examples: add a second generation repeater

The idealized quantum repeater from https://arxiv.org/pdf/0809.3629.pdf
is modelled with three repeater stations.

fixes: tqsd#91

examples/repeater/swap_and_distil.py

@@ -0,0 +1,119 @@
+from dataclasses import dataclass
+
+from qunetsim.components import Host
+from qunetsim.objects import Logger, Qubit
+from qunetsim.components import Network
+
+Logger.DISABLED = True
+
+
+@dataclass()
+class Ebit:
+    val: tuple[int, int]
+
+    def __str__(self):
+        return {
+            (0, 0): "phi+",
+            (0, 1): "psi+",
+            (1, 0): "phi-",
+            (1, 1): "psi-",
+        }[self.val]
+
+    @staticmethod
+    def from_bell_measurement(a: Qubit, b: Qubit):
+        a.cnot(b)
+        a.H()
+        return Ebit((a.measure(), b.measure()))
+
+
+def send_epr(host, right_host_id):
+    a, b = Qubit(host), Qubit(host)
+    a.H()
+    a.cnot(b)
+    host.send_qubit(right_host_id, b)
+    return a
+
+
+DATA_QUBITS = 10
+
+
+def boundary_protocol(host, left_host_id, right_host_id):
+    conn = None
+    qubit = []
+
+    # Send an EPR qubit right; receive an EPR qubit from the left.
+    for _ in range(DATA_QUBITS):
+        if left_host_id is not None:
+            conn = left_host_id
+            qubit.append(host.get_qubit(conn, wait=-1))
+        elif right_host_id is not None:
+            conn = right_host_id
+            qubit.append(send_epr(host, conn))
+        else:
+            raise ValueError('boundary node has no connection')
+
+    for i in range(DATA_QUBITS):
+        # Apply local ops to transform |ab> into |Φ+> (modulo phase).
+        msg = host.get_next_classical(conn, wait=-1).content
+
+        if left_host_id is None:
+            if msg == 'psi-':
+                qubit[i].Y()
+        elif right_host_id is None:
+            if msg == 'psi+':
+                qubit[i].X()
+            elif msg == 'phi-':
+                qubit[i].Z()
+
+        # confirm that the boundaries are correlated
+        print(f"{host.host_id} measures {qubit[i].measure()}")
+
+
+def repeater_protocol(host, left_host_id, right_host_id):
+    for _ in range(DATA_QUBITS):
+        # Swap. Send EPR right; receive from the left.
+        q_right = send_epr(host, right_host_id)
+        q_left = host.get_qubit(left_host_id, wait=-1)
+
+        # measure and broadcast
+        ebit = Ebit.from_bell_measurement(q_left, q_right)
+        host.send_broadcast(str(ebit))
+
+
+def main():
+    network = Network.get_instance()
+    nodes = ["Alice", "Polly", "Bob"]
+
+    network.start(nodes)
+    network.delay = 0.1
+
+    alice = Host("Alice")
+    alice.add_connection("Polly")
+    alice.start()
+
+    polly = Host("Polly")
+    polly.add_connection("Alice")
+    polly.add_connection("Bob")
+    polly.start()
+
+    bob = Host("Bob")
+    bob.add_connection("Polly")
+    bob.start()
+
+    network.add_host(alice)
+    network.add_host(polly)
+    network.add_host(bob)
+    network.start()
+
+    t1 = alice.run_protocol(boundary_protocol, (None, polly.host_id))
+    t2 = bob.run_protocol(boundary_protocol, (polly.host_id, None))
+    _ = polly.run_protocol(repeater_protocol, (alice.host_id, bob.host_id))
+
+    t1.join()
+    t2.join()
+
+    network.stop(True)
+
+
+if __name__ == "__main__":
+    main()