Method for controlling service demand of quantum entanglement generation in quantum network TECHNICAL FIELD The present disclosure generally relates to controlling service allocation. Particular embodiments relate to methods and network nodes for controlling service demand, and to a related computer program and computer-readable storage medium. BACKGROUND Classical communication networks such as the internet or the telephone network employ control methods to coordinate connections and data transfers in the network. In the domain of such networks, control methods dictate the rate at which data can be sent over the network by different application classes, depending on their requirements and priority levels. In classical networks, components called routers typically have switch capabilities and can direct traffic from different sources along various links depending on network conditions and the target destination of the traffic. By directing traffic in such a manner, routers temporarily allocate specific fractions of the bandwidth along pathways of links in the network to source-destination flows of information. Different flows have different bandwidth requirements, and the routers employ control methods to calculate allocations which maximize the utility of the network. In general, all major networks utilize the same control methods to ensure interoperability of smaller sub-networks with larger networks as well as to enforce a universal set of requirements for allowing devices to connect to the network. The state of the art of control of quantum networks primarily consists of two types of preliminaries: literature introducing a framework for developing a network control plane and in complement a small-scale experimental demonstration of quantum device control via a network controller. In literature, several articles have addressed the development of a quantum network stack. In the network stack framework, the two most relevant concepts are that of the physical and link layers. The physical layer of a network encompasses the quantum devices, associated electronics, and physical connections between devices. The role of the link layer is to coordinate the robust delivery of entanglement between nodes. A state-of-the-art demonstration consisted of fully automated control from the link layer directing the physical layer of a two-node quantum network. In this proof of principle demonstration quantum network applications were executed using fully programmed command routines. The meaning of the proof of principle demonstration is that the link layer has been shown, in a simple quantum network, to be an effective control plane. SUMMARY The inventors have identified that there nevertheless still remain concerns with reliability of the above-described proof of principle, and that the proof of principle is too restricted by applying only to two users. Thus, it is an aim for at least some embodiments according to the present disclosure to improve reliability of service delivery of entanglement between nodes. It is another aim for at least some embodiments according to the present disclosure to support larger numbers of users. Accordingly, there is provided in a first aspect of the present disclosure a computer- implemented method for controlling service demand in a network; the network comprising a plurality of network nodes configured to request a service of quantum entanglement generation, a plurality of resource nodes configured to deliver a service of quantum entanglement generation, a switch coupling the plurality of network nodes with the plurality of resource nodes, and a controller configured for controlling the switch; the method comprising, at the controller: - obtaining from at least a subset of the plurality of network nodes a plurality of service requests for quantum entanglement generation; - determining a central price for the service, based on the obtained plurality of service requests, based on a service capacity of the plurality of resource nodes and based on a plurality of previous session rates, taking into account a maximum rate of the switch; and - communicating the central price to at least the subset of the plurality of network nodes. In this way, the service requests may be directly controlled, by establishing a system to alter these service requests, e.g. requested rates. It may be assumed that the switch is configured to create a schedule based on received service requests, wherein the schedule is configured to control resource/service allocation. The rate control protocol operated in part by the above-described method can be thought of as controlling how the network nodes interact with the switch. For the network nodes it is a protocol dictating how to request service from the network. For the network it is a protocol for collaborating with the network nodes to ensure the possibility of providing stable service which approximately achieves a target quality of service metric. The method moreover allows taking into account an individual benefit each network node of the plurality of network nodes obtains from the allocated resource scheduling; and taking into account a collective benefit the plurality of network nodes obtains from the allocated resource scheduling. In a particular embodiment, the method further comprises, preferably at the controller or at the switch: - determining an allocation schedule for allocating the plurality of resource nodes to the plurality of network nodes, based on a plurality of session rates obtained from the plurality of network nodes. It is noted as an example that the rate control method may dictate how communication sessions should fix their rates. At each time slot, every communication