QuNetSim

https://tqsd.github.io/QuNetSim/
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### Full W State Example

Source: https://tqsd.github.io/QuNetSim/_sources/examples/send_w.rst.txt

A complete implementation showing the setup of the EQSN backend, network initialization, and host configuration.

```python
from qunetsim.backends import EQSNBackend
from qunetsim.components import Host
from qunetsim.components import Network
from qunetsim.objects import Logger

Logger.DISABLED = False


def main():
    network = Network.get_instance()
    nodes = ["Alice", "Bob", "Eve", "Dean"]
    back = EQSNBackend()
    network.start(nodes, back)

    network.delay = 0.1

    host_alice = Host('Alice', back)
    host_alice.add_connection('Bob')
    host_alice.add_connection('Eve')
    host_alice.start()

    host_bob = Host('Bob', back)
    host_bob.add_connection('Alice')
    host_bob.add_connection('Eve')
    host_bob.start()

    host_eve = Host('Eve', back)
    host_eve.add_connection('Bob')
    host_eve.add_connection('Dean')
    host_eve.add_connection('Alice')
    host_eve.start()
```

--------------------------------

### Full example: network setup and EPR exchange

Source: https://tqsd.github.io/QuNetSim/examples/send_epr.html

Complete example that sets up a three-node network (A, B, C), connects hosts, and runs the sender and receiver protocols for EPR pair exchange. Disables logging and starts the network before running the protocols.

```python
from qunetsim.components import Host
from qunetsim.components import Network
from qunetsim.objects import Logger

Logger.DISABLED = True


def protocol_1(host, receiver):
    """
    Sender protocol for sending 5 EPR pairs.

    Args:
        host (Host): The sender Host.
        receiver (str): The ID of the receiver of the EPR pairs.
    """
    # Here we write the protocol code for a host.
    for i in range(5):
        print('Sending EPR pair %d' % (i + 1))
        epr_id, ack_arrived = host.send_epr(receiver, await_ack=True)

        if ack_arrived:
            # Receiver got the EPR pair and ACK came back
            # safe to use the EPR pair.
            q = host.get_epr(receiver, q_id=epr_id)
            print('Host 1 measured: %d' % q.measure())
        else:
            print('The EPR pair was not properly established')
    print('Sender protocol done')


def protocol_2(host, sender):
    """
    Receiver protocol for receiving 5 EPR pairs.

    Args:
        host (Host): The sender Host.
        sender (str): The ID of the sender of the EPR pairs.
    """

    # Host 2 waits 5 seconds for the EPR to arrive.
    for _ in range(5):
        q = host.get_epr(sender, wait=5)
        # q is None if the wait time expired.
        if q is not None:
            print('Host 2 measured: %d' % q.measure())
        else:
            print('Host 2 did not receive an EPR pair')
    print('Receiver protocol done')


def main():
    network = Network.get_instance()
    nodes = ['A', 'B', 'C']
    network.start(nodes)

    host_A = Host('A')
    host_A.add_connection('B')
    host_A.start()

    host_B = Host('B')
    host_B.add_connection('A')
    host_B.add_connection('C')
    host_B.start()

    host_C = Host('C')
    host_C.add_connection('B')
    host_C.start()

    network.add_host(host_A)
    network.add_host(host_B)
    network.add_host(host_C)

    host_A.run_protocol(protocol_1, (host_C.host_id,))
    host_C.run_protocol(protocol_2, (host_A.host_id,))


if __name__ == '__main__':
    main()
```

--------------------------------

### Full example: send EPR pairs

Source: https://tqsd.github.io/QuNetSim/_sources/examples/send_epr.rst.txt

Complete example combining network setup, protocol definitions, and execution. Imports from qunetsim, disables logging, and runs the protocols.

```python
from qunetsim.components import Host
from qunetsim.components import Network
from qunetsim.objects import Logger

Logger.DISABLED = True


def protocol_1(host, receiver):
    """
    Sender protocol for sending 5 EPR pairs.

    Args:
        host (Host): The sender Host.
        receiver (str): The ID of the receiver of the EPR pairs.
    """
    # Here we write the protocol code for a host.
    for i in range(5):
        print('Sending EPR pair %d' % (i + 1))
        epr_id, ack_arrived = host.send_epr(receiver, await_ack=True)

        if ack_arrived:
            # Receiver got the EPR pair and ACK came back
            # safe to use the EPR pair.
            q = host.get_epr(receiver, q_id=epr_id)
            print('Host 1 measured: %d' % q.measure())
        else:
            print('The EPR pair was not properly established')
    print('Sender protocol done')


def protocol_2(host, sender):
    """
    Receiver protocol for receiving 5 EPR pairs.

    Args:
        host (Host): The sender Host.
        sender (str): The ID of the sender of the EPR pairs.
    """

    # Host 2 waits 5 seconds for the EPR to arrive.
    for _ in range(5):
        q = host.get_epr(sender, wait=5)
        # q is None if the wait time expired.
        if q is not None:
            print('Host 2 measured: %d' % q.measure())
        else:
            print('Host 2 did not receive an EPR pair')
    print('Receiver protocol done')


def main():
    network = Network.get_instance()
    nodes = ['A', 'B', 'C']
    network.start(nodes)

    host_A = Host('A')
    host_A.add_connection('B')
    host_A.start()

    host_B = Host('B')
    host_B.add_connection('A')
    host_B.add_connection('C')
    host_B.start()

    host_C = Host('C')
    host_C.add_connection('B')
    host_C.start()

    network.add_host(host_A)
    network.add_host(host_B)
    network.add_host(host_C)

    host_A.run_protocol(protocol_1, (host_C.host_id,))
    host_C.run_protocol(protocol_2, (host_A.host_id,))
```

