Implement RGate in Rust (Qiskit#12662)

* Implement RGate in Rust

* Update crates/circuit/src/operations.rs

Co-authored-by: Elena Peña Tapia <57907331+ElePT@users.noreply.github.com>

* Fix error in decomposition of RGate

There is an error in the expression for decomposition of the R gate
in the port to Rust.

This fixes the error and re-enables the skipped test that failed because
of the incorrect expression.

* Factor cloning the Param enum in Rust

To clone the enum, each variant must be handled separately.  This is currently used once,
but can be used each time a `Param` is cloned. In case more work needs to be done within
match arms, one might choose not to use this function, but rather clone in each of these
arms.

* Run cargo fmt

* Implement and use addition for enum Param

This handles `Float` and `ParameterExpression` variants uniformly.

---------

Co-authored-by: Elena Peña Tapia <57907331+ElePT@users.noreply.github.com>

crates/circuit/src/gate_matrix.rs

@@ -24,6 +24,19 @@ const fn c64(re: f64, im: f64) -> Complex64 {
 pub static ONE_QUBIT_IDENTITY: [[Complex64; 2]; 2] =
     [[c64(1., 0.), c64(0., 0.)], [c64(0., 0.), c64(1., 0.)]];
 
+#[inline]
+pub fn r_gate(theta: f64, phi: f64) -> [[Complex64; 2]; 2] {
+    let half_theta = theta / 2.;
+    let cost = c64(half_theta.cos(), 0.);
+    let sint = half_theta.sin();
+    let cosphi = phi.cos();
+    let sinphi = phi.sin();
+    [
+        [cost, c64(-sint * sinphi, -sint * cosphi)],
+        [c64(sint * sinphi, -sint * cosphi), cost],
+    ]
+}
+
 #[inline]
 pub fn rx_gate(theta: f64) -> [[Complex64; 2]; 2] {
     let half_theta = theta / 2.;

crates/circuit/src/imports.rs

@@ -157,7 +157,7 @@ static STDGATE_IMPORT_PATHS: [[&str; 2]; STANDARD_GATE_SIZE] = [
     // CRZGate = 31
     ["placeholder", "placeholder"],
     // RGate 32
-    ["placeholder", "placeholder"],
+    ["qiskit.circuit.library.standard_gates.r", "RGate"],
     // CHGate = 33
     ["qiskit.circuit.library.standard_gates.h", "CHGate"],
     // CPhaseGate = 34