session may be arranged to communicate a message to the switch, which message includes an updated rate as well as a number of single entanglement requests (requests for a single entangled pair) to be submitted to the controller. The number of single entanglement requests submitted may be a sample of a Bernoulli random variable with expectation value equal to the request rate of session s. The controller may be configured to store only the most recently declared rate request value but may be configured to maintain queues of the single entanglement requests. A separate queue of requests may be dedicated to each communication session s. The queues of requests may form the basis for calculating a schedule. As a scheduling method, Maximum Weight Scheduling may be used. Whenever there is sufficient demand, the switch may schedule the H sessions with the longest queues. Fewer than H sessions may be scheduled if there are insufficient requests queued. In a particular embodiment, an initial iteration of the central price is set at zero and an initial iteration of the previous session rates is set at a maximum possible rate for each corresponding session, ^ ^{^} ^ _^ ^{^} ^ ^{^} ^ _^^,^ . In a particular embodiment, the service capacity of the plurality of resource nodes is based on the number of resource nodes and is based on a plurality of respective probabilities of the plurality of resource nodes to generate quantum entanglement. In a particular embodiment, the plurality of service requests comprises a plurality of requests for desired delivery of a specified average rate of quantum entanglement generation from the plurality of resource nodes. Also, there is provided in a second aspect of the present disclosure a computer- implemented method for controlling service demand in a network; the network comprising a plurality of network nodes configured to request a service of quantum entanglement generation, a plurality of resource nodes configured to deliver a service of quantum entanglement generation, a switch coupling the plurality of network nodes with the plurality of resource nodes, and a controller configured for controlling the switch; wherein a subset or a totality of the plurality of nodes is involved in a plurality of sessions, wherein each session of the plurality of sessions is associated with a network node designated as a master network node to communicate with the controller and is further associated with another network node designated as a slave network node to refrain from communication with the controller; the method comprising, at a master network node of the plurality of network nodes, the master network node being associated with at least one session of the plurality of sessions: - obtaining from the controller a central price; and - for each session of the at least one session, receiving a previous node price from the respective slave network node associated with said session; - determining a new node price, based on a backlog of at least one previous transmitted service request for quantum entanglement generation from each session of the at least one session, and based on at least one most recent rate request from each session of the at least one session, taking into account a maximally supported rate for the network node; - for each session of the at least one session, determining a session rate, based on the central price and at least one previous node price, taking into account a minimum session rate value, a maximum session rate value, and a utility function encoding a target metric and a fairness metric; and - for each session of the at least one session, sending the session rate to the controller. It will be appreciated that considerations and advantages applying to any above- described embodiment may analogously apply to this embodiment, mutatis mutandis. This advantageously both distributes and reduces the computational efforts, as neither the central controller, nor each and every network node, but rather the central controller and a select group of network nodes is tasked with determining session rate, user price, and central price. It is also advantageous that the controller does not need to know the particular utility functions chosen by the network nodes – i.e. the controller does not need to have knowledge of the nodes’ target metric / notion of fairness. The rate control protocol operated in part by the above-described method can be thought of as controlling how the network nodes interact with the switch. As stated, for the network nodes it is a protocol dictating how to request service from the network. For the network it is a protocol for collaborating with the network nodes to ensure the possibility of providing stable service which approximately achieves a target quality of service metric. In a particular embodiment, the fairness metric is defined such that, if a session rate would be increased, then a sum of fractions of change of session rates would be non- positive. In a particular embodiment, an initial iteration of the session rate is set at a maximum session rate for each corresponding session, ^ ^{^} ^ _^ ^{^} ^ ^{^} ^ _^^,^ . Also, there is provided in a third aspect of the present disclosure computer- implemented method for controlling service demand in a network; the network comprising a plurality of network nodes configured to request a service of quantum entanglement generation, a plurality of resource nodes configured to deliver a service of quantum entanglement generation, a switch coupling the plurality of network nodes with the plurality of resource nodes, and a controller configured for controlling the switch; wherein a subset or a totality of the plurality of nodes is