Merge pull request #109 from qiboteam/identity_density_matrix

Identity density matrix

Author: scarrazza

src/qibojit/backends/gpu.py

@@ -156,6 +156,15 @@ def zero_density_matrix(self, nqubits):
         self.cp.cuda.stream.get_current_stream().synchronize()
         return state.reshape((n, n))
 
+    def identity_density_matrix(self, nqubits):
+        n = 1 << nqubits
+        kernel = self.gates.get(f"initial_state_kernel_{self.kernel_type}")
+        state = self.cp.eye(n, dtype=self.dtype)
+        kernel((1,), (1,), [state])
+        self.cp.cuda.stream.get_current_stream().synchronize()
+        state /= 2**nqubits
+        return state.reshape((n, n))
+
     def plus_state(self, nqubits):
         state = self.cp.ones(2**nqubits, dtype=self.dtype)
         state /= self.cp.sqrt(2**nqubits)

src/qibojit/backends/matrices.py

@@ -1,21 +1,9 @@
 import sys
+from functools import cached_property  # pylint: disable=E0611
 
 import numpy as np
 from qibo.backends.npmatrices import NumpyMatrices
 
-if sys.version_info.minor >= 8:
-    from functools import cached_property  # pylint: disable=E0611
-else:
-    # Custom ``cached_property`` because it is not available for Python < 3.8
-    from functools import lru_cache
-
-    def cached_property(func):  # pragma: no cover
-        @property
-        def wrapper(self):
-            return lru_cache()(func)(self)
-
-        return wrapper
-
 
 class CuQuantumMatrices(NumpyMatrices):
     # These matrices are used by the custom operators and may

src/qibojit/tests/test_ops.py

@@ -20,6 +20,13 @@ def test_zero_state(backend, dtype, is_matrix):
     backend.assert_allclose(final_state, target_state)
 
 
+def test_identity_density_matrix(backend, dtype):
+    set_precision(dtype, backend)
+    final_state = backend.identity_density_matrix(4)
+    target_state = np.eye(16, 16, dtype=dtype) / 16
+    backend.assert_allclose(final_state, target_state)
+
+
 @pytest.mark.parametrize("is_matrix", [False, True])
 def test_plus_state(backend, dtype, is_matrix):
     set_precision(dtype, backend)