From 8d57cf6a90f3b270cc0defe655f66c17f24a01f2 Mon Sep 17 00:00:00 2001
From: Ari Jordan <ari.jordan@gmx.ch>
Date: Tue, 10 Mar 2020 17:36:23 +0100
Subject: [PATCH 01/19] Add histogram plotting function for Simulator

---
 projectq/libs/__init__.py             |  2 +
 projectq/libs/hist/__init__.py        | 14 +++++
 projectq/libs/hist/_histogram.py      | 36 ++++++++++++
 projectq/libs/hist/_histogram_test.py | 82 +++++++++++++++++++++++++++
 pytest.ini                            |  1 +
 5 files changed, 135 insertions(+)
 create mode 100644 projectq/libs/hist/__init__.py
 create mode 100644 projectq/libs/hist/_histogram.py
 create mode 100644 projectq/libs/hist/_histogram_test.py

diff --git a/projectq/libs/__init__.py b/projectq/libs/__init__.py
index ee1451dcd..9c1d76507 100755
--- a/projectq/libs/__init__.py
+++ b/projectq/libs/__init__.py
@@ -11,3 +11,5 @@
 #   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 #   See the License for the specific language governing permissions and
 #   limitations under the License.
+
+from projectq.libs.hist import histogram
diff --git a/projectq/libs/hist/__init__.py b/projectq/libs/hist/__init__.py
new file mode 100644
index 000000000..de0c3d4ec
--- /dev/null
+++ b/projectq/libs/hist/__init__.py
@@ -0,0 +1,14 @@
+#   Copyright 2017 ProjectQ-Framework (www.projectq.ch)
+#
+#   Licensed under the Apache License, Version 2.0 (the "License");
+#   you may not use this file except in compliance with the License.
+#   You may obtain a copy of the License at
+#
+#       http://www.apache.org/licenses/LICENSE-2.0
+#
+#   Unless required by applicable law or agreed to in writing, software
+#   distributed under the License is distributed on an "AS IS" BASIS,
+#   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#   See the License for the specific language governing permissions and
+#   limitations under the License.
+from ._histogram import histogram
diff --git a/projectq/libs/hist/_histogram.py b/projectq/libs/hist/_histogram.py
new file mode 100644
index 000000000..41f94f618
--- /dev/null
+++ b/projectq/libs/hist/_histogram.py
@@ -0,0 +1,36 @@
+import matplotlib.pyplot as plt
+from projectq.backends import Simulator
+
+def histogram(sim, qureg):
+    #qubit_ids = set()
+    #for qb in qubit_list:
+    #qubit_ids.add(qb.id)
+    qubit_list = []
+    for q in qureg:
+        if(isinstance(q, list)):
+            for qb in q:
+                qubit_list.append(qb)
+        else:
+            qubit_list.append(q)
+    if len(qubit_list) > 5:
+        print(f'Warning: For {len(qubit_list)} qubits there are 2^{len(qubit_list)} different outcomes')
+        print("The resulting histogram may look bad and/or take too long")
+        print("Consider calling histogram() with a sublist of the qubits", flush=True)
+    outcome = [0] * len(qubit_list)
+    n_outcomes = (1 << len(qubit_list))
+    probabilities = {}
+    for i in range (n_outcomes):
+        for pos in range (len(qubit_list)):
+            if((1 << pos) & i):
+                outcome[pos] = 1
+            else:
+                outcome[pos] = 0
+        str1 = ""
+        probabilities[''.join([str(bit) for bit in outcome])] = sim.get_probability(outcome, qubit_list)
+    fig, axes = plt.subplots(figsize = (min(21.2, 2 + 0.6 * (1 << len(qubit_list))), 7))
+    names = list(probabilities.keys())
+    values = list(probabilities.values())
+    axes.bar(names, values)
+    fig.suptitle('Measurement Probabilities')
+    plt.show()
+    return(fig, axes, probabilities)
diff --git a/projectq/libs/hist/_histogram_test.py b/projectq/libs/hist/_histogram_test.py
new file mode 100644
index 000000000..c097e1e45
--- /dev/null
+++ b/projectq/libs/hist/_histogram_test.py
@@ -0,0 +1,82 @@
+from projectq import MainEngine
+from projectq.ops import H, C, X, Measure, All
+from projectq.backends import Simulator
+from projectq.libs import histogram
+import pytest
+from pytest import approx
+import matplotlib
+
+@pytest.fixture(scope="module")
+def matplotlib_setup():
+    old_backend = matplotlib.get_backend()
+    matplotlib.use('agg') # avoid showing the histogram plots
+    yield
+    matplotlib.use(old_backend)
+
+def test_qubit(matplotlib_setup):
+    sim = Simulator()
+    eng = MainEngine(sim)
+    qb = eng.allocate_qubit()
+    eng.flush()
+    fig, axes, prob = histogram(sim, qb)
+    assert prob["0"] == approx(1)
+    assert prob["1"] == approx(0)
+    H | qb
+    eng.flush()
+    fig, axes, prob = histogram(sim, qb)
+    assert prob["0"] == approx(0.5)
+    Measure | qb
+    eng.flush()
+    fig, axes, prob = histogram(sim, qb)
+    assert prob["0"] == approx(1) or prob["1"] == approx(1)
+
+
+def test_qureg(matplotlib_setup):
+    sim = Simulator()
+    eng = MainEngine(sim)
+    qr = eng.allocate_qureg(3)
+    eng.flush()
+    fig, axes, prob = histogram(sim, qr)
+    assert prob["000"] == approx(1)
+    assert prob ["110"] == approx(0)
+    H | qr[0]
+    C(X, 1) | (qr[0], qr[1])
+    H | qr[2]
+    eng.flush()
+    fig, axes, prob = histogram(sim, qr)
+    assert prob["110"] == approx(0.25)
+    assert prob["100"] == approx(0)
+    All(Measure) | qr
+    eng.flush()
+    fig, axes, prob = histogram(sim, qr)
+    assert prob["000"] == approx(1) or prob["001"] == approx(1) or prob["110"] == approx(1) or prob["111"] == approx(1)
+    assert prob["000"] + prob["001"] + prob["110"] + prob["111"] == approx(1)
+
+
+def test_combination(matplotlib_setup):
+    sim = Simulator()
+    eng = MainEngine(sim)
+    qr = eng.allocate_qureg(2)
+    qb = eng.allocate_qubit()
+    eng.flush()
+    fig, axes, prob = histogram(sim, [qr, qb])
+    assert prob["000"] == approx(1)
+    H | qr[0]
+    C(X, 1)| (qr[0], qr[1])
+    H | qb
+    Measure | qr[0]
+    eng.flush()
+    fig, axes, prob = histogram(sim, [qr, qb])
+    assert (prob["000"] == approx(0.5) and prob["001"] == approx(0.5)) or (prob["110"] == approx(0.5) and prob["111"] == approx(0.5))
+    assert prob["100"] == approx(0)
+    Measure | qb
+
+
+def test_too_many_qubits():
+    sim = Simulator()
+    eng = MainEngine(sim)
+    qr = eng.allocate_qureg(6)
+    eng.flush()
+    fig, axes, prob = histogram(sim, qr)
+    assert prob["000000"] == approx(1)
+    All(Measure)
diff --git a/pytest.ini b/pytest.ini
index fab634b12..d17d4ce2e 100755
--- a/pytest.ini
+++ b/pytest.ini
@@ -4,3 +4,4 @@ testpaths = projectq
 filterwarnings =
     error
     ignore:the matrix subclass is not the recommended way:PendingDeprecationWarning
+    ignore:Matplotlib is currently using agg, which is a non-GUI backend, so cannot show the figure.