involved in a plurality of sessions, wherein each session of the plurality of sessions is associated with a network node designated as a master network node to communicate with the controller and is further associated with another network node designated as a slave network node to refrain from communication with the controller; the method comprising, at a slave network node of the plurality of network nodes, the slave network node being associated with at least one session of the plurality of sessions: - obtaining from the controller a central price; - determining a new node price, based on a backlog of at least one previous transmitted service request for quantum entanglement generation from each session of the at least one session, and based on at least one most recent rate request from each session of the at least one session, taking into account a maximally supported rate for the network node; and - for each session of the at least one session, sending the new node price to the respective master network node associated with said session. It will be appreciated that considerations and advantages applying to any above- described embodiment may analogously apply to this embodiment, mutatis mutandis. In a particular embodiment, the controller is externally connected with the plurality of quantum entanglement generation resource nodes and internally comprises only non- quantum resources. In a particular embodiment, the switch couples the plurality of network nodes with the plurality of resource nodes via a crossbar topology, wherein any resource node of the plurality of resource nodes is allocatable to any subset of network nodes of the plurality of network nodes. In a particular embodiment, the plurality of physical resources comprises a plurality of heralding stations. In an alternative embodiment, the plurality of physical resources comprises a plurality of photon sources, and the switch is configured for transmitting photons, and each network node of the plurality of network nodes comprises at least a photon detector. Also, there is provided in a fourth aspect of the present disclosure computer program comprising instructions configured for, when executed by at least one processor, performing the method of any preceding claim. It will be appreciated that considerations and advantages applying to any above- described embodiment may analogously apply to this embodiment, mutatis mutandis. Also, there is provided in a fifth aspect of the present disclosure computer-readable storage medium storing the computer program of claim 11. Also, there is provided in a sixth aspect of the present disclosure controller for controlling service demand in a network; the network comprising a plurality of network nodes configured to request a service of quantum entanglement generation, a plurality of resource nodes configured to deliver a service of quantum entanglement generation, and a switch coupling the plurality of network nodes with the plurality of resource nodes; the controller comprising: - a receiver configured for obtaining from at least a subset of the plurality of network nodes a plurality of service requests for quantum entanglement generation; - a calculation unit configured for determining a central price for the service, based on the obtained plurality of service requests, based on a service capacity of the plurality of resource nodes and based on a plurality of previous session rates, taking into account a maximum rate of the switch; and - a transmitter configured for communicating the central price to at least the subset of the plurality of network nodes. It will be appreciated that considerations and advantages applying to any above- described embodiment may analogously apply to this embodiment, mutatis mutandis. Also, there is provided in a seventh aspect of the present disclosure network node for controlling service demand in a network; the network comprising a plurality of network nodes configured to request a service of quantum entanglement generation, a plurality of resource nodes configured to deliver a service of quantum entanglement generation, a switch coupling the plurality of network nodes with the plurality of resource nodes, and a controller configured for controlling the switch; wherein a subset or a totality of the plurality of nodes is involved in a plurality of sessions, wherein each session of the plurality of sessions is associated with a network node designated as a master network node to communicate with the controller and is further associated with another network node designated as a slave network node to refrain from communication with the controller; the network node being designated as a master network node and being associated with at least one session of the plurality of sessions; the network node comprising: - a receiver configured for: - obtaining from the controller a central price; and - for each session of the at least one session, receiving a previous node price from the respective slave network node associated with said session; - a calculation unit configured for - determining a new node price, based on a backlog of at least one previous transmitted service request for quantum entanglement generation from each session of the at least one session, and based on at least one most recent rate request from each session of the at least one session, taking into account a maximally supported rate for the network node; and - for each session of the at least one session, determining a session rate, based on the central price and at least one previous node price, taking into account a minimum session rate value, a maximum session rate value, and a utility function encoding a target metric and a fairness