--------------------------------

### B92 QKD full example: imports and network setup

Source: https://tqsd.github.io/QuNetSim/_sources/examples/QKD_B92.rst.txt

Full B92 QKD example using QuNetSim. It imports the required components, disables logging, sets a wait time, defines Eve's sniffing function, and starts building the network with hosts Alice, Bob, and Eve.

```python
# This example implements the B92 QKD protocol using the QuNetSim package

from qunetsim.components import Host, Network
from qunetsim.objects import Qubit
from qunetsim.objects import Logger
from random import randint, random

Logger.DISABLED = True
wait_time = 60


def eve_sniffing_quantum(sender, receiver, qubit):
    # Eve will manipulate only part of the qubits she intercepts
    # She chooses the base in which she measures at random.
    if sender == 'Alice':
        r = random()
        if r > 0.5:
            base = randint(0, 1)
            if base == 1:
                qubit.H()
            qubit.measure(non_destructive=True)


def build_network_b92(eve_interception):

    network = Network.get_instance()

    nodes = ['Alice', 'Bob', 'Eve']
    network.start(nodes)

    host_alice = Host('Alice')
    host_bob = Host('Bob')
    host_eve = Host('Eve')
```

--------------------------------

### Set up network hosts

Source: https://tqsd.github.io/QuNetSim/examples/QKD_BB84.html

Adds three hosts to the network to set up the topology Alice <--> Bob <--> Eve. This is the initial setup for the QKD example.

```python
network.add_host(host_alice)
network.add_host(host_bob)
network.add_host(host_eve)
```

--------------------------------

### start()

Source: https://tqsd.github.io/QuNetSim/components/host.html

Starts the host.

```APIDOC
## start()

### Description
Starts the host.

### Method
start

### Parameters
None

### Response
No return value.
```

--------------------------------

### start(nodes=None, backend=None)

Source: https://tqsd.github.io/QuNetSim/components/network.html

Starts the network.

```APIDOC
## start(nodes=None, backend=None)

### Description
Starts the network.

### Method
start

### Parameters
- **nodes** (optional) - Not documented in source.
- **backend** (optional) - Not documented in source.

```

--------------------------------

### Install QuNetSim via pip

Source: https://tqsd.github.io/QuNetSim/install.html

Install the package using pip and use the template script to generate an example.

```bash
pip install qunetsim
```

```bash
template
```

--------------------------------

### Import libraries and configure QKD example

Source: https://tqsd.github.io/QuNetSim/examples/QKD_BB84.html

Imports the required libraries and sets up the QKD example: imports `numpy`, `random`, and the `qunetsim` components/objects, disables the logger, and sets `wait_time = 10`.

```python
import numpy as np
import random

from qunetsim.components import Host
from qunetsim.components import Network
from qunetsim.objects import Qubit
from qunetsim.objects import Logger

Logger.DISABLED = True

wait_time = 10
```

--------------------------------

### Full Linear Protocol Example

Source: https://tqsd.github.io/QuNetSim/examples/send_data.html

A complete implementation of the linear protocol style, including network setup, host configuration, and the qubit transmission loop.

```python
from qunetsim.components import Host
from qunetsim.components import Network
from qunetsim.objects import Qubit


def main():
    network = Network.get_instance()
    nodes = ["Alice", "Bob", "Eve", "Dean"]
    network.start(nodes)
    network.delay = 0.1

    host_alice = Host('Alice')
    host_alice.add_connection('Bob')
    host_alice.start()

    host_bob = Host('Bob')
    host_bob.add_connection('Alice')
    host_bob.add_connection('Eve')
    host_bob.start()

    host_eve = Host('Eve')
    host_eve.add_connection('Bob')
    host_eve.add_connection('Dean')
    host_eve.start()

    host_dean = Host('Dean')
    host_dean.add_connection('Eve')
    host_dean.start()

    network.add_host(host_alice)
    network.add_host(host_bob)
    network.add_host(host_eve)
    network.add_host(host_dean)

    for _ in range(10):
        # Create a qubit owned by Alice
        q = Qubit(host_alice)
        # Put the qubit in the excited state
        q.X()
        # Send the qubit and await an ACK from Dean
        q_id, _ = host_alice.send_qubit('Dean', q, await_ack=True)

        # Get the qubit on Dean's side from Alice
        q_rec = host_dean.get_qubit('Alice', q_id)

        # Ensure the qubit arrived and then measure and print the results.
        if q_rec is not None:
            m = q_rec.measure()
            print("Results of the measurements for q_id are ", str(m))
        else:
            print('q_rec is none')

    # Stop the network at the end of the example
    network.stop(stop_hosts=True)

if __name__ == '__main__':
    main()
```