From 29989359dbaee89c285ba23503a008503373750d Mon Sep 17 00:00:00 2001
From: Ari Jordan 80055203 <ari.jordan@huawei.com>
Date: Sat, 14 Mar 2020 11:20:39 +0000
Subject: [PATCH 02/19] Apply suggestion to projectq/libs/__init__.py

---
 projectq/libs/__init__.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/projectq/libs/__init__.py b/projectq/libs/__init__.py
index 9c1d76507..a153103d4 100755
--- a/projectq/libs/__init__.py
+++ b/projectq/libs/__init__.py
@@ -12,4 +12,3 @@
 #   See the License for the specific language governing permissions and
 #   limitations under the License.
 
-from projectq.libs.hist import histogram

From 24accda2516541dfb9fd11e1cb57654b4f1d589a Mon Sep 17 00:00:00 2001
From: Ari Jordan 80055203 <ari.jordan@huawei.com>
Date: Sat, 14 Mar 2020 11:21:03 +0000
Subject: [PATCH 03/19] Apply suggestion to projectq/libs/hist/__init__.py

---
 projectq/libs/hist/__init__.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/projectq/libs/hist/__init__.py b/projectq/libs/hist/__init__.py
index de0c3d4ec..219348fc0 100644
--- a/projectq/libs/hist/__init__.py
+++ b/projectq/libs/hist/__init__.py
@@ -1,4 +1,4 @@
-#   Copyright 2017 ProjectQ-Framework (www.projectq.ch)
+#   Copyright 2020 ProjectQ-Framework (www.projectq.ch)
 #
 #   Licensed under the Apache License, Version 2.0 (the "License");
 #   you may not use this file except in compliance with the License.

From 0aae42c3158dc48dd21a70eaa44dfdfa224593d9 Mon Sep 17 00:00:00 2001
From: Ari Jordan 80055203 <ari.jordan@huawei.com>
Date: Sat, 14 Mar 2020 11:26:03 +0000
Subject: [PATCH 04/19] Update _histogram.py

---
 projectq/libs/hist/_histogram.py | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/projectq/libs/hist/_histogram.py b/projectq/libs/hist/_histogram.py
index 41f94f618..8ad689479 100644
--- a/projectq/libs/hist/_histogram.py
+++ b/projectq/libs/hist/_histogram.py
@@ -1,3 +1,17 @@
+#   Copyright 2020 ProjectQ-Framework (www.projectq.ch)
+#
+#   Licensed under the Apache License, Version 2.0 (the "License");
+#   you may not use this file except in compliance with the License.
+#   You may obtain a copy of the License at
+#
+#       http://www.apache.org/licenses/LICENSE-2.0
+#
+#   Unless required by applicable law or agreed to in writing, software
+#   distributed under the License is distributed on an "AS IS" BASIS,
+#   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#   See the License for the specific language governing permissions and
+#   limitations under the License.
+
 import matplotlib.pyplot as plt
 from projectq.backends import Simulator
 

From 0e870de10dcc37adfbde0c6426cab8cb1bb2c128 Mon Sep 17 00:00:00 2001
From: Ari Jordan 80055203 <ari.jordan@huawei.com>
Date: Sat, 14 Mar 2020 11:28:08 +0000
Subject: [PATCH 05/19] Apply suggestion to projectq/libs/hist/_histogram.py

---
 projectq/libs/hist/_histogram.py | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/projectq/libs/hist/_histogram.py b/projectq/libs/hist/_histogram.py
index 8ad689479..837eea61c 100644
--- a/projectq/libs/hist/_histogram.py
+++ b/projectq/libs/hist/_histogram.py
@@ -22,8 +22,7 @@ def histogram(sim, qureg):
     qubit_list = []
     for q in qureg:
         if(isinstance(q, list)):
-            for qb in q:
-                qubit_list.append(qb)
+            qubit_list.extend(q)
         else:
             qubit_list.append(q)
     if len(qubit_list) > 5:

From 360efb62dc8e664d1e8ac483958105010b72f888 Mon Sep 17 00:00:00 2001
From: Ari Jordan 80055203 <ari.jordan@huawei.com>
Date: Sat, 14 Mar 2020 11:28:20 +0000
Subject: [PATCH 06/19] Apply suggestion to projectq/libs/hist/_histogram.py