metric; and - a transmitter configured for, for each session of the at least one session, sending the session rate to the controller. It will be appreciated that considerations and advantages applying to any above- described embodiment may analogously apply to this embodiment, mutatis mutandis. For example, in a particular embodiment, the fairness metric is defined such that, if a session rate would be increased, then a sum of fractions of change of session rates would be non-positive. Also, there is provided in an eighth aspect of the present disclosure network node for controlling service demand in a network; the network comprising a plurality of network nodes configured to request a service of quantum entanglement generation, a plurality of resource nodes configured to deliver a service of quantum entanglement generation, a switch coupling the plurality of network nodes with the plurality of resource nodes, and a controller configured for controlling the switch; wherein a subset or a totality of the plurality of nodes is involved in a plurality of sessions, wherein each session of the plurality of sessions is associated with a network node designated as a master network node to communicate with the controller and is further associated with another network node designated as a slave network node to refrain from communication with the controller; the network node being designated as a slave network node and being associated with at least one session of the plurality of sessions; the network node comprising: - a receiver configured for obtaining from the controller a central price; - a calculation unit configured for determining a new node price, based on a backlog of at least one previous transmitted service request for quantum entanglement generation from each session of the at least one session, and based on at least one most recent rate request from each session of the at least one session, taking into account a maximally supported rate for the network node; and - a transmitter configured for, for each session of the at least one session, sending the new node price to the respective master network node associated with said session. It will be appreciated that considerations and advantages applying to any above- described embodiment may analogously apply to this embodiment, mutatis mutandis. BRIEF DESCRIPTION OF THE DRAWINGS The embodiments described above are merely provided for illustrative purposes and are not meant to limit the present invention, which will be more fully understood with the help of the following detailed description and the appended drawing, in which: Figure 1 schematically illustrates a specific example 100 of a controller embodiment 102 and various network node embodiments 111-117 according to the present disclosure. DETAILED DESCRIPTION Figure 1 schematically illustrates a specific example 100 of a controller embodiment 102 and various network node embodiments 111-117 according to the present disclosure. The figure shows an entanglement generation switch 101 (abbreviated as an EGS) with, in this specific example, H = 3 physical resources 106-108 connected to N = 7 network nodes 111-117. From a logical perspective, the switch 101 may comprise a control layer and a physical layer. The control layer may be hosted by a controller 102 (e.g. embodied on a processor of switch 101) which may run a control method together with the network nodes 111-117, as is represented in the figure by the processors shown schematically at the top in each network node 111-117. The physical layer may be defined by the switch fabric 103, the physical resources 106- 108, and the connections between these components. Each network node 111-117 may be connected to the switch fabric 103 by a physical channel and to the control layer by a digital channel. The control layer may pass scheduling information to the switch fabric 103 and prices for rate allocation to the network nodes 111-117. Success or failure flags from the entanglement generation attempts of network nodes 111-117 that have been allocated a resource from among the physical resources 106-108 may be passed from the control layer to the network nodes 111-117. The switch fabric 103 may comprise switching means 104-105 configured for effecting the coupling between the network nodes 111-117 and the physical resources 106-108. It will be understood that the specific number of physical resources, as well as the specific number of network nodes shown in Figure 1 is merely illustrative and that either or both may be different in other examples. In the following description, various specific embodiments will be described in detail, in order to illustrate the present invention. It is to be understood that the following description should not be interpreted to limit the present invention, which is determined by the scope of the claims. It is also to be understood that network comprising a plurality of network nodes configured to request a service of quantum entanglement generation may be termed a quantum network. With respect to the rate of the switch, this may be interpreted as simply that: the rate at which the switch operates. Alternatively, it may be interpreted as meaning the fact that timeslots of the switch have a fixed duration and hence allocation, scheduling happens at a fixed rate, that is also interpretable as the rate of the switch. Requested and delivered entanglement rates are defined with respect to the fixed time slot duration as number of pairs delivered per time slot. Various embodiments according to the present disclosure propose an architecture for the control of an important component of near-term quantum networks, which may be identified as an entanglement