantum nodes operates independently without inter-node communication.

[0065] The benefit of a system as described by the method is that the quantum nodes can operate truly independent from each other, meaning that they can be added and / or removed from the quantum network without needing to adjust the rest of the network. This can only be achieved if there exists no inter-node communication, such that the quantum nodes are only allowed to communicate with the midpoint of the quantum network, which would therefore act as a master and the quantum nodes would act as slaves.

[0066] In a further example, the classical message about the validation of the entanglement attempt comprises information of an outcome of the measurement at the midpoint, information of the attempt validation, and / or an indicator to wait an indicated time period before engaging in subsequent entanglement generation.

[0067] The classical message received by the quantum node from the midpoint is used to indicate to the respective quantum nodes whether the entanglement has been successful or not. To do so, said message should at least comprise one of the above- mentioned pieces of information. The outcome of the measurement at the midpoint may already give a first indication whether the entanglement has been successful or not, however this leaves the quantum node to still perform a secondary check with the state-validation information of the quantum node. Therefore, more complete information about the attempt is comprised in a message from the midpoint containing information of the attempt validation. Lastly, the classical message sent by the midpoint may also only or together with any of the above contain an indicator, indicating the quantum node to wait an indicated time period before engaging in subsequent entanglement generation. A message only containing the latter could indicate to a node that the entanglement generation has been successful, such that instead of engaging in new entanglement generation, it should focus on the quantum operations of the communication protocol.

[0068] In another example, the state-validation information of the classical signal sent by a quantum node comprises timing information, a charge and a resonance check status, a phonon side band measurement information, an attempt identifier including a quantum node identifier, an indicator of whether a state is stored in the qubit of said quantum node, and / or frequency information.

[0069] Here, specific examples are mentioned on pieces of information of a quantum node that may be used for state-validation. The specific type of information depends on the exact implementation of the quantum network. For instance, photon information, such as timing, phonon side bands, and / or frequency may be useful when the remote quantum nodes have an optical interface to the quantum network.

[0070] In yet another example, the quantum signal comprises at least one photon. In another example, the quantum signal comprises a flying qubit encoded in a basis such as time-bin, polarization, frequency or photon-number.

[0071] The transmitted quantum signal may be sent electrically, but in this particular example, it is sent optically. The advantage of such systems is that less losses are present and that the signals can be transmitted over longer distances as compared to other methods.

[0072] In another aspect of the disclosure, irrespective of the communication protocol, pertains to a method of local status determination for quantum nodes in a quantum network comprising the steps of: receiving, by the midpoint, a message over a first communication interface from a distant quantum node, that the quantum node is offline for a predetermined time; sending, by the midpoint, another message over a first communication interface to another distant quantum node, that it should go offline for a predetermined time.

[0073] Another example pertains to a method of local status determination for quantum nodes in a quantum network comprising the steps of: sending, by the midpoint, a message over a first communication interface to at least two distant quantum nodes, that the quantum nodes should go offline for a predetermined time; receiving, by the midpoint, another message over a first communication interface from each of the at least two distant quantum nodes, what their status is.

[0074] Yet another example pertains to a method of local status determination for quantum nodes in a quantum network comprising the steps of: receiving, by a distant quantum node, a message over a first communication interface from a midpoint, that the node should go offline for a predetermined time; sending, to the midpoint, another message over a first communication interface from the distant quantum node, what its status is.

[0075] Even another example pertains to a method of local status determination for quantum nodes in a quantum network comprising the steps of: receiving, by the midpoint, a message over a first communication interface from a distant quantum node, that the node should is offline for a predetermined time; sending, by the midpoint, another message over a first communication interface to another distant quantum node, that it should go offline for a predetermined time. These last four examples relate to several new options for the status determination of the quantum nodes in a quantum network according to the disclosure. However, it should be noted that these methods are not limited to relying on or using these communication methods. These methods can be utilized with other communication schemes.

[0076] The above-described status methods are beneficial for improving the overall cleanliness of the network communication. For instance, in some cases it may be required for distant quantum nodes to perform local preparation or calibration routines, which causes the node to be unable to participate in the communication. Because of the availability of two communication interfaces, the midpoint could still be informed of this “offline” status. Going offline should thus be understood to no longer utilize the communication interface, thereby causing some silence on said communication interface.

[0077] At every wall clock time the distant quantum node is offline the midpoint could be using scarce resources or resource power trying to communicate with said quantum node. Furthermore, because entanglement is tried to be established between two quantum nodes, the partner quantum node may also be silenced during the offline time of one of them. This eases the load on the communication interfaces and spares resources at the midpoint.