---
 projectq/libs/hist/_histogram.py | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/projectq/libs/hist/_histogram.py b/projectq/libs/hist/_histogram.py
index 837eea61c..7da3fa091 100644
--- a/projectq/libs/hist/_histogram.py
+++ b/projectq/libs/hist/_histogram.py
@@ -16,9 +16,6 @@
 from projectq.backends import Simulator
 
 def histogram(sim, qureg):
-    #qubit_ids = set()
-    #for qb in qubit_list:
-    #qubit_ids.add(qb.id)
     qubit_list = []
     for q in qureg:
         if(isinstance(q, list)):

From a537eb856e16736b14b48d6b73080f44f7f018fe Mon Sep 17 00:00:00 2001
From: Ari Jordan <ari.jordan@gmx.ch>
Date: Mon, 16 Mar 2020 15:10:38 +0100
Subject: [PATCH 07/19] Add docstrings

---
 projectq/libs/hist/__init__.py        |  6 ++++
 projectq/libs/hist/_histogram.py      | 19 +++++++++-
 projectq/libs/hist/_histogram_test.py | 52 ++++++++++++++++++---------
 3 files changed, 60 insertions(+), 17 deletions(-)

diff --git a/projectq/libs/hist/__init__.py b/projectq/libs/hist/__init__.py
index 219348fc0..088766263 100644
--- a/projectq/libs/hist/__init__.py
+++ b/projectq/libs/hist/__init__.py
@@ -11,4 +11,10 @@
 #   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 #   See the License for the specific language governing permissions and
 #   limitations under the License.
+
+"""
+contains a function to plot measurement outcome probabilities
+as a histogram for the simulator
+"""
+
 from ._histogram import histogram
diff --git a/projectq/libs/hist/_histogram.py b/projectq/libs/hist/_histogram.py
index 7da3fa091..0ffb1aab4 100644
--- a/projectq/libs/hist/_histogram.py
+++ b/projectq/libs/hist/_histogram.py
@@ -16,6 +16,24 @@
 from projectq.backends import Simulator
 
 def histogram(sim, qureg):
+    """
+    Make a measurement outcome probability histogram for the given qubits
+
+    Args:
+        sim (Simulator): The simulator to call get_probability
+        qureg (list of qubits and/or quregs): The qubits,
+        for which to make the histogram
+
+    Returns:
+        A tuple (fig, axes, probabilities), where:
+        fig: The histogram as figure
+        axes: The axes of the histogram
+        probabilities (dict): A dictionary mapping outcomes as string
+        to their probabilities
+
+    Note:
+        Don't forget to flush() before using this function.
+    """
     qubit_list = []
     for q in qureg:
         if(isinstance(q, list)):
@@ -42,5 +60,4 @@ def histogram(sim, qureg):
     values = list(probabilities.values())
     axes.bar(names, values)
     fig.suptitle('Measurement Probabilities')
-    plt.show()
     return(fig, axes, probabilities)
diff --git a/projectq/libs/hist/_histogram_test.py b/projectq/libs/hist/_histogram_test.py
index c097e1e45..703d488a6 100644
--- a/projectq/libs/hist/_histogram_test.py
+++ b/projectq/libs/hist/_histogram_test.py
@@ -1,10 +1,24 @@
+#   Copyright 2020 ProjectQ-Framework (www.projectq.ch)
+#
+#   Licensed under the Apache License, Version 2.0 (the "License");
+#   you may not use this file except in compliance with the License.
+#   You may obtain a copy of the License at
+#
+#       http://www.apache.org/licenses/LICENSE-2.0
+#
+#   Unless required by applicable law or agreed to in writing, software
+#   distributed under the License is distributed on an "AS IS" BASIS,
+#   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#   See the License for the specific language governing permissions and
+#   limitations under the License.
+
 from projectq import MainEngine
 from projectq.ops import H, C, X, Measure, All
 from projectq.backends import Simulator
-from projectq.libs import histogram
+from projectq.libs.hist import histogram
 import pytest
-from pytest import approx
 import matplotlib
+import matplotlib.pyplot as plt
 
 @pytest.fixture(scope="module")
 def matplotlib_setup():
@@ -19,16 +33,17 @@ def test_qubit(matplotlib_setup):
     qb = eng.allocate_qubit()
     eng.flush()
     fig, axes, prob = histogram(sim, qb)
-    assert prob["0"] == approx(1)
-    assert prob["1"] == approx(0)
+    assert prob["0"] == pytest.approx(1)
+    assert prob["1"] == pytest.approx(0)
     H | qb
     eng.flush()
     fig, axes, prob = histogram(sim, qb)
-    assert prob["0"] == approx(0.5)
+    assert prob["0"] == pytest.approx(0.5)
     Measure | qb
     eng.flush()
     fig, axes, prob = histogram(sim, qb)
-    assert prob["0"] == approx(1) or prob["1"] == approx(1)
+    assert prob["0"] == pytest.approx(1) or prob["1"] == pytest.approx(1)
+    plt.show()
 
 
 def test_qureg(matplotlib_setup):
@@ -37,20 +52,22 @@ def test_qureg(matplotlib_setup):
     qr = eng.allocate_qureg(3)
     eng.flush()
     fig, axes, prob = histogram(sim, qr)
-    assert prob["000"] == approx(1)
-    assert prob ["110"] == approx(0)
+    assert prob["000"] == pytest.approx(1)
+    assert prob ["110"] == pytest.approx(0)
     H | qr[0]
     C(X, 1) | (qr[0], qr[1])
     H | qr[2]
     eng.flush()
     fig, axes, prob = histogram(sim, qr)
-    assert prob["110"] == approx(0.25)
-    assert prob["100"] == approx(0)
+    assert prob["110"] == pytest.approx(0.25)
+    assert prob["100"] == pytest.approx(0)
     All(Measure) | qr
     eng.flush()
     fig, axes, prob = histogram(sim, qr)
-    assert prob["000"] == approx(1) or prob["001"] == approx(1) or prob["110"] == approx(1) or prob["111"] == approx(1)
-    assert prob["000"] + prob["001"] + prob["110"] + prob["111"] == approx(1)
+    assert prob["000"] == pytest.approx(1) or prob["001"] == pytest.approx(1) \
+        or prob["110"] == pytest.approx(1) or prob["111"] == pytest.approx(1)
+    assert prob["000"] + prob["001"] + prob["110"] + prob["111"] == pytest.approx(1)
+    plt.show()
 