generation switch (EGS). In a sense, these embodiments provide a control protocol, functioning as a method which may instantiate a new class of methods describing control of such EGSs. The disclosed architecture may consider an EGS to comprise a control layer, a physical layer and an interface between the two layers. Listing 1 below gives a formalized overview of an example architecture embodying a controller and various network nodes according to the present invention, as well as showing operational steps of several example method embodiments according to the present disclosure. In terms of the quantum network stack, the control layer of an EGS may be at the link layer (see reference [1] below) level of a quantum network. At the physical level, an EGS may be considered to be a central service station in a quantum network with physical connections, such as optical fibers, to N quantum nodes, which may be equipped with one or more communication qubits. The EGS may be equipped with H sets of physical resources, which may be allocated to subsets of nodes for the purposes of entanglement generation. For the sake of convenience, the physical resources to be allocated may e.g. be considered to be heralding stations and the subsets of nodes may be considered to be node pairings. However, various disclosed embodiments may also be compatible with any other entanglement generation protocol which happens to rely on access to a physical resource. The control layer of the EGS may include both the switch processor and the set of network protocols followed by the switch and the network nodes. In an example, the function of the switch processor may be considered to be to coordinate the service that the EGS provides to the network; in other words, it may receive requests from network nodes, it may communicate service information to the nodes and it may calculate a resource allocation schedule for the switch. The set of network protocols may help to enable reliable service by the EGS and may include the disclosed rate control protocol, the disclosed scheduling method used to determine the allocation of resources, and the disclosed entanglement generation protocol employed by the switch. The rate control protocol may be a method runnable by both the network nodes and the switch processor which may incorporate real-time service information from the switch to adjust the service requests sent from the network nodes to the switch. In this manner, various embodiments according to the present disclosure may help to ensure that the switch resources may be distributed fairly amongst all the node requests and may help to optimize use of the available resources. The switch processor is preferably work-conserving, which means that all physical resources are preferably assigned at each time slot whenever there is sufficient demand for entanglement from the nodes. Pairs of nodes may demand service from the switch by requesting specific rates of entanglement delivery, where the rate may be defined as the average number of entangled pairs delivered per time slot of the switch. In the following description, the notation I _k = {1,2,⋯, k} will be used to refer to an index set of k indices and the n - th time slot of the switch will be marked by t _n . Denote the rate of entanglement requested by a pair of nodes (i, j) ∈ I _N × I _N by λ _(i,j) (t _n ) = . To each distinct pair of nodes, a unique session id s may be associated, as in the following definition. Definition 1.1 (Session id). The set of session id’s is, For each unique session id s, the two involved network nodes may agree upon which one of the nodes is designated to communicate the entanglement requests to the switch. This node may be designated as a master node to communicate with the controller, while the other node may be designated as a slave node to refrain from communicating with the controller. Listing 1 – Control Architecture for Entanglement Generation Switches Physical Layer: The entanglement generation switch may consist of the following components: 1. A set of H physical resources which can be allocated to facilitate entanglement generation; 2. Physical connections such as optical fibers between nodes and the central switch; 3. An optical switch, preferably with a crossbar topology, wherein the switch is configured to mediate the allocation of physical resources to pairs of end nodes; 4. The controller, which may also be termed a switch processor, which is a classical processor. Control Layer: The control layer may be hosted by the switch processor and may coordinate the physical layer. In conjunction with classical processors at the nodes it may: 1. Calculate the duration of time a physical resource is allocated for by the switch schedule, t _Sched ; 2. Calculate a schedule for the allocation of the physical resources to nodes; 3. Execute the rate control method for updating the rates of entanglement generation requested by each session. The rate control method: – may require two-way communication between the designated communication node of each session and the switch; – may require only one-way communication between the two nodes of a communication session, directed from the secondary node to the designated communication node; – may coordinate the rates requested by communication sessions to balance the individual benefit each session s obtains from the entanglement delivery rate λ _s (t _n ) against the social utility of the set of network rates {λ _s (t _n ) ∀ s}. Interface between Control and Physical Layers: The control method may expose the following physical properties of the EGS which may be accounted for in the control method: 1. The fidelity of entanglement requested by