--------------------------------

### Install QuNetSim on Windows

Source: https://tqsd.github.io/QuNetSim/install.html

Commands to clone the repository, set up a virtual environment, install dependencies, and run the templater script on Windows.

```bash
git clone git@github.com:tqsd/QuNetSim.git
```

```bash
python3 -m venv venv
.\venv\Scripts\activate
python -m pip install --upgrade pip
python -m pip install -r .\QuNetSim\requirements.txt
```

```bash
python3 .\QuNetSim\templater.py
```

```bash
python3 <name of your file>.py
```

--------------------------------

### Full CHSH game example with backend setup

Source: https://tqsd.github.io/QuNetSim/examples/chsh.html

Complete CHSH game example that imports and sets up the backend (ProjectQ or EQSN) to support rotation operators. The code is displayed with line numbers in the source, but the code content is not fully visible in the provided text.

```python

```

--------------------------------

### QuNetSim Network Template Example

Source: https://tqsd.github.io/QuNetSim/_sources/quick_start.rst.txt

A generated template file demonstrating protocol definitions and network setup for a 4-node fully connected network.

```python
from qunetsim.components.host import Host
from qunetsim.components.network import Network
from qunetsim.objects import Qubit
from qunetsim.objects import Logger
Logger.DISABLED = True


def protocol_1(host, receiver):
    # Here we write the protocol code for a host.
    for i in range(5):
        q = Qubit(host)
        # Apply Hadamard gate to the qubit
        q.H()
        print('Sending qubit %d.' % (i+1))
        # Send qubit and wait for an acknowledgement
        host.send_qubit(receiver, q, await_ack=True)
        print('Qubit %d was received by %s.' % (i+1, receiver))


def protocol_2(host, sender):
    # Here we write the protocol code for another host.
    for _ in range(5):
        # Wait for a qubit from Alice for 10 seconds.
        q = host.get_qubit(sender, wait=10)
        # Measure the qubit and print the result.
        print('%s received a qubit in the %d state.' % (host.host_id, q.measure()))


def main():
   network = Network.get_instance()
   nodes = ['A', 'B', 'C', 'D']
   network.start(nodes)

   host_A = Host('A')
   host_A.add_connection('B')
   host_A.add_connection('C')
   host_A.add_connection('D')
   host_A.start()
   host_B = Host('B')
   host_B.add_connection('A')
   host_B.add_connection('C')
   host_B.add_connection('D')
   host_B.start()
   host_C = Host('C')
   host_C.add_connection('A')
   host_C.add_connection('B')
   host_C.add_connection('D')
   host_C.start()
   host_D = Host('D')
   host_D.add_connection('A')
   host_D.add_connection('B')
   host_D.add_connection('C')
   host_D.start()

   network.add_host(host_A)
   network.add_host(host_B)
   network.add_host(host_C)
   network.add_host(host_D)

   host_A.run_protocol(protocol_1, (host_D.host_id,))
   host_D.run_protocol(protocol_2, (host_A.host_id,))

if __name__ == '__main__':
   main()
```

--------------------------------

### Install QuNetSim on Linux/Mac OS X

Source: https://tqsd.github.io/QuNetSim/install.html

Commands to clone the repository, set up a virtual environment, install dependencies, and run the templater script.

```bash
git clone git@github.com:tqsd/QuNetSim.git
```

```bash
python3 -m venv venv
source ./venv/bin/activate
pip install --upgrade pip
pip install -r ./QuNetSim/requirements.txt
```

```bash
export PYTHONPATH=$PYTHONPATH:$PWD/QuNetSim/
```

```bash
python3 ./QuNetSim/templater.py
```

```bash
python3 <name of your file>.py
```

--------------------------------

### Imports for entanglement routing example

Source: https://tqsd.github.io/QuNetSim/_sources/examples/entanglement_routing.rst.txt

Import the required modules for the example: Host, Network, Logger, networkx, and time.

```python
from qunetsim.components import Host
from qunetsim.components import Network
from qunetsim.objects import Logger
import networkx
import time
```