 
 def test_combination(matplotlib_setup):
@@ -60,16 +77,18 @@ def test_combination(matplotlib_setup):
     qb = eng.allocate_qubit()
     eng.flush()
     fig, axes, prob = histogram(sim, [qr, qb])
-    assert prob["000"] == approx(1)
+    assert prob["000"] == pytest.approx(1)
     H | qr[0]
     C(X, 1)| (qr[0], qr[1])
     H | qb
     Measure | qr[0]
     eng.flush()
     fig, axes, prob = histogram(sim, [qr, qb])
-    assert (prob["000"] == approx(0.5) and prob["001"] == approx(0.5)) or (prob["110"] == approx(0.5) and prob["111"] == approx(0.5))
-    assert prob["100"] == approx(0)
+    assert (prob["000"] == pytest.approx(0.5) and prob["001"] == pytest.approx(0.5)) \
+        or (prob["110"] == pytest.approx(0.5) and prob["111"] == pytest.approx(0.5))
+    assert prob["100"] == pytest.approx(0)
     Measure | qb
+    plt.show()
 
 
 def test_too_many_qubits():
@@ -78,5 +97,6 @@ def test_too_many_qubits():
     qr = eng.allocate_qureg(6)
     eng.flush()
     fig, axes, prob = histogram(sim, qr)
-    assert prob["000000"] == approx(1)
+    assert prob["000000"] == pytest.approx(1)
     All(Measure)
+    plt.show()

From baf739dc56b3e102c919a18ca4c3d720fde30dc2 Mon Sep 17 00:00:00 2001
From: Ari Jordan <ari.jordan@gmx.ch>
Date: Mon, 16 Mar 2020 15:30:17 +0100
Subject: [PATCH 08/19] All suggestions applied.

---
 projectq/libs/hist/_histogram.py | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/projectq/libs/hist/_histogram.py b/projectq/libs/hist/_histogram.py
index 0ffb1aab4..2ae87a7a7 100644
--- a/projectq/libs/hist/_histogram.py
+++ b/projectq/libs/hist/_histogram.py
@@ -17,7 +17,7 @@
 
 def histogram(sim, qureg):
     """
-    Make a measurement outcome probability histogram for the given qubits
+    Make a measurement outcome probability histogram for the given qubits.
 
     Args:
         sim (Simulator): The simulator to call get_probability
@@ -41,9 +41,9 @@ def histogram(sim, qureg):
         else:
             qubit_list.append(q)
     if len(qubit_list) > 5:
-        print(f'Warning: For {len(qubit_list)} qubits there are 2^{len(qubit_list)} different outcomes')
-        print("The resulting histogram may look bad and/or take too long")
-        print("Consider calling histogram() with a sublist of the qubits", flush=True)
+        print('Warning: For ', len(qubit_list), ' qubits there are 2^', len(qubit_list),  ' different outcomes.', sep='')
+        print("The resulting histogram may look bad and/or take too long.")
+        print("Consider calling histogram() with a sublist of the qubits.", flush=True)
     outcome = [0] * len(qubit_list)
     n_outcomes = (1 << len(qubit_list))
     probabilities = {}

From e575cc2ce204f07fed7641a00dc491b136aac322 Mon Sep 17 00:00:00 2001
From: Damien Nguyen <damien1@huawei.com>
Date: Tue, 17 Mar 2020 08:16:47 +0000
Subject: [PATCH 09/19] Minor fixes for _histogram.py

---
 projectq/libs/hist/_histogram.py | 33 +++++++++++++++++++-------------
 1 file changed, 20 insertions(+), 13 deletions(-)

diff --git a/projectq/libs/hist/_histogram.py b/projectq/libs/hist/_histogram.py
index 2ae87a7a7..56b6cc2f0 100644
--- a/projectq/libs/hist/_histogram.py
+++ b/projectq/libs/hist/_histogram.py
@@ -13,7 +13,7 @@
 #   limitations under the License.
 
 import matplotlib.pyplot as plt
-from projectq.backends import Simulator
+
 
 def histogram(sim, qureg):
     """
@@ -22,7 +22,7 @@ def histogram(sim, qureg):
     Args:
         sim (Simulator): The simulator to call get_probability
         qureg (list of qubits and/or quregs): The qubits,
-        for which to make the histogram
+            for which to make the histogram
 
     Returns:
         A tuple (fig, axes, probabilities), where:
@@ -32,32 +32,39 @@ def histogram(sim, qureg):
         to their probabilities
 
