add su4 gate

Author: refraction-ray

CHANGELOG.md

@@ -10,6 +10,8 @@
 
 - Add exp and expm for torch backend.
 
+- Add `su4` as a generic parameterized two-qubit gates.
+
 ## v1.4.0
 
 ### Added

tensorcircuit/abstractcircuit.py

@@ -53,6 +53,7 @@
     "any",
     "exp",
     "exp1",
+    "su4",
 ]
 mpogates = ["multicontrol", "mpo"]
 gate_aliases = [

tensorcircuit/gates.py

@@ -870,6 +870,43 @@ def exponential_gate_unity(
     return Gate(mat, name="exp1-" + name)
 
 
+def su4_gate(theta: Tensor, name: str = "su(4)") -> Gate:
+    r"""
+    Two-qubit general SU(4) gate.
+
+    :param theta: the angle tensor (15 components) of the gate.
+    :type theta: Tensor
+    :param name: the name of the gate.
+    :type name: str
+    :return: a gate object.
+    :rtype: Gate
+    """
+    theta = num_to_tensor(theta)
+    pauli_ops = array_to_tensor(
+        _ix_matrix,
+        _iy_matrix,
+        _iz_matrix,
+        _xi_matrix,
+        _xx_matrix,
+        _xy_matrix,
+        _xz_matrix,
+        _yi_matrix,
+        _yx_matrix,
+        _yy_matrix,
+        _yz_matrix,
+        _zi_matrix,
+        _zx_matrix,
+        _zy_matrix,
+        _zz_matrix,
+    )
+    generator = backend.sum(
+        backend.stack([theta[i] * pauli_ops[i] for i in range(15)]), axis=0
+    )
+    mat = backend.expm(-1j * generator)
+    mat = backend.reshape2(mat)
+    return Gate(mat, name=name)
+
+
 exp1_gate = exponential_gate_unity
 # exp1 = exponential_gate_unity
 rzz_gate = partial(exp1_gate, unitary=_zz_matrix, half=True)
@@ -974,6 +1011,7 @@ def meta_vgate() -> None:
         "rzz",
         "rxx",
         "ryy",
+        "su4",
     ]:
         for funcname in [f, f + "gate"]:
             setattr(thismodule, funcname, GateVF(getattr(thismodule, f + "_gate"), f))

tests/test_circuit.py

@@ -260,6 +260,18 @@ def test_expectation(backend):
     )
 
 
+@pytest.mark.parametrize("backend", [lf("npb"), lf("tfb"), lf("jaxb"), lf("torchb")])
+def test_su4gate(backend, highp):
+    def f(param):
+        c = tc.Circuit(8)
+        for d in range(6):
+            for i in range(8):
+                c.su4(i, (i + 1) % 8, theta=param[d, i])
+        return tc.backend.norm(c.state())
+
+    np.testing.assert_allclose(f(tc.backend.ones([6, 8, 15])), 1, atol=1e-7)
+
+
 @pytest.mark.parametrize("backend", [lf("npb"), lf("tfb"), lf("jaxb"), lf("cpb")])
 def test_exp1(backend):
     @partial(tc.backend.jit, jit_compile=True)