--------------------------------

### Full Entanglement Routing Implementation

Source: https://tqsd.github.io/QuNetSim/examples/entanglement_routing.html

A complete example showing the generation of entanglement, a custom routing algorithm using networkx, and the setup of a network with multiple hosts.

```python
from qunetsim.components import Host
from qunetsim.components import Network
from qunetsim.objects import Logger
import networkx
import time


def generate_entanglement(host):
    """
    Generate entanglement if the host has nothing to process (i.e. is idle).
    """
    while True:
        if host.is_idle():
            host_connections = host.get_connections()
            for connection in host_connections:
                if connection['type'] == 'quantum':
                    num_epr_pairs = len(host.get_epr_pairs(connection['connection']))
                    if num_epr_pairs < 4:
                        host.send_epr(connection['connection'], await_ack=True)
        time.sleep(5)


def routing_algorithm(di_graph, source, target):
    """
    Entanglement based routing function. Note: any custom routing function must
    have exactly these three parameters and must return a list ordered by the steps
    in the route.

    Args:
        di_graph (networkx DiGraph): The directed graph representation of the network.
        source (str): The sender ID
        target (str: The receiver ID
    Returns:
        (list): The route ordered by the steps in the route.
    """
    # Generate entanglement network
    entanglement_network = nx.DiGraph()
    nodes = di_graph.nodes()
    for node in nodes:
        host = network.get_host(node)
        host_connections = host.get_connections()
        for connection in host_connections:
            if connection['type'] == 'quantum':
                num_epr_pairs = len(host.get_epr_pairs(connection['connection']))
                if num_epr_pairs == 0:
                    entanglement_network.add_edge(host.host_id, connection['connection'], weight=1000)
                else:
                    entanglement_network.add_edge(host.host_id, connection['connection'], weight=1. / num_epr_pairs)

    try:
        route = nx.shortest_path(entanglement_network, source, target, weight='weight')
        print('-------' + str(route) + '-------')
        return route
    except Exception as e:
        Logger.get_instance().error(e)


def main():
    network = Network.get_instance()
    network.quantum_routing_algo = routing_algorithm
    nodes = ['A', 'node_1', 'node_2', 'B']
    network.use_hop_by_hop = False
    network.set_delay = 0.2
    network.start(nodes)

    A = Host('A')
    A.add_connection('node_1')
    A.add_connection('node_2')
    A.start()

    node_1 = Host('node_1')
    node_1.add_connection('A')
    node_1.add_connection('B')
    node_1.start()

    node_2 = Host('node_2')
    node_2.add_connection('A')
    node_2.add_connection('B')
    node_2.start()

    B = Host('B')
    B.add_connection('node_1')
    B.add_connection('node_2')
    B.start()

    hosts = [A, node_1, node_2, B]
    for h in hosts:
        network.add_host(h)

    node_1.run_protocol(generate_entanglement)
    node_2.run_protocol(generate_entanglement)

    print('---- BUILDING ENTANGLEMENT   ----')
    # Let the network build up entanglement
    time.sleep(15)
    print('---- DONE BUILDING ENTANGLEMENT   ----')

    choices = ['00', '11', '10', '01']
    for _ in range(5):
        print('----  sending superdense  ----')
        A.send_superdense(B.host_id, random.choice(choices), await_ack=True)
        time.sleep(1)

    # Let the network run for 40 seconds
    time.sleep(40)
    print('stopping')
    network.stop(stop_hosts=True)
```

--------------------------------

### Full example: send data qubits from Alice to Dean

Source: https://tqsd.github.io/QuNetSim/_sources/examples/send_data.rst.txt

Complete runnable example that configures the network and sends 10 qubits from Alice to Dean. It includes imports, network delay setting, and the sending loop. The send_qubit call uses await_ack=True but ignores the ACK boolean.

```python
from qunetsim.components import Host
from qunetsim.components import Network
from qunetsim.objects import Qubit


def main():
    network = Network.get_instance()
    nodes = ["Alice", "Bob", "Eve", "Dean"]
    network.start(nodes)
    network.delay = 0.1

    host_alice = Host('Alice')
    host_alice.add_connection('Bob')
    host_alice.start()

    host_bob = Host('Bob')
    host_bob.add_connection('Alice')
    host_bob.add_connection('Eve')
    host_bob.start()

    host_eve = Host('Eve')
    host_eve.add_connection('Bob')
    host_eve.add_connection('Dean')
    host_eve.start()

    host_dean = Host('Dean')
    host_dean.add_connection('Eve')
    host_dean.start()

    network.add_host(host_alice)
    network.add_host(host_bob)
    network.add_host(host_eve)
    network.add_host(host_dean)

    for _ in range(10):
        # Create a qubit owned by Alice
        q = Qubit(host_alice)
        # Put the qubit in the excited state
        q.X()
        # Send the qubit and await an ACK from Dean
        q_id, _ = host_alice.send_qubit('Dean', q, await_ack=True)

        # Get the qubit on Dean's side from Alice
        q_rec = host_dean.get_qubit('Alice', q_id)
```

--------------------------------

### Expected output of the eavesdropping example

Source: https://tqsd.github.io/QuNetSim/_sources/examples/packet_sniffing.rst.txt

Shows the expected console output when running the example. Classical messages are prefixed with '** Bob was here :) **' and qubits are measured as 0 due to Bob's X operation.

```bash
Alice sends: Hi Eve.
Eve Received classical: ** Bob was here :) ** Hi Eve..
Alice sends: Hi Eve.
Eve Received classical: ** Bob was here :) ** Hi Eve..
Alice sends: Hi Eve.
Eve Received classical: ** Bob was here :) ** Hi Eve..
Alice sends: Hi Eve.
Eve Received classical: ** Bob was here :) ** Hi Eve..
Alice sends: Hi Eve.
Eve Received classical: ** Bob was here :) ** Hi Eve..
Alice sends qubit in the |1> state
Eve measured: 0.
Alice sends qubit in the |1> state
Eve measured: 0.
Alice sends qubit in the |1> state
Eve measured: 0.
Alice sends qubit in the |1> state
Eve measured: 0.
Alice sends qubit in the |1> state
Eve measured: 0.
```