nodes; 2. The probability p _gen that a pair of nodes successfully generates entanglement when allocated a physical resource; 3. t _Sched , the length of a time slot of the resource allocation schedule; 4. t _Comm , the time delay associated with one way communication between the switch and the furthest linked node; Note that t _Sched may be implicitly accounted for in the control method, mostly due to the definition of the rate, which may be defined as “number of pairs received over time interval t _Sched ”. It may also be accounted for in the sense that once begun, the method is assumed to run once completely each time interval t _Sched . Note that p _gen may be accounted for in the method by parameter ^ _^^^^^^ in equation 1.11 below (^ _^^^^^^ = ^ ⋅ ^ _^^^ ) and by the parameter ^ ^{^} ^ _^ ^{^} ^ ^{^} ^ _^^,^ (= ^ ⋅ ^ _^^^ ) in equation 1.13, where ^ is the maximum number of times a session can be scheduled per time slot. In a further developed embodiment of the control method, time delay may be taken into account by considering any one or more of the following parameters in addition to the parameters above: 3. t _Sched (u, v, h), the length of a time slot allocating resource h to node pair (u, v), instead of the above t _Sched . 4. tComm(u), the time delay associated with one way communication between the switch and node u, instead of the above t _Comm . 5. t _Switch (u, v, h), the time delay associated with aligning the optical switch to allocate physical resource h to node pair (u, v); 6. t _Calibrate (u, v, h), potentially periodic downtime associated with re-calibration of switch components, which may depend on node pairs (u, v) or on physical resources h. With regards to the parameter t _Calibrate , any calibration time may for the sake of expediency be thought of as absorbed into ^ _^^^^^ . Associate with each session id a random variable a _s (t _n ) which follows a Bernoulli distribution with expectation value E[a _s (t _n )] = λ _s (t _n ) (i.e. the expected value is the entanglement request rate of session s). Each time slot, the designated communication node of each session s may create a number a _s (t _n ) of individual requests for bi-partite entanglement generation by sampling a Bernoulli random number generator and may send these requests to the switch. In a preferred example, the requests may be sent with the distribution expectation value included in the message header so that the switch processor may track the values {λ _s (t _n ) ∀ s}. The switch processor may maintain a classical queue of the requests which may be separated into ∣S∣ virtual queues, each corresponding to a unique session id and considered to have an infinite buffer (of course, in a practical implementation the buffer size will be finite, but this has no relevant impact). The queues may form the basis for a maximum weight scheduling method, by which the switch may allocate use of the heralding stations to individual sessions at each time step. Definition 1.2 (Max Weight Scheduling). Let q _s represent the virtual queue length corresponding to the session s. Let x ^∗ be the maximum number of heralding stations that can be allocated to a single session per time slot. The switch control selects a schedule M(t _n+1 ) for the following time step such that, where M ^(h) is the h - th schedule of the set of all feasible schedules of size 0 ≤ ∣M ^(h) ∣ ≤ H and specifies whether session s is selected in the h - th possible schedule. Hence The switch may allocate x heralding stations in the following time step to session s if M _s (t _n +1) = x and q _s (t _n ) ≥ x, i.e., there are at least x requests queued from session s at time t _n . For various embodiments according to the present disclosure, maximum weight scheduling may reduce to allocating a heralding station to each of the H sessions with the largest queues, with random tie-breaking and the possibility of allocating more than one heralding station to a single session s if q _s (t _n ) exceeds the next largest session queue by more than 1 request and x ^∗ the maximum number of resources that can be allocated to a single session exceeds 1. When a session s is scheduled to use a heralding station during time step t _n , the nodes composing the session will attempt heralded entanglement generation during the allocated time. The probability that a scheduled session succeeds in producing heralded entanglement before the end of the time-slot can be modelled as a Bernoulli random variable with expectation value p _gen . Therefore, a session s that is allocated use of M _s (t _n ) heralding stations during time step t _n produces a number of entangled pairs g _s (t _n ) which is the realization of a binomial random variable, g _s (t _n ) ∼ Bin(M _s (t _n ), p _gen ), with expectation value E[g _s (t _n )] = pgen ⋅ Ms(tn). Requests are removed from a session queue only when heralded entanglement generation succeeds, hence g _s (t _n ) requests depart from each virtual queue q _s (t _n ) at each time step. The queue dynamics for each session are described by the following equation, where [z] ⁺ = max(z, 0). A distinction between an EGS and other possible configurations of switches in a quantum network is that an EGS has only classical and no quantum resources, meaning it has no qubits for communication or quantum memories. The role of the EGS may be considered to be to allocate the use of individual resources to subsets of end nodes. A crossbar topology is preferred for the switch, meaning that any resource can be allocated to an arbitrary subset of nodes. One way in which the crossbar topology of the switch could be physically implemented is by a low-loss optical switch. The optical switching necessary