[0078] The four methods as described above, allow either the midpoint or the distant quantum node to initiate and communicate its offline state to the other. In that case, the midpoint can instruct the partner quantum node at the same time it is requesting a quantum node to go offline or after receiving an offline message from another quantum node.

[0079] Additionally, with the communication method according to the disclosure, the classical signal may be used to communicate a status among the nodes in the quantum network. In such case, a repeat offline message could be sent every wall clock time, but alternatively a new status request may be sent after the predetermined time has elapsed.

[0080] These methods for distributing the status in the quantum network may also be used in combination with a memory-equipped midpoint. It may occur during the communication that the memory at the midpoint is unavailable, because of a variety of reasons, including that there is a state already stored in memory or that there is some calibration of the memory required. In such case, the classical signal sent by the midpoint may include, after an entanglement attempt, an indication to one or both of the quantum nodes that they should not send (hybrid) signals for the subsequent wall clock times. This could be for a pre-determined number of wall clock times, or until a follow-up message is received. In these examples, “offline” should be truly considered as a temporal silencing of the communication interface.

[0081] SHORT DESCRIPTION OF THE FIGURES

[0082] The disclosure will now be discussed with reference to the drawing:

[0083] Fig. 1 shows a quantum network comprising a plurality of independent distant quantum nodes and one midpoint.

[0084] Fig. 2 shows the timing of the quantum part of the communication between two distant quantum nodes and one midpoint.

[0085] Fig. 3 shows the steps taken at the midpoint side and the quantum node side during the communication.

[0086] DETAILED DESCRIPTION

[0087] For a proper understanding of the disclosure, in the detailed description below corresponding elements or parts of the disclosure will be denoted with identical reference numerals as used in the drawings.

[0088] Throughout the disclosure the term independent quantum node is used. It should be noted that the quantum nodes may have access to the shared clock by the midpoint, and that they communicate back and forth with the midpoint. However, a common clock does not make the nodes interdependent, because it is only the midpoint that sets and determines this common clock, meaning there are no adjustments made based on feedback given by the quantum nodes. This results in the beneficial effect that an entire node connection can be added or removed, without needing to adjust the clock or any other quantum nodes.

[0089] The disclosure pertains to a method for real-time state-validation of the entanglement between at least two independent quantum nodes 100i-100e at a midpoint node 150 between the at least two independent and probabilistic quantum nodes 100i-100e. In standard entanglement protocols each quantum node needs to communicate with any other quantum node 100i-100e to establish entanglement between those respective nodes 100i-100e. Here, a lot of information is passed back- and-forth, straining the technical requirements of the quantum nodes 100i-100e, because of the possibility of decoherence of the quantum state.

[0090] With the method according to the disclosure, the measurement success message of heralded entanglement is upgraded, such that the need of post-processing at the quantum nodes 100i-100e is removed.

[0091] In Fig. 1 an extendible quantum network 10 that uses the method for real-time state-validation of entanglement between at least two independent distant quantum nodes 100i-100e is shown. Herein, each quantum node 100i-100e comprises at least a qubit, and the quantum network 10 comprises a midpoint 150, comprising a measurement apparatus for measuring a herald in a quantum signal. Furthermore, the quantum network 10 comprises a digital logic unit for calculating the measurement result.

[0092] The qubit of the quantum node 100i-100e is at least a communication qubit, but may also further comprise a memory qubit.

[0093] A key aspect of the method is that the signal sent from the distant quantum nodes 100i-100e to the midpoint 150 is a hybrid signal, which comprises a quantum signal part and a classical signal part. The quantum signal part contains information of the quantum state of the quantum node 100i-100e, and the classical signal part contains information of the state-validation of said quantum state.

[0094] In prior art method, the latter piece of information is kept by quantum nodes 100i-100e, such that every quantum node needs to obtain quantum state information from any other node it is intended to entangle with, whereafter logic and computation operations need to be performed on its own quantum state information and the quantum state information of the other node 100i-100e. The results are subsequently communicated from one node to the other and vice versa at the same time. One can thus say that the signals in prior art methods, therefore, need to travel the distance between the quantum nodes 100i - 100e four times (twice for every quantum node 100i- 1006).

