From 9ac49a38d48940fdd190487725e783ceb3e3fb6b Mon Sep 17 00:00:00 2001
From: Nicolas Vogt <45093608+nfwvogt@users.noreply.github.com>
Date: Thu, 18 Nov 2021 19:14:47 +0100
Subject: [PATCH] Fix_multi_qubit_zz (#133)

* update dependencies

* updated dependencies

* fixing bug in MultiQubitZZ gate

* Update multi_qubit_gate_operations.rs

Co-authored-by: kbarkhqs <65908581+kbarkhqs@users.noreply.github.com>
---
 roqoqo/src/operations/multi_qubit_gate_operations.rs | 12 +++++++++++-
 .../operations/multi_qubit_gate_operations.rs        | 12 ++++++------
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/roqoqo/src/operations/multi_qubit_gate_operations.rs b/roqoqo/src/operations/multi_qubit_gate_operations.rs
index 75dc939a..daca65dc 100644
--- a/roqoqo/src/operations/multi_qubit_gate_operations.rs
+++ b/roqoqo/src/operations/multi_qubit_gate_operations.rs
@@ -10,6 +10,8 @@
 // express or implied. See the License for the specific language governing permissions and
 // limitations under the License.
 
+use std::panic;
+
 use crate::operations;
 use crate::prelude::*;
 use crate::Circuit;
@@ -127,7 +129,15 @@ impl OperateGate for MultiQubitZZ {
         let cos: Complex64 = Complex64::new((self.theta.float()? / 2.0).cos(), 0.0);
         let sin: Complex64 = Complex64::new(0.0, -(self.theta.float()? / 2.0).sin());
         for i in 0..dim {
-            array[(i, i)] = cos + sin;
+            // Fix the signs of the imaginary part due to the ZZZ..ZZ product
+            let prefactor: f64 = (0..self.qubits.len())
+                .map(|q| match i.div_euclid(2usize.pow(q as u32)) % 2 {
+                    0 => 1.0,
+                    1 => -1.0,
+                    _ => panic!("Internal division error MuliQubitZZ"),
+                })
+                .product();
+            array[(i, i)] = cos + prefactor * sin;
         }
         Ok(array)
     }
diff --git a/roqoqo/tests/integration/operations/multi_qubit_gate_operations.rs b/roqoqo/tests/integration/operations/multi_qubit_gate_operations.rs
index 29a9c01a..fe444cd5 100644
--- a/roqoqo/tests/integration/operations/multi_qubit_gate_operations.rs
+++ b/roqoqo/tests/integration/operations/multi_qubit_gate_operations.rs
@@ -337,14 +337,14 @@ fn test_matrix_output_multi_qubit_zz(qubits: Vec<usize>) {
         ],
         [
             Complex64::new(0.0, 0.0),
-            Complex64::new(f, (-1.0) * f),
+            Complex64::new(f, f),
             Complex64::new(0.0, 0.0),
             Complex64::new(0.0, 0.0)
         ],
         [
             Complex64::new(0.0, 0.0),
             Complex64::new(0.0, 0.0),
-            Complex64::new(f, (-1.0) * f),
+            Complex64::new(f, f),
             Complex64::new(0.0, 0.0)
         ],
         [
@@ -376,7 +376,7 @@ fn test_matrix_output_three_multi_qubit_zz(qubits: Vec<usize>) {
         ],
         [
             Complex64::new(0.0, 0.0),
-            Complex64::new(f, (-1.0) * f),
+            Complex64::new(f, f),
             Complex64::new(0.0, 0.0),
             Complex64::new(0.0, 0.0),
             Complex64::new(0.0, 0.0),
@@ -387,7 +387,7 @@ fn test_matrix_output_three_multi_qubit_zz(qubits: Vec<usize>) {
         [
             Complex64::new(0.0, 0.0),
             Complex64::new(0.0, 0.0),
-            Complex64::new(f, (-1.0) * f),
+            Complex64::new(f, f),
             Complex64::new(0.0, 0.0),
             Complex64::new(0.0, 0.0),
             Complex64::new(0.0, 0.0),
@@ -409,7 +409,7 @@ fn test_matrix_output_three_multi_qubit_zz(qubits: Vec<usize>) {
             Complex64::new(0.0, 0.0),
             Complex64::new(0.0, 0.0),
             Complex64::new(0.0, 0.0),
-            Complex64::new(f, (-1.0) * f),
+            Complex64::new(f, f),
             Complex64::new(0.0, 0.0),
             Complex64::new(0.0, 0.0),
             Complex64::new(0.0, 0.0)
@@ -442,7 +442,7 @@ fn test_matrix_output_three_multi_qubit_zz(qubits: Vec<usize>) {
             Complex64::new(0.0, 0.0),
             Complex64::new(0.0, 0.0),
             Complex64::new(0.0, 0.0),
-            Complex64::new(f, (-1.0) * f)
+            Complex64::new(f, f)
         ],
     ];
     let unit = gate.unitary_matrix().unwrap();