     Note:
-        Don't forget to flush() before using this function.
+        Don't forget to call eng.flush() before using this function.
     """
     qubit_list = []
     for q in qureg:
-        if(isinstance(q, list)):
+        if isinstance(q, list):
             qubit_list.extend(q)
         else:
             qubit_list.append(q)
+
     if len(qubit_list) > 5:
-        print('Warning: For ', len(qubit_list), ' qubits there are 2^', len(qubit_list),  ' different outcomes.', sep='')
+        print('Warning: For {0} qubits there are 2^{0} different outcomes'.
+              format(len(qubit_list)))
         print("The resulting histogram may look bad and/or take too long.")
-        print("Consider calling histogram() with a sublist of the qubits.", flush=True)
+        print("Consider calling histogram() with a sublist of the qubits.",
+              flush=True)
+
     outcome = [0] * len(qubit_list)
     n_outcomes = (1 << len(qubit_list))
     probabilities = {}
-    for i in range (n_outcomes):
-        for pos in range (len(qubit_list)):
-            if((1 << pos) & i):
+    for i in range(n_outcomes):
+        for pos in range(len(qubit_list)):
+            if (1 << pos) & i:
                 outcome[pos] = 1
             else:
                 outcome[pos] = 0
-        str1 = ""
-        probabilities[''.join([str(bit) for bit in outcome])] = sim.get_probability(outcome, qubit_list)
-    fig, axes = plt.subplots(figsize = (min(21.2, 2 + 0.6 * (1 << len(qubit_list))), 7))
+        probabilities[''.join([str(bit) for bit in outcome
+                               ])] = sim.get_probability(outcome, qubit_list)
+
+    # Empirical figure size for up to 5 qubits
+    fig, axes = plt.subplots(figsize=(min(21.2, 2
+                                          + 0.6 * (1 << len(qubit_list))), 7))
     names = list(probabilities.keys())
     values = list(probabilities.values())
     axes.bar(names, values)
     fig.suptitle('Measurement Probabilities')
-    return(fig, axes, probabilities)
+    return (fig, axes, probabilities)

From 00468a776704722e8a88c3589a025a1745be9764 Mon Sep 17 00:00:00 2001
From: Damien Nguyen <damien1@huawei.com>
Date: Tue, 17 Mar 2020 08:24:13 +0000
Subject: [PATCH 10/19] Minor modifications to _histogram_test.py

---
 projectq/libs/hist/_histogram_test.py | 70 +++++++++++++--------------
 1 file changed, 35 insertions(+), 35 deletions(-)

diff --git a/projectq/libs/hist/_histogram_test.py b/projectq/libs/hist/_histogram_test.py
index 703d488a6..8038009b1 100644
--- a/projectq/libs/hist/_histogram_test.py
+++ b/projectq/libs/hist/_histogram_test.py
@@ -12,91 +12,91 @@
 #   See the License for the specific language governing permissions and
 #   limitations under the License.
 
+import pytest
+import matplotlib
+import matplotlib.pyplot as plt
+
 from projectq import MainEngine
 from projectq.ops import H, C, X, Measure, All
 from projectq.backends import Simulator
 from projectq.libs.hist import histogram
-import pytest
-import matplotlib
-import matplotlib.pyplot as plt
+
 
 @pytest.fixture(scope="module")
 def matplotlib_setup():
     old_backend = matplotlib.get_backend()
-    matplotlib.use('agg') # avoid showing the histogram plots
+    matplotlib.use('agg')  # avoid showing the histogram plots
     yield
     matplotlib.use(old_backend)
 
+
 def test_qubit(matplotlib_setup):
     sim = Simulator()
     eng = MainEngine(sim)
-    qb = eng.allocate_qubit()
+    qubit = eng.allocate_qubit()
     eng.flush()
-    fig, axes, prob = histogram(sim, qb)
+    _, _, prob = histogram(sim, qubit)
     assert prob["0"] == pytest.approx(1)
     assert prob["1"] == pytest.approx(0)
-    H | qb
+    H | qubit
     eng.flush()
-    fig, axes, prob = histogram(sim, qb)
+    _, _, prob = histogram(sim, qubit)
     assert prob["0"] == pytest.approx(0.5)
-    Measure | qb
+    Measure | qubit
     eng.flush()
-    fig, axes, prob = histogram(sim, qb)
+    _, _, prob = histogram(sim, qubit)
     assert prob["0"] == pytest.approx(1) or prob["1"] == pytest.approx(1)
-    plt.show()
 
 
 def test_qureg(matplotlib_setup):
     sim = Simulator()
     eng = MainEngine(sim)
-    qr = eng.allocate_qureg(3)
+    qureg = eng.allocate_qureg(3)
     eng.flush()
-    fig, axes, prob = histogram(sim, qr)
+    _, _, prob = histogram(sim, qureg)
     assert prob["000"] == pytest.approx(1)
-    assert prob ["110"] == pytest.approx(0)
-    H | qr[0]
-    C(X, 1) | (qr[0], qr[1])
-    H | qr[2]
+    assert prob["110"] == pytest.approx(0)
+    H | qureg[0]
+    C(X, 1) | (qureg[0], qureg[1])
+    H | qureg[2]
     eng.flush()
-    fig, axes, prob = histogram(sim, qr)
+    _, _, prob = histogram(sim, qureg)
     assert prob["110"] == pytest.approx(0.25)
     assert prob["100"] == pytest.approx(0)
-    All(Measure) | qr
+    All(Measure) | qureg
     eng.flush()
-    fig, axes, prob = histogram(sim, qr)
+    _, _, prob = histogram(sim, qureg)
     assert prob["000"] == pytest.approx(1) or prob["001"] == pytest.approx(1) \
         or prob["110"] == pytest.approx(1) or prob["111"] == pytest.approx(1)
-    assert prob["000"] + prob["001"] + prob["110"] + prob["111"] == pytest.approx(1)
-    plt.show()
+    assert prob["000"] + prob["001"] + prob["110"] + prob[
+        "111"] == pytest.approx(1)
 
 
 def test_combination(matplotlib_setup):
     sim = Simulator()
     eng = MainEngine(sim)
-    qr = eng.allocate_qureg(2)
-    qb = eng.allocate_qubit()
+    qureg = eng.allocate_qureg(2)
+    qubit = eng.allocate_qubit()
     eng.flush()
-    fig, axes, prob = histogram(sim, [qr, qb])
+    _, _, prob = histogram(sim, [qureg, qubit])
     assert prob["000"] == pytest.approx(1)
-    H | qr[0]
-    C(X, 1)| (qr[0], qr[1])
-    H | qb
-    Measure | qr[0]
+    H | qureg[0]
+    C(X, 1) | (qureg[0], qureg[1])
+    H | qubit
+    Measure | qureg[0]
     eng.flush()
-    fig, axes, prob = histogram(sim, [qr, qb])
+    _, _, prob = histogram(sim, [qureg, qubit])
     assert (prob["000"] == pytest.approx(0.5) and prob["001"] == pytest.approx(0.5)) \
         or (prob["110"] == pytest.approx(0.5) and prob["111"] == pytest.approx(0.5))
     assert prob["100"] == pytest.approx(0)
-    Measure | qb
-    plt.show()
+    Measure | qubit
 