[0095] The method of the disclosure utilizes a midpoint node 150, which is located in between the distant quantum nodes 100i -100e, which receives both the quantum state information as well as the state-validation information. Therefore, the to-be- communicated signal has become a hybrid signal. This way, the hybrid signal only needs to travel back-and-forth once from the quantum node 100i-100e to the midpoint and back. This reduces the latency of the communication tremendously, such that the technical requirements of the quantum nodes no longer need to be strained as much as in prior art methods, where a greater chance of decoherence of the quantum node 100i-100e is present.

[0096] In Fig. 2 the timeline of the communication between two distant quantum nodes 100i- IOO2 is shown, wherein specifically the quantum signal part is shown. Herein, it is shown that the midpoint 150 is positioned in between the two distant quantum nodes IOO1-IOO2, and does not necessarily need to be exactly in the center of the two. Furthermore, an allowed wall clock time 200 is shown having allowed wall clock pulses 200i, 2OO2, and 2OO3. This allowed wall clock time 200 is used by the two distant quantum nodes IOO1-IOO2 to cue that an entanglement attempt can be initiated. The allowed wall clock time is a timing run on the internal clocks of the midpoint 150 and the quantum nodes IOO1-IOO2, indicating when an entanglement attempt can be initiated.

[0097] Not shown is an additional heartbeat signal that may be sent by the midpoint 150 towards the two quantum nodes IOO1-IOO2, aligned to the wall clock time of the midpoint solely. This way, the two quantum nodes IOO1-IOO2 may adjust their internal clocks based on the obtained heartbeat signal. This heartbeat signal ensures that the midpoint 150 can act as a master, dictating the time in the quantum network 10, and that the distant quantum nodes IOO1-IOO2 act as slaves. Only this way, effective communication can be established without cluttering the communication channels between the midpoint and the quantum nodes IOO1-IOO2 with signals of which the origin is unclear, making sure that all clocks in the quantum network 10 are aligned.

[0098] Upon receival of the first wall clock pulse 200i from the midpoint by the quantum nodes IOO1-IOO2, the quantum nodes IOO1-IOO2 are cued to initiate an entanglement attempt. This process may be timed with a delay at every individual quantum node IOO1-IOO2 based on a difference between the centralized wall clock pulse 200i and its own timing and / or based on information of the time needed to transfer a response signal back to the midpoint. This time is indicated in the figure with 311 i and 312i, respectively. This is to ensure that said response signal from both quantum nodes IOO1-IOO2 arrives within a predefined heralding time window 302i at the midpoint 150. It should be noted at this point that the process of entanglement attempt initiation occurs probabilistically due to the implementation at the quantum node 100i- IOO2. Furthermore, an entanglement attempt is understood in the art as the generation of a quantum state and the emission of a signal at both quantum nodes, such that the two quantum states may become entangled following an interference of the two quantum signals.

[0099] The emission of the response signal (quantum signal part of the hybrid signal) is shown to occur at time 3011 and 302i, respectively, and is also known as the trigger.

[0100] A mathematic expression is as foWows. trig ger = allowed wall clock pulse + heatbeat — travel time

[0101] With this implementation at least the quantum signal part arrived within a predefined heralding time window 302i and preferably concurrently, which gives the midpoint the first piece of information on whether the entanglement attempt was successful.

[0102] Not shown in the figure, but in a similar fashion, but with less strict time tolerance, the classical signal part of the hybrid signal is sent from the distant quantum node IOO1-IOO2 upon entanglement attempt initiation towards the midpoint 150. This classical signal part does not need to arrive concurrently and neither needs to arrive within the heralding time window 302i of the quantum signal part. This releases some strain on the quantum network 10, yet still allows for transfer of the state-validation information of the entanglement attempt at the quantum node IOO1-IOO2 to the midpoint.

[0103] With this second piece of information, the midpoint can check whether both the timing and the quantum states were correct in order to validate that the two quantum nodes IOO1-IOO2 have been entangled. A message from the midpoint 150 is subsequently sent to the quantum nodes IOO1-IOO2 comprising information on the entanglement attempt, such that the distant quantum nodes100i-1002 may or may not proceed with quantum computations, quantum operations, or other communication protocols. This way, the two quantum nodes IOO1-IOO2 only communicate with the midpoint 150, and there is not inter-node communication, thereby reducing the latency in the communication and removing the need for post-processing at the quantum nodes IOO1-IOO2. The latter makes both the technical aspects of the quantum nodes 100 and the quantum network 10 easier compared to prior art protocols, and allows for nodes to be independently added or removed to the network 10, where the hardware / control to realize the entanglement are not shared between the nodes 100.