--------------------------------

### Full eavesdropping example with imports and Logger disabled

Source: https://tqsd.github.io/QuNetSim/_sources/examples/packet_sniffing.rst.txt

Complete runnable example that includes imports, disables the logger, and defines all functions. It uses 'amount_transmit' instead of 'amount_to_transmit' and includes a type check for Message in the classical sniffing function.

```python
from qunetsim.components import Host
from qunetsim.components import Network
from qunetsim.objects import Message
from qunetsim.objects import Qubit
from qunetsim.objects import Logger

Logger.DISABLED = True

amount_transmit = 5


def alice(host):
    for _ in range(amount_transmit):
        s = 'Hi Eve.'
        print("Alice sends: %s" % s)
        host.send_classical('Eve', s, await_ack=True)

    for _ in range(amount_transmit):
        print("Alice sends qubit in the |1> state")
        q = Qubit(host)
        q.X()
        host.send_qubit('Eve', q, await_ack=True)


def bob_sniffing_quantum(sender, receiver, qubit):
    # Bob applies an X operation to all qubits that are routed through him
    qubit.X()


def bob_sniffing_classical(sender, receiver, msg):
    # Bob modifies the message content of all classical messages routed through him
    if isinstance(msg, Message):
        msg.content = "** Bob was here :) ** " + msg.content
```

--------------------------------

### Full Example for Anonymous Transfer Protocol in QuNetSim

Source: https://tqsd.github.io/QuNetSim/_sources/examples/anonymous_transfer.rst.txt

Complete QuNetSim example implementing the anonymous transfer protocol. It defines functions for distribution, sending, receiving, and intermediate nodes, plus the main function that sets up the network and hosts. It requires the QuNetSim library.

```python
import time

from qunetsim.components import Host
from qunetsim.components import Network
import random

from qunetsim.objects import Qubit


def distribute(host, nodes):
    # distribute=True => sender doesn't keep part of the GHZ
    host.send_ghz(nodes, distribute=True)


def sender(host, distributor, r, epr_id):
    q = host.get_ghz(distributor, wait=10)
    b = random.choice(['0', '1'])
    host.send_broadcast(b)
    if b == '1':
        q.Z()

    host.add_epr(r, q, q_id=epr_id)
    sending_qubit = Qubit(host)
    sending_qubit.X()
    print('Sending %s' % sending_qubit.id)
    # Generate EPR if none shouldn't change anything, but if there is
    # no shared entanglement between s and r, then there should
    # be a mistake in the protocol
    host.send_teleport(r, sending_qubit, generate_epr_if_none=False, await_ack=False)
    host.empty_classical()


def receiver(host, distributor, s, epr_id):
    q = host.get_ghz(distributor, wait=10)
    b = random.choice(['0', '1'])
    host.send_broadcast(b)

    messages = []
    # Await broadcast messages from all parties
    while len(messages) < 3:
        messages = host.classical
        messages = [x for x in messages if x.content != 'ACK']
        time.sleep(0.5)

    print([m.content for m in messages])
    parity = int(b)
    for m in messages:
        if m.sender != s:
            parity = parity ^ int(m.content)

    if parity == 1:
        q.Z()

    print('established secret EPR')
    host.add_epr(s, q, q_id=epr_id)
    q = host.get_qubit(s, wait=10)
    host.empty_classical()
    print('Received qubit %s in the %d state' % (q.id, q.measure()))


def node(host, distributor):
    q = host.get_ghz(distributor, wait=10)
    if q is None:
        print('failed')
        return
    q.H()
    m = q.measure()
    host.send_broadcast(str(m))


def main():
    network = Network.get_instance()
    nodes = ['A', 'B', 'C', 'D', 'E']
    network.start(nodes)

    host_A = Host('A')
    host_A.add_connections(['B', 'C', 'D', 'E'])
    host_A.start()
    host_B = Host('B')
    host_B.add_c_connections(['C', 'D', 'E'])
    host_B.start()
    host_C = Host('C')
    host_C.add_c_connections(['B', 'D', 'E'])
    host_C.start()
    host_D = Host('D')
    host_D.add_c_connections(['B', 'C', 'E'])
    host_D.start()
    host_E = Host('E')
    host_E.add_c_connections(['B', 'C', 'D'])
    host_E.start()

    network.add_hosts([host_A, host_B, host_C, host_D, host_E])
```

--------------------------------

### Entanglement routing example: generate_entanglement, routing_algorithm, and main()

Source: https://tqsd.github.io/QuNetSim/_sources/examples/entanglement_routing.rst.txt