 
 def test_too_many_qubits():
     sim = Simulator()
     eng = MainEngine(sim)
-    qr = eng.allocate_qureg(6)
+    qureg = eng.allocate_qureg(6)
     eng.flush()
-    fig, axes, prob = histogram(sim, qr)
+    _, _, prob = histogram(sim, qureg)
     assert prob["000000"] == pytest.approx(1)
     All(Measure)
-    plt.show()

From 2503c2a7d56c9d09dba3398f6366b3f6f4accc63 Mon Sep 17 00:00:00 2001
From: Damien Nguyen <damien1@huawei.com>
Date: Tue, 17 Mar 2020 08:51:51 +0000
Subject: [PATCH 11/19] Check that some message was printed in case of too many
 qubits

---
 projectq/libs/hist/_histogram_test.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/projectq/libs/hist/_histogram_test.py b/projectq/libs/hist/_histogram_test.py
index 8038009b1..f4a0ab4ef 100644
--- a/projectq/libs/hist/_histogram_test.py
+++ b/projectq/libs/hist/_histogram_test.py
@@ -92,11 +92,13 @@ def test_combination(matplotlib_setup):
     Measure | qubit
 
 
-def test_too_many_qubits():
+def test_too_many_qubits(matplotlib_setup, capsys):
     sim = Simulator()
     eng = MainEngine(sim)
     qureg = eng.allocate_qureg(6)
     eng.flush()
+    l_ref = len(capsys.readouterr().out)
     _, _, prob = histogram(sim, qureg)
+    assert len(capsys.readouterr().out) > l_ref
     assert prob["000000"] == pytest.approx(1)
     All(Measure)

From f95a54f9a93354b60153fb8655d6df6e0f9c9a64 Mon Sep 17 00:00:00 2001
From: Damien Nguyen <damien1@huawei.com>
Date: Wed, 25 Mar 2020 16:25:48 +0100
Subject: [PATCH 12/19] Update __init__.py

Trigger Travis-CI
---
 projectq/libs/__init__.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/projectq/libs/__init__.py b/projectq/libs/__init__.py
index a153103d4..ee1451dcd 100755
--- a/projectq/libs/__init__.py
+++ b/projectq/libs/__init__.py
@@ -11,4 +11,3 @@
 #   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 #   See the License for the specific language governing permissions and
 #   limitations under the License.
-

From edf4bdd447157a3951e35524bae77b1d7dfbc04f Mon Sep 17 00:00:00 2001
From: Damien Nguyen <damien1@huawei.com>
Date: Wed, 25 Mar 2020 17:06:51 +0100
Subject: [PATCH 13/19] Add print_function for Python 2.7

---
 projectq/libs/hist/_histogram.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/projectq/libs/hist/_histogram.py b/projectq/libs/hist/_histogram.py
index 56b6cc2f0..570379ad7 100644
--- a/projectq/libs/hist/_histogram.py
+++ b/projectq/libs/hist/_histogram.py
@@ -12,6 +12,7 @@
 #   See the License for the specific language governing permissions and
 #   limitations under the License.
 
+from __future__ import print_function
 import matplotlib.pyplot as plt
 
 

From b46705e60ec1e958ccaabbf61fff36d59504ca03 Mon Sep 17 00:00:00 2001
From: Damien Nguyen <damien1@huawei.com>
Date: Wed, 25 Mar 2020 17:11:54 +0100
Subject: [PATCH 14/19] Remove unsupported 'flush' keyword for Python 2.7

---
 projectq/libs/hist/_histogram.py | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/projectq/libs/hist/_histogram.py b/projectq/libs/hist/_histogram.py
index 570379ad7..15ac62454 100644
--- a/projectq/libs/hist/_histogram.py
+++ b/projectq/libs/hist/_histogram.py
@@ -46,8 +46,7 @@ def histogram(sim, qureg):
         print('Warning: For {0} qubits there are 2^{0} different outcomes'.
               format(len(qubit_list)))
         print("The resulting histogram may look bad and/or take too long.")
-        print("Consider calling histogram() with a sublist of the qubits.",
-              flush=True)
+        print("Consider calling histogram() with a sublist of the qubits.")
 
     outcome = [0] * len(qubit_list)
     n_outcomes = (1 << len(qubit_list))

From ad566703e80a18f75b26e9c8fb8c3b038615fb96 Mon Sep 17 00:00:00 2001
From: Damien Nguyen <damien1@huawei.com>
Date: Mon, 20 Apr 2020 13:11:06 +0200
Subject: [PATCH 15/19] Update bellpair example with histogram plot of
 probabilities

---
 examples/bellpair_circuit.py | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/examples/bellpair_circuit.py b/examples/bellpair_circuit.py
index 108e224fa..96c001ffe 100755
--- a/examples/bellpair_circuit.py
+++ b/examples/bellpair_circuit.py
@@ -1,13 +1,20 @@
+import matplotlib.pyplot as plt
+
 from projectq import MainEngine
 from projectq.backends import CircuitDrawer
+from projectq.setups.default import get_engine_list
+from projectq.libs.hist import histogram
 
 from teleport import create_bell_pair
 
 # create a main compiler engine
 drawing_engine = CircuitDrawer()
-eng = MainEngine(drawing_engine)
+eng = MainEngine(engine_list = get_engine_list() + [drawing_engine])
 