crates/circuit/src/operations.rs

@@ -239,7 +239,7 @@ static STANDARD_GATE_NUM_QUBITS: [u32; STANDARD_GATE_SIZE] = [
     1, 1, 1, 2, 2, 2, 3, 1, 1, 1, // 0-9
     2, 2, 1, 0, 1, 1, 1, 1, 1, 1, // 10-19
     1, 1, 1, 2, 2, 2, 1, 1, 1, 34, // 20-29
-    34, 34, 34, 2, 2, 2, 2, 2, 3, 2, // 30-39
+    34, 34, 1, 2, 2, 2, 2, 2, 3, 2, // 30-39
     2, 2, 34, 34, 34, 2, 34, 34, 34, 34, // 40-49
     34, 34, 34, // 50-52
 ];
@@ -249,7 +249,7 @@ static STANDARD_GATE_NUM_PARAMS: [u32; STANDARD_GATE_SIZE] = [
     0, 0, 0, 0, 0, 0, 0, 1, 1, 1, // 0-9
     0, 0, 0, 1, 0, 0, 1, 3, 0, 0, // 10-19
     0, 0, 0, 0, 2, 2, 1, 2, 3, 34, // 20-29
-    34, 34, 34, 0, 1, 0, 0, 0, 0, 3, // 30-39
+    34, 34, 2, 0, 1, 0, 0, 0, 0, 3, // 30-39
     1, 3, 34, 34, 34, 0, 34, 34, 34, 34, // 40-49
     34, 34, 34, // 50-52
 ];
@@ -514,7 +514,12 @@ impl Operation for StandardGate {
                 _ => None,
             },
             Self::CRXGate | Self::CRYGate | Self::CRZGate => todo!(),
-            Self::RGate => todo!(),
+            Self::RGate => match params {
+                [Param::Float(theta), Param::Float(phi)] => {
+                    Some(aview2(&gate_matrix::r_gate(*theta, *phi)).to_owned())
+                }
+                _ => None,
+            },
             Self::CHGate => todo!(),
             Self::CPhaseGate => todo!(),
             Self::CSGate => todo!(),
@@ -957,7 +962,21 @@ impl Operation for StandardGate {
                 )
             }),
             Self::CRXGate | Self::CRYGate | Self::CRZGate => todo!(),
-            Self::RGate => todo!(),
+            Self::RGate => Python::with_gil(|py| -> Option<CircuitData> {
+                let theta_expr = clone_param(&params[0], py);
+                let phi_expr1 = add_param(&params[1], -PI2, py);
+                let phi_expr2 = multiply_param(&phi_expr1, -1.0, py);
+                let defparams = smallvec![theta_expr, phi_expr1, phi_expr2];
+                Some(
+                    CircuitData::from_standard_gates(
+                        py,
+                        1,
+                        [(Self::UGate, defparams, smallvec![Qubit(0)])],
+                        FLOAT_ZERO,
+                    )
+                    .expect("Unexpected Qiskit python bug"),
+                )
+            }),
             Self::CHGate => todo!(),
             Self::CPhaseGate => todo!(),
             Self::CSGate => todo!(),
@@ -997,14 +1016,37 @@ impl Operation for StandardGate {
 
 const FLOAT_ZERO: Param = Param::Float(0.0);
 
+// Return explictly requested copy of `param`, handling
+// each variant separately.
+fn clone_param(param: &Param, py: Python) -> Param {
+    match param {
+        Param::Float(theta) => Param::Float(*theta),
+        Param::ParameterExpression(theta) => Param::ParameterExpression(theta.clone_ref(py)),
+        Param::Obj(_) => unreachable!(),
+    }
+}
+
 fn multiply_param(param: &Param, mult: f64, py: Python) -> Param {
     match param {
         Param::Float(theta) => Param::Float(*theta * mult),
         Param::ParameterExpression(theta) => Param::ParameterExpression(
             theta
                 .clone_ref(py)
                 .call_method1(py, intern!(py, "__rmul__"), (mult,))
-                .expect("Parameter expression for global phase failed"),
+                .expect("Multiplication of Parameter expression by float failed."),
+        ),
+        Param::Obj(_) => unreachable!(),
+    }
+}
+
+fn add_param(param: &Param, summand: f64, py: Python) -> Param {
+    match param {
+        Param::Float(theta) => Param::Float(*theta + summand),
+        Param::ParameterExpression(theta) => Param::ParameterExpression(
+            theta
+                .clone_ref(py)
+                .call_method1(py, intern!(py, "__add__"), (summand,))
+                .expect("Sum of Parameter expression and float failed."),
         ),
         Param::Obj(_) => unreachable!(),
     }

qiskit/circuit/library/standard_gates/r.py

@@ -20,6 +20,7 @@
 from qiskit.circuit.gate import Gate
 from qiskit.circuit.quantumregister import QuantumRegister
 from qiskit.circuit.parameterexpression import ParameterValueType
+from qiskit._accelerate.circuit import StandardGate
 
 
 class RGate(Gate):
@@ -49,6 +50,8 @@ class RGate(Gate):
             \end{pmatrix}
     """
 
+    _standard_gate = StandardGate.RGate
+
     def __init__(
         self,
         theta: ParameterValueType,

qiskit/circuit/quantumcircuit.py

@@ -4649,9 +4649,7 @@ def r(
         Returns:
             A handle to the instructions created.
         """
-        from .library.standard_gates.r import RGate
-
-        return self.append(RGate(theta, phi), [qubit], [], copy=False)
+        return self._append_standard_gate(StandardGate.RGate, [theta, phi], qargs=[qubit])
 
     def rv(
         self,