These three functions form the complete entanglement routing example: a background protocol that generates EPR pairs when a host is idle, a custom routing algorithm that builds a weighted entanglement network and uses nx.shortest_path, and a main() that sets up a 4-node network, runs the protocol, and sends superdense-coded messages. The routing function must accept exactly (di_graph, source, target) and return a route list; the example uses network.quantum_routing_algo, use_hop_by_hop=False, and set_delay=0.2.

```python
    def generate_entanglement(host):
        """
        Generate entanglement if the host has nothing to process (i.e. is idle).
        """
        while True:
            if host.is_idle():
                host_connections = host.get_connections()
                for connection in host_connections:
                    if connection['type'] == 'quantum':
                        num_epr_pairs = len(host.get_epr_pairs(connection['connection']))
                        if num_epr_pairs < 4:
                            host.send_epr(connection['connection'], await_ack=True)
            time.sleep(5)
```

```python
    def routing_algorithm(di_graph, source, target):
        """
        Entanglement based routing function. Note: any custom routing function must
        have exactly these three parameters and must return a list ordered by the steps
        in the route.

        Args:
            di_graph (networkx DiGraph): The directed graph representation of the network.
            source (str): The sender ID
            target (str: The receiver ID
        Returns:
            (list): The route ordered by the steps in the route.
        """
        # Generate entanglement network
        entanglement_network = nx.DiGraph()
        nodes = di_graph.nodes()
        for node in nodes:
            host = network.get_host(node)
            host_connections = host.get_connections()
            for connection in host_connections:
                if connection['type'] == 'quantum':
                    num_epr_pairs = len(host.get_epr_pairs(connection['connection']))
                    if num_epr_pairs == 0:
                        entanglement_network.add_edge(host.host_id, connection['connection'], weight=1000)
                    else:
                        entanglement_network.add_edge(host.host_id, connection['connection'], weight=1. / num_epr_pairs)

        try:
            route = nx.shortest_path(entanglement_network, source, target, weight='weight')
            print('-------' + str(route) + '-------')
            return route
        except Exception as e:
            Logger.get_instance().error(e)
```

```python
    def main():
        network = Network.get_instance()
        network.quantum_routing_algo = routing_algorithm
        nodes = ['A', 'node_1', 'node_2', 'B']
        network.use_hop_by_hop = False
        network.set_delay = 0.2
        network.start(nodes)

        A = Host('A')
        A.add_connection('node_1')
        A.add_connection('node_2')
        A.start()

        node_1 = Host('node_1')
        node_1.add_connection('A')
        node_1.add_connection('B')
        node_1.start()

        node_2 = Host('node_2')
        node_2.add_connection('A')
        node_2.add_connection('B')
        node_2.start()

        B = Host('B')
        B.add_connection('node_1')
        B.add_connection('node_2')
        B.start()

        hosts = [A, node_1, node_2, B]
        for h in hosts:
            network.add_host(h)

        node_1.run_protocol(generate_entanglement)
        node_2.run_protocol(generate_entanglement)

        print('---- BUILDING ENTANGLEMENT   ----')
        # Let the network build up entanglement
        time.sleep(15)
        print('---- DONE BUILDING ENTANGLEMENT   ----')

        choices = ['00', '11', '10', '01']
        for _ in range(5):
            print('----  sending superdense  ----')
            A.send_superdense(B.host_id, random.choice(choices), await_ack=True)
            time.sleep(1)

        # Let the network run for 40 seconds
        time.sleep(40)
        print('stopping')
        network.stop(stop_hosts=True)
```

--------------------------------

### Network Configuration

Source: https://tqsd.github.io/QuNetSim/examples/entanglement_routing.html

Initializes the network nodes and connections for the entanglement routing example.

```python
nodes = ['A', 'node_1', 'node_2', 'B']
network.use_hop_by_hop = False
network.set_delay = 0.1
network.start(nodes)

A = Host('A')
A.add_connection('node_1')
A.add_connection('node_2')
A.start()

node_1 = Host('node_1')
node_1.add_connection('A')
node_1.add_connection('B')
node_1.start()

node_2 = Host('node_2')
node_2.add_connection('A')
node_2.add_connection('B')
node_2.start()

B = Host('B')
B.add_connection('node_1')
B.add_connection('node_2')
B.start()

hosts = [A, node_1, node_2, B]
for h in hosts:
    network.add_host(h)
```