-create_bell_pair(eng)
+qb0, qb1 = create_bell_pair(eng)
 
 eng.flush()
 print(drawing_engine.get_latex())
+
+histogram(eng.backend, [qb0, qb1])
+plt.show()

From fcbc9bb7edeb744f97167c931a6c08876ef4f265 Mon Sep 17 00:00:00 2001
From: Damien Nguyen <damien1@huawei.com>
Date: Mon, 20 Apr 2020 13:43:09 +0200
Subject: [PATCH 16/19] Add support for other backends

---
 projectq/libs/hist/_histogram.py | 34 ++++++++++++++++++++------------
 1 file changed, 21 insertions(+), 13 deletions(-)

diff --git a/projectq/libs/hist/_histogram.py b/projectq/libs/hist/_histogram.py
index 15ac62454..0ad980892 100644
--- a/projectq/libs/hist/_histogram.py
+++ b/projectq/libs/hist/_histogram.py
@@ -15,13 +15,15 @@
 from __future__ import print_function
 import matplotlib.pyplot as plt
 
+from projectq.backends import Simulator
 
-def histogram(sim, qureg):
+
+def histogram(backend, qureg):
     """
     Make a measurement outcome probability histogram for the given qubits.
 
     Args:
-        sim (Simulator): The simulator to call get_probability
+        backend (BasicEngine): A ProjectQ backend
         qureg (list of qubits and/or quregs): The qubits,
             for which to make the histogram
 
@@ -48,17 +50,23 @@ def histogram(sim, qureg):
         print("The resulting histogram may look bad and/or take too long.")
         print("Consider calling histogram() with a sublist of the qubits.")
 
-    outcome = [0] * len(qubit_list)
-    n_outcomes = (1 << len(qubit_list))
-    probabilities = {}
-    for i in range(n_outcomes):
-        for pos in range(len(qubit_list)):
-            if (1 << pos) & i:
-                outcome[pos] = 1
-            else:
-                outcome[pos] = 0
-        probabilities[''.join([str(bit) for bit in outcome
-                               ])] = sim.get_probability(outcome, qubit_list)
+    if hasattr(backend, 'get_probabilities'):
+        probabilities = backend.get_probabilities(qureg)
+    elif isinstance(backend, Simulator):
+        outcome = [0] * len(qubit_list)
+        n_outcomes = (1 << len(qubit_list))
+        probabilities = {}
+        for i in range(n_outcomes):
+            for pos in range(len(qubit_list)):
+                if (1 << pos) & i:
+                    outcome[pos] = 1
+                else:
+                    outcome[pos] = 0
+            probabilities[''.join([str(bit) for bit in outcome
+                                   ])] = backend.get_probability(
+                                       outcome, qubit_list)
+    else:
+        raise RuntimeError('Unable to retrieve probabilities from backend')
 
     # Empirical figure size for up to 5 qubits
     fig, axes = plt.subplots(figsize=(min(21.2, 2

From e694f3affdbea032c6e26781b5b349692291bf98 Mon Sep 17 00:00:00 2001
From: Damien Nguyen <damien1@huawei.com>
Date: Mon, 20 Apr 2020 13:44:35 +0200
Subject: [PATCH 17/19] Update IBM examples with histogram plots

---
 examples/ibm.py              | 28 ++++++-----
 examples/ibmq_tutorial.ipynb | 90 ++++++++++++++++++++++++++++--------
 2 files changed, 89 insertions(+), 29 deletions(-)

diff --git a/examples/ibm.py b/examples/ibm.py
index 11a81a832..eafd50b80 100755
--- a/examples/ibm.py
+++ b/examples/ibm.py
@@ -1,8 +1,11 @@
-import projectq.setups.ibm
+import matplotlib.pyplot as plt
+import getpass
+
+from projectq import MainEngine
 from projectq.backends import IBMBackend
+from projectq.libs.hist import histogram
 from projectq.ops import Measure, Entangle, All
-from projectq import MainEngine
-import getpass
+import projectq.setups.ibm
 
 
 def run_entangle(eng, num_qubits=3):
@@ -29,9 +32,12 @@ def run_entangle(eng, num_qubits=3):
     eng.flush()
 
     # access the probabilities via the back-end:
-    results = eng.backend.get_probabilities(qureg)
-    for state in results:
-        print("Measured {} with p = {}.".format(state, results[state]))
+    # results = eng.backend.get_probabilities(qureg)
+    # for state in results:
+    #     print("Measured {} with p = {}.".format(state, results[state]))
+    # or plot them directly:
+    histogram(eng.backend, qureg)
+    plt.show()
 
     # return one (random) measurement outcome.
     return [int(q) for q in qureg]
@@ -39,11 +45,11 @@ def run_entangle(eng, num_qubits=3):
 
