GitList
Browse code

Add a function to produce a state from a classical bitstring

also add relevant tests.

Joseph Weston authored on 09/11/2019 00:48:19
Showing 2 changed files
qsim/state.py
History View file @ 384e466
... ... 
@@ -8,4 +8,34 @@ The computational basis for :math:`n` qubits is ordered by the number represente
8 8 
 by the associated classical bitstring.
9 9 
 """
10 10
11 
-__all__ = []
11 
+import numpy as np
12 
+
13 
+__all__ = ["from_classical"]
14 
+
15 
+
16 
+def from_classical(bitstring):
17 
+    """Return a quantum state corresponding to a classical bitstring.
18 
+
19 
+    Parameters
20 
+    ----------
21 
+    bitstring : sequence of bits
22 
+        Can be a string like "01011", or a sequence of
23 
+        integers.
24 
+
25 
+    Returns
26 
+    -------
27 
+    state : ndarray[complex]
28 
+        The state vector in the computational basis.
29 
+        Has :math:`2^n` components.
30 
+    """
31 
+
32 
+    bitstring = "".join(map(str, bitstring))
33 
+    n = len(bitstring)
34 
+    try:
35 
+        index = int(bitstring, base=2)
36 
+    except ValueError:
37 
+        raise ValueError("Input is not a classical bitstring") from None
38 
+
39 
+    state = np.zeros(2 ** n, dtype=complex)
40 
+    state[index] = 1
41 
+    return state
tests/test_state.py
History View file @ 384e466
12 42 
new file mode 100644
... ... 
@@ -0,0 +1,49 @@
1 
+import string
2 
+from functools import partial
3 
+
4 
+from hypothesis import given
5 
+import hypothesis.strategies as st
6 
+import numpy as np
7 
+import pytest
8 
+
9 
+import qsim.state
10 
+
11 
+
12 
+MAX_QUBITS = 5
13 
+
14 
+n_qubits = st.integers(1, MAX_QUBITS)
15 
+classical_bitstrings = n_qubits.flatmap(
16 
+    lambda n: st.integers(0, 2 ** n).map(partial("{:0{n}b}".format, n=n))
17 
+)
18 
+invalid_bitstrings = n_qubits.flatmap(
19 
+    lambda n: st.text(alphabet=string.digits, min_size=n, max_size=n)
20 
+).filter(lambda s: any(b not in "01" for b in s))
21 
+
22 
+
23 
+@given(classical_bitstrings)
24 
+def test_from_classical(bitstring):
25 
+
26 
+    state = qsim.state.from_classical(bitstring)
27 
+
28 
+    i = int(bitstring, base=2)
29 
+
30 
+    assert np.issubdtype(state.dtype, np.dtype(complex))
31 
+    assert state.shape == (2 ** len(bitstring),)
32 
+    assert np.linalg.norm(state) == 1
33 
+    assert abs(state[i]) == 1
34 
+
35 
+
36 
+@given(classical_bitstrings)
37 
+def test_from_classical_works_on_integer_lists(bitstring):
38 
+
39 
+    int_list = [int(b) for b in bitstring]
40 
+
41 
+    assert np.all(
42 
+        qsim.state.from_classical(bitstring) == qsim.state.from_classical(int_list)
43 
+    )
44 
+
45 
+
46 
+@given(invalid_bitstrings)
47 
+def test_from_classical_raises_on_bad_input(bitstring):
48 
+    with pytest.raises(ValueError):
49 
+        qsim.state.from_classical(bitstring)