--------------------------------

### Full W State Distribution Example

Source: https://tqsd.github.io/QuNetSim/examples/send_w.html

A complete script setting up the network, configuring hosts, distributing the W state, and performing measurements.

```python
from qunetsim.backends import EQSNBackend
from qunetsim.components import Host
from qunetsim.components import Network
from qunetsim.objects import Logger

Logger.DISABLED = False


def main():
    network = Network.get_instance()
    nodes = ["Alice", "Bob", "Eve", "Dean"]
    back = EQSNBackend()
    network.start(nodes, back)

    network.delay = 0.1

    host_alice = Host('Alice', back)
    host_alice.add_connection('Bob')
    host_alice.add_connection('Eve')
    host_alice.start()

    host_bob = Host('Bob', back)
    host_bob.add_connection('Alice')
    host_bob.add_connection('Eve')
    host_bob.start()

    host_eve = Host('Eve', back)
    host_eve.add_connection('Bob')
    host_eve.add_connection('Dean')
    host_eve.add_connection('Alice')
    host_eve.start()

    host_dean = Host('Dean', back)
    host_dean.add_connection('Eve')
    host_dean.start()

    network.add_host(host_alice)
    network.add_host(host_bob)
    network.add_host(host_eve)
    network.add_host(host_dean)

    share_list = ["Bob", "Eve", "Dean"]
    q_id1, ack_received = host_alice.send_w(share_list, await_ack=True)

    print("Alice received ACK from all? " + str(ack_received))

    q1 = host_alice.get_w('Alice', q_id1, wait=10)
    q2 = host_bob.get_w('Alice', q_id1, wait=10)
    q3 = host_eve.get_w('Alice', q_id1, wait=10)
    q4 = host_dean.get_w('Alice', q_id1, wait=10)

    print("System density matrix:")
    print(q1._qubit[0].data)

    m1 = q1.measure()
    m2 = q2.measure()
    m3 = q3.measure()
    m4 = q4.measure()

    print("\nResults of measurements are %d, %d, %d, %d." % (m1, m2, m3, m4))

    network.stop(True)
    exit()


if __name__ == '__main__':
    main()
```

--------------------------------

### Set up network with three nodes

Source: https://tqsd.github.io/QuNetSim/_sources/examples/send_epr.rst.txt

Creates a network with nodes A, B, and C, adds connections, and starts hosts. The network is a line A<==>B<==>C with both classical and quantum connections.

```python
network = Network.get_instance()
nodes = ['A', 'B', 'C']
network.start(nodes)

host_A = Host('A')
host_A.add_connection('B')
host_A.start()

host_B = Host('B')
host_B.add_connection('A')
host_B.add_connection('C')
host_B.start()

host_C = Host('C')
host_C.add_connection('B')
host_C.start()

network.add_host(host_A)
network.add_host(host_B)
network.add_host(host_C)

# Network is A<==>B<==>C;
# Note: we use 'A<==>B' to represent a classical and quantum connection
#       we use 'A<-->B' to represent a classical only connection
#       we use 'A<~~>B' to represent a quantum only connection
#       All connections are added uni-directionally, so '<' and '>'
#       represent the direction of the flow of traffic.
```

--------------------------------

### Configure Network and Hosts

Source: https://tqsd.github.io/QuNetSim/examples/send_data.html

Initializes the network instance and sets up four hosts with connections forming a linear topology: Alice <==> Bob <==> Eve <==> Dean.

```python
network = Network.get_instance()
nodes = ["Alice", "Bob", "Eve", "Dean"]
network.start(nodes)

host_alice = Host('Alice')
host_alice.add_connection('Bob')
host_alice.start()

host_bob = Host('Bob')
host_bob.add_connection('Alice')
host_bob.add_connection('Eve')
host_bob.start()

host_eve = Host('Eve')
host_eve.add_connection('Bob')
host_eve.add_connection('Dean')
host_eve.start()

host_dean = Host('Dean')
host_dean.add_connection('Eve')
host_dean.start()

# Network will be:
# Alice <==> Bob <==> Eve <==> Dean
# Note: we use 'A<==>B' to represent a classical and quantum connection
#       we use 'A<-->B' to represent a classical only connection
#       we use 'A<~~>B' to represent a quantum only connection
#       All connections are added uni-directionally, so '<' and '>'
```

--------------------------------

### Full BB84 QKD Example

Source: https://tqsd.github.io/QuNetSim/_sources/examples/QKD_BB84.rst.txt

The full BB84 QKD example with Alice and Eve. It includes the encryption/decryption functions, the QKD protocol, and the message sending functions. The source states this is the full example below the previous fragments.

```python
    import numpy as np
    import random

    from qunetsim.components import Host
    from qunetsim.components import Network
    from qunetsim.objects import Qubit
    from qunetsim.objects import Logger

    Logger.DISABLED = True

    wait_time = 10


    # !! Warning: this Crypto algorithm is really bad!
    # !! Warning: Do not use it as a real Crypto Algorithm!

    # key has to be a string
    def encrypt(key, text):
        encrypted_text = ""
        for char in text:
            encrypted_text += chr(ord(key) ^ ord(char))
        return encrypted_text


    def decrypt(key, encrypted_text):
        return encrypt(key, encrypted_text)


    def alice_qkd(alice, msg_buff, secret_key, receiver):
        sequence_nr = 0
        # iterate over all bits in the secret key.
        for bit in secret_key:
            ack = False
            while not ack:
                print("Alice sent %d key bits" % (sequence_nr + 1))
                # get a random base. 0 for Z base and 1 for X base.
                base = random.randint(0, 1)

                # create qubit
                q_bit = Qubit(alice)

                # Set qubit to the bit from the secret key.
                if bit == 1:
                    q_bit.X()

                # Apply basis change to the bit if necessary.
                if base == 1:
                    q_bit.H()

                # Send Qubit to Bob
                alice.send_qubit(receiver, q_bit, await_ack=True)

                # Get measured basis of Bob
                message = alice.get_next_classical_message(receiver, msg_buff, sequence_nr)