[0104] Further allowed emission times 2OO2-2OO3 are shown in Fig. 1 , indicating that this process occurs repeatedly until a successful entanglement has been established. The process of entanglement attempt initiation, quantum signal emission 3012- 3022 by the quantum nodes IOO1-IOO2, and receival 3022-3023 by the midpoint 150 are thus also shown to be repeated as time progresses.

[0105] It may thus be understood from these Figs, that the midpoint may require a master reference clock. This clock is known as the central clock. The controller of the midpoint has access to the central clock and any time-based actions taken by the midpoint are based on this clock. This is an efficient way to ensure that all quantum nodes run at a same timing.

[0106] Alternatively, the midpoint sends a timing signal, derived from the central clock to each node independently. This is the distributed clock. With a distributed clock, each quantum node has its own clock running as well, adjustment and corrections may need to be performed every once in a while to ensure all clocks operate at the same time. These corrections could be performed with algorithms, such as the Berkeley or Cristian algorithm. The advantage is, is that there is no single point of failure,

[0107] In Fig. 2 the quantum signal parts are shown to arrive within a predefined heralding window, and in the figure this is exemplified with concurrent arrival. This particular configuration is useful when the midpoint 150 is not equipped with a quantum memory, because in that case the quantum signal part needs to be measured and processed directly upon receival.

[0108] However, in case the midpoint 150 does comprise a quantum memory, more efficient resource allocation may be possible, since upon receival of a signal from one quantum node 100i, which now can be stored, it may temporarily close said connection and focus on signals coming from another quantum node IOO2. In that particular case, the quantum signal parts may even purposely be emitted alternatingly. Thus, the method according to the disclosure can both function with midpoints 150 either having or not having a quantum memory.

[0109] In Fig. 3 the steps according to the disclosure are both shown on the midpoint 150 and on the quantum node 100 side, wherein arrows are utilized to indicate the progress of the communication scheme. First optionally, the midpoint 150 may send a centralized clock signal to indicate to the quantum node 100 to start an entanglement attempt. Upon receiving that clock signal, the quantum node 100 may delay or immediately start a sequence of operations to initiate an entanglement generation attempt.

[0110] After the entanglement attempt generation a hybrid signal, comprising at least one quantum signal part containing information on the quantum state of the quantum node 100, and a classical signal part containing information on the state-validation, is sent to the midpoint 150, which are being received by the midpoint, respectively.

[0111] Subsequently, the midpoint 150 measures the quantum signal part of the hybrid message and optionally prior to the measurement stores it in its quantum memory, to obtain information on the quantum state of the distant quantum node 100.

[0112] With the information of the measurement and the state-validation information of the classical signal part, the midpoint 150 can validate the entanglement attempt with another distant quantum node(s) 100. This validation is performed by logic and / or computation operations on both pieces of information.

[0113] After determining whether the entanglement was successful or not, the midpoint sends a validation of the entanglement attempt message to the respective distant quantum nodes 100, which in turn receives said validation of the entanglement attempt message. This message contains information on whether the entanglement has been successful or not and allows the distant quantum node 100 to determine whether or not to continue with the quantum operation related to the entanglement.

[0114] With the method according to the disclosure, effective communication to validate entanglement between at least two distant quantum nodes 100 can be established. Herein, there is no need for inter-node communication, since all communication is performed through the midpoint 150. This ensures that less latency is present and therefore less strain is put on the technical requirements of the quantum nodes 100. Key herein is that the message sent from the quantum node 100 to the midpoint is a hybrid signal and comprises a quantum signal part, containing information on the quantum state of the respective quantum node 100, and a classical signal part, containing information on state-validation, which two parts are conjointly used by the midpoint to determine the validity of the entanglement attempt. REFERENCE NUMBERS

[0115] 10 quantum network

[0116] 100 quantum node

[0117] 100i-1002-1 oo3- first, second, third, etc. quantum node

[0118] 150 midpoint

[0119] 200 allowed wall clock time 200

[0120] 2OO1-2OO2-2OO3- first, second, third, etc. allowed wall clock pulses

[0121] 301 emission of the quantum signal part of the hybrid signal from the first quantum node

[0122] 3011-3012-3013- first, second, third, etc. emission of the quantum signal part of the hybrid signal from the first quantum node

[0123] 302 receival of quantum signal part of the hybrid signal from the second quantum node

[0124] 302I-3022-3023- first, second, third, etc. receival of quantum signal part of the hybrid signal from the second quantum node

