Add SiSWAP gate

doc/source/api-reference/qibo.rst

@@ -687,6 +687,13 @@ iSwap (iSWAP)
     :members:
     :member-order: bysource
 
+Square root of iSwap (SiSWAP)
+"""""""""""""""""""""""""""""
+
+.. autoclass:: qibo.gates.SiSWAP
+    :members:
+    :member-order: bysource
+
 f-Swap (FSWAP)
 """"""""""""""
 

src/qibo/backends/__init__.py

@@ -114,6 +114,7 @@ def create(self, dtype):
         self.CSXDG = self.matrices.CSXDG
         self.SWAP = self.matrices.SWAP
         self.iSWAP = self.matrices.iSWAP
+        self.SiSWAP = self.matrices.SiSWAP
         self.FSWAP = self.matrices.FSWAP
         self.ECR = self.matrices.ECR
         self.SYC = self.matrices.SYC

src/qibo/backends/npmatrices.py

@@ -194,6 +194,18 @@ def iSWAP(self):
             [[1, 0, 0, 0], [0, 0, 1j, 0], [0, 1j, 0, 0], [0, 0, 0, 1]], dtype=self.dtype
         )
 
+    @cached_property
+    def SiSWAP(self):
+        return self.np.array(
+            [
+                [1, 0, 0, 0],
+                [0, 1 / self.np.sqrt(2), 1j / self.np.sqrt(2), 0],
+                [0, 1j / self.np.sqrt(2), 1 / self.np.sqrt(2), 0],
+                [0, 0, 0, 1],
+            ],
+            dtype=self.dtype,
+        )
+
     @cached_property
     def FSWAP(self):
         return self.np.array(

src/qibo/gates/gates.py

@@ -1476,7 +1476,7 @@ def qasm_label(self):
 
 
 class iSWAP(Gate):
-    """The iswap gate.
+    """The iSWAP gate.
 
     Corresponds to the following unitary matrix
 
@@ -1507,6 +1507,33 @@ def qasm_label(self):
         return "iswap"
 
 
+class SiSWAP(Gate):
+    """The :math:`\\sqrt{\\text{iSWAP}}}` gate.
+
+    Corresponds to the following unitary matrix
+
+    .. math::
+        \\begin{pmatrix}
+        1 & 0 & 0 & 0 \\\\
+        0 & 1/\\sqrt{2} & i/\\sqrt{2} & 0 \\\\
+        0 & i/\\sqrt{2} & 1/\\sqrt{2} & 0 \\\\
+        0 & 0 & 0 & 1 \\\\
+        \\end{pmatrix}
+
+    Args:
+        q0 (int): the first qubit to be swapped id number.
+        q1 (int): the second qubit to be swapped id number.
+    """
+
+    def __init__(self, q0, q1):
+        super().__init__()
+        self.name = "siswap"
+        self.draw_label = "si"
+        self.target_qubits = (q0, q1)
+        self.init_args = [q0, q1]
+        self.unitary = True
+
+
 class FSWAP(Gate):
     """The fermionic swap gate.
 

tests/test_gates_gates.py

@@ -711,6 +711,17 @@ def test_iswap(backend):
     assert gates.iSWAP(0, 1).unitary
 
 
+def test_siswap(backend):
+    final_state = apply_gates(backend, [gates.X(1), gates.SiSWAP(0, 1)], nqubits=2)
+    target_state = np.zeros_like(final_state)
+    target_state[1] = 1.0 / np.sqrt(2)
+    target_state[2] = 1.0j / np.sqrt(2)
+    backend.assert_allclose(final_state, target_state)
+
+    assert not gates.SiSWAP(0, 1).clifford
+    assert gates.SiSWAP(0, 1).unitary
+
+
 def test_fswap(backend):
     nqubits = 2
     initial_state = random_statevector(2**nqubits, backend=backend)