for dynamically allocating the switch resources may be considered to be an instantaneous process and the control method may be considered to not have to incorporate delay. However, various further developed embodiments may incorporate delays (possibly dependent on the particular nodes and resources to be allocated) due to the optical switching time (t _Switch (u, v, h)), finite communication time (t _Comm (u)), and down-time associated with the need for network calibration (t _Calibrate (u, v, h)) at regular intervals. Two examples of physical resources that the switch may possess are heralding stations and photon sources. The physical entanglement generation protocol must be tailored to the physical resource. For the sake of conciseness, the following description will mainly assume that the resources are heralding stations and will only briefly describe the format of protocol in the case of photon sources. However, it will of course be understood that the principles underlying the present disclosure are of course not necessarily limited to heralding stations and/or photon sources. A heralding station may comprise of two input channels connected to a 50/50 beam splitter, which is then connected by two output channels to a pair of photon detectors. Once a station is allocated to a pair of nodes, the nodes may use it to attempt a protocol called heralded bipartite entanglement generation. This is a probabilistic protocol which swaps communication qubit-photon entanglement of the two participating nodes by performing a Bell state measurement (BSM). The result is the probabilistic creation of an entangled pair between two nodes, which succeeds with success probability p _gen . A requirement for the success of heralded bi-partite entanglement generation is that the photons from the two nodes arrive simultaneously at the heralding station. Hence, precise coordination of timing in the network is required. The advantage of heralded entanglement generation is that when successful, it outputs a heralding signal or flag which indicates that the protocol was a success; this classical information can be sent to notify nodes when they have created shared entanglement. In the case where the physical resources are instead photon sources the entanglement generation protocol requires the switch to send entangled photons to network nodes, which must be equipped with detector setups to detect received photons. In the present description, the EGS may preferably be equipped to generate bi-partite entanglement. It is however possible to extend the notion of an EGS equipped with only classical physical resources such as linear optics instruments in order to be compatible with the generation of multi-partite entanglement. One possibility is to consider an appropriate multi-partite version of the heralding protocol, such as in reference [2] (see below). EGSs may for example be used in metropolitan scale networks, meaning that the maximum distance of any physical link in the network does not exceed a scale of tens of kilometers. At this distance scale, all links in the network may be considered to be direct links and the use of components such as repeater stations does not have to be considered. However, it will be understood that in practical implementations, such components may prove to be necessary in certain situations. The basic interaction between nodes and an EGS is that individual pairs of nodes submit requests to the switch for the delivery of a specified average rate of entanglement generation. Specifically, consider the switch to be a slotted time system with time steps indexed as {t _n } of duration t _Sched , and the rate of entanglement delivery to be defined with respect to the average number of pairs delivered per the unit of time t _Sched . The duration of time slots is preferably fixed based on physical characteristics of the network nodes and the fidelity of entanglement requested by the network nodes. The lengths of links connecting the nodes to the switch set communication delays due to the finite speed of information transfer and should also be accounted for in the time slot duration; the time slot duration should be calculated to satisfy t _Sched > 2 ⋅ t _Comm to account for round trip communication over the longest link in the EGS network. In this analysis it may be assumed that the network nodes are homogeneous, in the sense that all communication qubits consist of the same type of physical qubit. Nodes may however have differing numbers of quantum resources such as memories or communication qubits. Furthermore, it may be assumed that all requests for entanglement are for a fixed fidelity of bi-partite entangled pair. For any specific physical platform (such as Nitrogen Vacancy centers, trapped ions, all photonic processors, see references [3, 4], [5, 6], [7, 8] below) for communication qubits, fixing the request fidelity level can be used to determine a suitable length for the switch time- slots. When the pair of nodes corresponding to communication session s requests an entanglement delivery rate of ^ _^ , they are requesting that an average of λ _s entangled pairs be delivered per time slot. The EGS may use a scheduling method to allocate the use of heralding stations to pairs of end nodes. Associated with the switch is a set of maximum service rates, known as the capacity region of the switch (i.e. the service capacity of the switch). When the set of rates requested by all pairs of end nodes lies within the capacity region of the switch, there exists a scheduling method under which the time average of the entanglement delivery rate realized for each pair of nodes matches the request rate. The rate control protocol may