[0125] 303 receival of quantum signal part of the hybrid signal by the midpoint

[0126] 303I-3032-3033- first, second, third, etc. receival of quantum signal part by the midpoint

[0127] 305 heralding time window

[0128] 3111-3112-3113- first, second, third, etc. transfer time of first quantum node to the midpoint

[0129] 312I-3122-3123- first, second, third, etc. transfer time of second quantum node to the midpoint

Claims

CLAIMS1. A method for real-time state-validation of entanglement between at least two independent distant quantum nodes in an extendible quantum network, each quantum node comprises at least a communication qubit, the quantum network comprises a midpoint, comprising a measurement apparatus for measuring a quantum signal, and a digital logic unit for calculating the measurement result, the method comprising the steps of:1) receiving, by the midpoint, from each of the at least two independent distant quantum nodes, a hybrid signal comprising at least one quantum signal and a classical signal, wherein the classical signal comprises state-validation information of said at least one quantum signal;2) measuring, by the measurement apparatus at the midpoint, each of the at least one quantum signal of each independent distant quantum node to herald entanglement between two of the at least two independent distant quantum nodes;3) validating, by the midpoint, the entanglement between the at least two independent distant quantum nodes based on the outcome of the measurement in step 2 and on the state-validation information of the classical signal;4) sending, by the midpoint, to each of the at least two independent distant quantum nodes, a classical message about the validation of the entanglement attempt based on the information obtained about the entanglement in step 3.

2. The method according to claim 1 , wherein the quantum signal part of the hybrid signal from each of the at least two independent distant quantum nodes according to step 1) is received within a predefined heralding time window at the midpoint.

3. The method according to any of the preceding claims, wherein receiving the classical signal part of the hybrid signal from each of the at least two independent distant quantum nodes by the midpoint according to step 1) is limited to a predefined time period.

4. The method according to any of the preceding claims, wherein the midpoint comprises a quantum memory.

5. The method according to any of the preceding claims, wherein the midpoint further comprises a clock signal generator to distribute a centralized timing signal in the quantum network.

6. The method according to any of the preceding claims, wherein measuring, by the measurement apparatus at the midpoint, of step 2) is performed through a projective measurement of the herald in the quantum signal, preferably a Bell-state measurement.

7. The method according to any of the preceding claims, wherein validating the entanglement between two of the at least two independent distant quantum nodes of step 3) is performed through logic operations and computation on the output of the measurements of the measurement apparatus combined with the state-validation information in the classical signal of the hybrid signal.

8. The method according to any of the preceding claims, further comprising the step of:- correcting, by the midpoint, the at least one quantum signal in real-time based on the classical signal,9. The method according to any of the preceding claims, further comprising the step of:- obtaining, by the midpoint, information in real-time on which of the at least two independent distant quantum nodes in the quantum network to entangle.

10. The method according to any of the preceding claims, wherein the midpoint is co-located with one of the at least two independent distant quantum nodes.

11. A method for real-time state-validation of entanglement between a midpoint and at least two independent distant quantum nodes in an extendible quantum network:i) sending, by one of the at least two independent distant quantum nodes, to the midpoint, a hybrid signal comprising a quantum signal and a classical signal, wherein the classical signal comprises state-validation information of said quantum signal; ii) receiving, by one of the at least two independent distant quantum nodes, to the midpoint, a classical message about a validation of the entanglement attempt based on the state-validation measurement; iii) determining, based on the information in the classical message about the validation of the entanglement attempt whether or not to continue quantum operations with said quantum node.

12. The method according to claim 11 , wherein the sending of the hybrid signal by one of the at least two independent distant quantum nodes of step i) is performed probabilistically.

13. The method according to claims 11-12, wherein the sending of the hybrid signal by one of the at least two independent distant quantum nodes of step i) is performed based on a centralized timing signal.

14. The method according to claims 11-13, wherein each of the at least two quantum nodes operates independently without inter-node communication.

15. The method according to claims 11-14, wherein the classical message about the validation of the entanglement attempt comprises information of an outcome of the measurement at the midpoint, information of the attempt validation, and / or an indicator to wait an indicated time period before engaging in subsequent entanglement generation.

16. The method according to claims 11-15, wherein the state validation information comprises timing information, a charge and a resonance check status, a phonon side band measurement information, an attempt identifier including a quantum node identifier, an indicator of whether a state is stored in the quantum memory of said quantum node, and / or frequency information.

17. The method according to claims 11-16, wherein the quantum signal comprises at least one photon.

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