From 8d80f251d43aae81d480a1cbeec7ce01a8d21d05 Mon Sep 17 00:00:00 2001
From: Nicolas Vogt <45093608+nfwvogt@users.noreply.github.com>
Date: Thu, 18 Nov 2021 16:18:00 +0100
Subject: [PATCH] Topology_links_interface (#132)

* update dependencies

* updated dependencies

* WIP: two_qubit_edges

* Added two_qubit_edges to Device trait
---
 CHANGELOG.md                        |  1 +
 roqoqo/src/devices.rs               | 14 ++++++++++++++
 roqoqo/tests/integration/devices.rs | 25 +++++++++++++++++++++----
 3 files changed, 36 insertions(+), 4 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6edbe3cc..0d43a470 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,7 @@ This changelog track changes to the qoqo project starting at version 0.5.0
 ### Added not released
 
 * MultiQubitZZ gate. Rotation under a multi-qubit product of Pauli Z operators.
+* `two_qubit_edges` function in Device trait. Used to create a simple graph-library-agnostic representation of the connectivity graph of a device.
 
 ## 0.8.1
 
diff --git a/roqoqo/src/devices.rs b/roqoqo/src/devices.rs
index 746717e8..ad480100 100644
--- a/roqoqo/src/devices.rs
+++ b/roqoqo/src/devices.rs
@@ -111,6 +111,20 @@ pub trait Device {
     /// Returns the number of qubits the device supports.
     fn number_qubits(&self) -> usize;
 
+    /// Returns the list of pairs of qubits linked with a native two-qubit-gate in the device.
+    ///
+    /// A pair of qubits is considered linked by a native two-qubit-gate if the device
+    /// can implement a two-qubit-gate btween the two qubits without decomposing it
+    /// into a sequence of gates that involves a third qubit of the device.
+    /// The two-qubit-gate also has to form a universal set together with the available
+    /// single qubit gates.
+    ///
+    /// The returned vectors is a simple, graph-library independent, representation of
+    /// the undirected connectivity graph of the device.
+    /// It can be used to construct the connectivity graph in a graph library of the users
+    /// choice from a list of edges and can be used for applications like routing in quantum algorithms.
+    fn two_qubit_edges(&self) -> Vec<(usize, usize)>;
+
     /// Changes the device topology based on a Pragma operation.
     ///
     /// Specific devices and backends can allow changes to the device topology.
diff --git a/roqoqo/tests/integration/devices.rs b/roqoqo/tests/integration/devices.rs
index 590e819b..7912d967 100644
--- a/roqoqo/tests/integration/devices.rs
+++ b/roqoqo/tests/integration/devices.rs
@@ -42,10 +42,6 @@ impl TestDevice {
 }
 
 impl Device for TestDevice {
-    fn number_qubits(&self) -> usize {
-        self.number_qubits
-    }
-
     fn single_qubit_gate_time(&self, hqslang: &str, qubit: &usize) -> Option<f64> {
         match self.single_qubit_gates.get(&hqslang.to_string()) {
             Some(x) => x.get(&qubit).map(|x| *x),
@@ -67,6 +63,20 @@ impl Device for TestDevice {
     fn qubit_decoherence_rates(&self, qubit: &usize) -> Option<Array2<f64>> {
         self.rates.get(&qubit).map(|x| x.to_owned())
     }
+
+    fn number_qubits(&self) -> usize {
+        self.number_qubits
+    }
+
+    fn two_qubit_edges(&self) -> Vec<(usize, usize)> {
+        let mut edges: Vec<(usize, usize)> = Vec::new();
+        for row in 0..self.number_qubits {
+            for column in row + 1..self.number_qubits {
+                edges.push((row, column));
+            }
+        }
+        edges
+    }
 }
 
 /// Basic functional test
@@ -128,6 +138,13 @@ fn it_works() {
         Some(0.8f64)
     );
     assert_eq!(device.multi_qubit_gate_time("Other", &[0, 1, 2]), None);
+
+    let test_edges = vec![(0, 1), (0, 2), (1, 2)];
+    let edges = device.two_qubit_edges();
+    assert_eq!(test_edges.len(), edges.len());
+    for edge in edges {
+        assert!(test_edges.contains(&edge));
+    }
 }
 
 /// Basic functional test