be considered to be a distributed method for solving a Network Utility Maximization (NUM) problem on an EGS. NUM is a framework for deriving the optimal service rates in a network with respect to a specific target performance metric and a notion of fairness. Here, the NUM problem is at the communication session level, as opposed to at the user level. An example performance metric and associated notion of fairness are throughput and proportional fairness. Throughput in the context of an EGS is the total rate of entanglement delivery; it is equal to the sum of the rates delivered to each communication session. The term proportional fairness refers to a structure of the optimization problem such that if one of the session rates is increased by a certain amount, then the sum of the fractions by which the different users’ rates change is non-positive (see reference [9] below). In this framework, a quantity called the utility is associated with each communication session, s. The utility is a strictly concave function f _s (λ _s ) which quantifies the benefit to communication session s of receiving rate allocation λ _s . Strict concavity of the utility functions enforces that the utility of each source is non-decreasing, but the amount of utility to be gained by increasing a large rate is less than the utility to be gained by increasing a smaller rate. The utility of each communication session is balanced against the cost to the network of allocating rate λ _s to session s. The optimization problem is to maximize the sum utility of the communication sessions, subject to the constraints of the network costs. The choice of utility functions may preferably encode both the target metric and the notion of fairness. For example, by using a (weighted) log utility function, f _s (λ _s ) = w _s log(λ _s ), throughput as a target metric and proportional fairness are encoded into the optimization problem. A formal presentation of the problem with abstract utility functions is as follows. Define S(u) ∶= {s ∶ u ∈ s} to be the set of sessions in which node u participates. The complimentary set U(s) ∶= {u ∶ u ∈ s} defines the pair of nodes that constitute session s. Define the feasible rate region of session s as follows, where λ ^min s ≥ 0 and λ ^max H _erald,s = x ⋅ p _gen , where x ∈ [1, H] is the maximum number of times a single session can be scheduled per time slot. The feasible region for all rate vectors λ is then given by, We derive a method which solves the following problem: The objective functions of interest, f _s , in equation 1.6 are strictly concave. To ensure feasibility and satisfy the Slater constraint qualification (see reference [10] below), it is necessary that the rate vector with components equal to the minimal rates of each session is an interior point of the constraint set, The full set of constraints considered is highly adaptable. The role of two of the constraints is to enforce that the rate allocation remains within the capacity region of the switch. The role of the other two constraints is to incorporate physically motivated restrictions on the capabilities of the end nodes. Taken together, constraint 1.7 and the upper bound on each session rate in equation 1.4 encode the capacity region of the switch. The lower bound on each session rate in equation 1.4 encodes that each communication session specifies a minimum usable rate, which the session requires in order for the received entanglement to be useful for the targeted application. The final constraint, equation 1.8, is an upper bound on the maximum rate that can be requested by any one node, across all of its communication sessions. This constraint describes a situation in which the network nodes have restricted quantum resources. In such a situation, each node can only support some maximum value of entanglement generation across all of its communication sessions. Examples of ways in which the quantum resources at a network node may be limited include a finite number of communication qubits, a finite number of quantum memories, or a transfer time from communication qubit to quantum memory that is slow compared to the switch timescale. An example of the rate control protocol for an EGS is presented below in Listing 2. The protocol controls the rate requested by each communication session and is made up of two sub methods: a method run by the central switch and a method run by each node in the network. Listing 2 – Rate Control Protocol Central Switch’s Method: At times tn = t1, t2, ⋯, the central switch: 1. receives rates λ _s (t _n ) from all communication sessions s ∈ S; 2. computes a new centralized price, 3. broadcasts the new price p _c (t _n+1 ) to all communication sessions s ∈ S. Node u’s Method: At times t _n = t ₁ , t ₂ , ⋯, node u: 1. receives from the switch the centralized price p _c (t _n ); 2. marks the subset of communication sessions COMM(u) ⊆ S(u) which involve node u for which it is the designated communication node of the session; 3. receives from every partner node u′ the price p _u′ (t _n ) for each session s = (u, u′) ∈ COMM(u) 4. computes a new node price, 5. communicates the new price ^ _^ (^ _^^^ ) to the partner node from every session s ∈ S(u) \COMM(u) in which u is not the designated communication node; 6. computes the new session rate for every session s ∈ COMM(u), where [z] ^M m = max ( min(z, M), m) and p(t _i ) = (p _c (t _i ), p _u (t _i ) ∀ u) is the vector of prices pertaining to time-slot t _i ; 7. communicates the new session rate λ _s (t _n+1 ) to the central switch, for every session s ∈ COMM(u). REFERENCES [1] Axel Dahlberg et al. “A link layer protocol for quantum networks”. 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