```

--------------------------------

### Initialize QuNetSim network with hosts Alice, Bob, and Eve

Source: https://tqsd.github.io/QuNetSim/_sources/examples/packet_sniffing.rst.txt

Sets up the network and hosts. Alice connects to Bob, Bob connects to Alice and Eve, and Eve connects to Bob. Eve has a delay of 0.2. This is the first step in the eavesdropping example.

```python
network = Network.get_instance()
nodes = ["Alice", "Bob", "Eve"]
network.start(nodes)
network.delay = 0.0

host_alice = Host('Alice')
host_alice.add_connection('Bob')
host_alice.start()

host_bob = Host('Bob')
host_bob.add_connection('Alice')
host_bob.add_connection('Eve')
host_bob.start()

host_eve = Host('Eve')
host_eve.add_connection('Bob')
host_eve.delay = 0.2
host_eve.start()

network.add_host(host_alice)
network.add_host(host_bob)
network.add_host(host_eve)
```

--------------------------------

### Main network setup with sniffing

Source: https://tqsd.github.io/QuNetSim/_sources/examples/packet_sniffing.rst.txt

Sets up the network with Alice, Bob, and Eve. Bob is configured with quantum and classical relay sniffing functions. Alice runs the alice protocol and Eve runs the eve protocol, then the network stops.

```python
def main():
    network = Network.get_instance()
    nodes = ["Alice", "Bob", "Eve"]
    network.start(nodes)
    network.delay = 0.0

    host_alice = Host('Alice')
    host_alice.add_connection('Bob')
    host_alice.start()

    host_bob = Host('Bob')
    host_bob.add_connection('Alice')
    host_bob.add_connection('Eve')
    host_bob.start()

    host_eve = Host('Eve')
    host_eve.add_connection('Bob')
    host_eve.delay = 0.2
    host_eve.start()

    network.add_host(host_alice)
    network.add_host(host_bob)
    network.add_host(host_eve)

    host_bob.q_relay_sniffing = True
    host_bob.q_relay_sniffing_fn = eve_sniffing_quantum

    host_bob.c_relay_sniffing = True
    host_bob.c_relay_sniffing_fn = bob_sniffing_classical

    t1 = host_alice.run_protocol(alice)
    t2 = host_eve.run_protocol(eve)

    t1.join()
    t2.join()

    network.stop(True)
    exit()
```

--------------------------------

### Initialize EQSN Backend

Source: https://tqsd.github.io/QuNetSim/_sources/backends.rst.txt

Example of explicitly setting the EQSN backend by creating a backend object and passing it to the network and host instances.

```python
import numpy as np
# import the eqsn backend
from qunetsim.backends import EQSNBackend

# create the EQSN backend object
backend = EQSNBackend()

# Initialize a network and define the host IDs in the network
network = Network.get_instance()
nodes = ['Alice', 'Bob', 'Eve']

# Start the network with the hosts and explicitly set the backend
network.start(nodes, backend)

# Initialize the host Alice with the right backend
host_alice = Host('Alice', backend)

host_alice.add_connection('Bob')
host_alice.start()

# Initialize the host Bob with the right backend
host_bob = Host('Bob', backend)

host_bob.add_connection('Alice')
host_bob.add_connection('Eve')
host_bob.start()

# Initialize the host Eve with the right backend
host_eve = Host('Eve', backend)
host_eve.add_connection('Bob')
host_eve.start()

# Add the hosts to the network
network.add_host(host_alice)
network.add_host(host_bob)
network.add_host(host_eve)
```

--------------------------------

### Initialize QuNetSim network with three hosts

Source: https://tqsd.github.io/QuNetSim/_sources/examples/QKD_BB84.rst.txt

Sets up a three-party QuNetSim network with hosts Alice, Bob, and Eve. Alice connects to Bob, Bob connects to both Alice and Eve, and Eve connects to Bob, forming a linear topology. The network is started and all hosts are added before any QKD protocol runs.

```python
import numpy as np

# Initialize a network
network = Network.get_instance()

# Define the host IDs in the network
nodes = ['Alice', 'Bob', 'Eve']

# Start the network with the defined hosts
network.start(nodes)

# Initialize the host Alice
host_alice = Host('Alice')

# Add a one-way connection (classical and quantum) to Bob
host_alice.add_connection('Bob')

# Start listening
host_alice.start()

host_bob = Host('Bob')
# Bob adds his own one-way connection to Alice and Eve
host_bob.add_connection('Alice')
host_bob.add_connection('Eve')
host_bob.start()

host_eve = Host('Eve')
host_eve.add_connection('Bob')
host_eve.start()

# Add the hosts to the network
# The network is: Alice <--> Bob <--> Eve
network.add_host(host_alice)
network.add_host(host_bob)
network.add_host(host_eve)
```
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