 if __name__ == "__main__":
     #devices commonly available :
-    #ibmq_16_melbourne (15 qubit)
-    #ibmq_essex (5 qubit)
-    #ibmq_qasm_simulator (32 qubits)
-    device = None #replace by the IBM device name you want to use
-    token = None  #replace by the token given by IBMQ
+    # ibmq_16_melbourne (15 qubit)
+    # ibmq_essex (5 qubit)
+    # ibmq_qasm_simulator (32 qubits)
+    device = None # replace by the IBM device name you want to use
+    token = None  # replace by the token given by IBMQ
     if token is None:
         token = getpass.getpass(prompt='IBM Q token > ')
     if device is None:
diff --git a/examples/ibmq_tutorial.ipynb b/examples/ibmq_tutorial.ipynb
index 464612ca5..358bdcf9e 100644
--- a/examples/ibmq_tutorial.ipynb
+++ b/examples/ibmq_tutorial.ipynb
@@ -2,14 +2,18 @@
  "cells": [
   {
    "cell_type": "markdown",
-   "metadata": {},
+   "metadata": {
+    "collapsed": false
+   },
    "source": [
     "# Running ProjectQ code on IBM Q devices"
    ]
   },
   {
    "cell_type": "markdown",
-   "metadata": {},
+   "metadata": {
+    "collapsed": false
+   },
    "source": [
     "In this tutorial, we will see how to run code on IBM Q devices directly from within ProjectQ. All that is needed is an IBM Q Experience user account. To sign up, visit https://quantumexperience.ng.bluemix.net/.\n",
     "\n",
@@ -19,10 +23,24 @@
     "First, we import all necessary operations (`Entangle`, measurement), the back-end (`IBMBackend`), and the main compiler engine (`MainEngine`). The Entangle operation is defined as a Hadamard gate on the first qubit (creates an equal superposition of |0> and |1>), followed by controlled NOT gates acting on all other qubits controlled on the first."
    ]
   },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "%matplotlib inline\n",
+    "import matplotlib.pyplot as plt"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": 1,
-   "metadata": {},
+   "metadata": {
+    "collapsed": false
+   },
    "outputs": [],
    "source": [
     "import projectq.setups.ibm\n",
@@ -33,7 +51,9 @@
   },
   {
    "cell_type": "markdown",
-   "metadata": {},
+   "metadata": {
+    "collapsed": false
+   },
    "source": [
     "Next, we instantiate a main compiler engine using the IBM Q back-end and the predefined compiler engines which take care of the qubit placement, translation of operations, etc.:"
    ]
@@ -41,7 +61,9 @@
   {
    "cell_type": "code",
    "execution_count": 2,
-   "metadata": {},
+   "metadata": {
+    "collapsed": false
+   },
    "outputs": [],
    "source": [
     "eng = MainEngine(IBMBackend(use_hardware=True, num_runs=1024,\n",
@@ -51,7 +73,9 @@
   },
   {
    "cell_type": "markdown",
-   "metadata": {},
+   "metadata": {
+    "collapsed": false
+   },
    "source": [
     "If `use_hardware` is set to `False`, it will use the IBM Q simulator instead. `num_runs` specifies the number of samples to collect for statistics, `verbose=True` would output additional information which may be helpful for debugging, and the device parameter lets users choose between the two devices (\"ibmqx4\" and \"ibmqx5\").\n",
     "\n",
@@ -61,7 +85,9 @@
   {
    "cell_type": "code",
    "execution_count": 3,
-   "metadata": {},
+   "metadata": {
+    "collapsed": false
+   },
    "outputs": [
     {
      "name": "stdout",
@@ -105,9 +131,12 @@
     "    eng.flush()\n",
     "\n",
     "    # access the probabilities via the back-end:\n",
-    "    results = eng.backend.get_probabilities(qureg)\n",
-    "    for state in results:\n",
-    "        print(\"Measured {} with p = {}.\".format(state, results[state]))\n",
+    "    # results = eng.backend.get_probabilities(qureg)\n",
+    "    # for state in results:\n",
+    "    #     print(\"Measured {} with p = {}.\".format(state, results[state]))\n",
+    "    # or plot them directly:\n",
+    "    histogram(eng.backend, qureg)\n",
+    "    plt.show()\n",
     "\n",
     "    # return one (random) measurement outcome.\n",
     "    return [int(q) for q in qureg]\n",
@@ -117,7 +146,9 @@
   },
   {
    "cell_type": "markdown",
-   "metadata": {},
+   "metadata": {
+    "collapsed": false
+   },
    "source": [
     "## Retrieving a timed-out execution\n",
     "Sometimes, the queue is very long and the waiting times may exceed the limit of 5 minutes. In this case, ProjectQ will raise an exception which contains the job ID, as could be seen above, where the job ID was `5b557df2306393003b746da2`."
@@ -125,7 +156,9 @@
   },
   {
    "cell_type": "markdown",
-   "metadata": {},
+   "metadata": {
+    "collapsed": false
+   },
    "source": [
     "In order to still retrieve all results at a later point in time, one can simply re-run the entire program using a slightly modified back-end:"
    ]
@@ -133,7 +166,9 @@
   {
    "cell_type": "code",
    "execution_count": 4,
-   "metadata": {},
+   "metadata": {
+    "collapsed": false
+   },
    "outputs": [
     {
      "name": "stdout",
@@ -193,7 +228,9 @@
   },
   {
    "cell_type": "markdown",
-   "metadata": {},
+   "metadata": {
+    "collapsed": false
+   },
    "source": [
     "## Entangling more qubits: Using ibmqx5\n",
     "\n",
@@ -209,7 +246,9 @@
   {
    "cell_type": "code",
    "execution_count": 5,
-   "metadata": {},
+   "metadata": {
+    "collapsed": false
+   },
    "outputs": [],
    "source": [
     "import projectq.setups.ibm16  # import setup which contains the grid mapper\n",
@@ -220,7 +259,9 @@
   },
   {
    "cell_type": "markdown",
-   "metadata": {},
+   "metadata": {
+    "collapsed": false
+   },
    "source": [
     "and then re-run the example from before via `run_entangle(eng, num_qubits)`. If an execution times out, it can also be retrieved at a later point by providing the additional `retrieve_execution=\"execution_id\"` parameter to the IBMBackend (but this time with `device='ibmqx5'`)."
    ]
@@ -228,7 +269,9 @@
   {
    "cell_type": "code",
    "execution_count": 6,
-   "metadata": {},
+   "metadata": {
+    "collapsed": false
+   },
    "outputs": [
     {
      "name": "stdout",
@@ -488,8 +531,18 @@
  ],
  "metadata": {
   "kernelspec": {
+   "argv": [
+    "python",
+    "-m",
+    "ipykernel_launcher",
+    "-f",
+    "{connection_file}"
+   ],
    "display_name": "Python 3",
+   "env": null,
+   "interrupt_mode": "signal",
    "language": "python",
+   "metadata": null,
    "name": "python3"
   },
   "language_info": {
@@ -503,7 +556,8 @@
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
    "version": "3.6.5"
-  }
+  },
+  "name": "ibmq_tutorial.ipynb"
  },
  "nbformat": 4,
  "nbformat_minor": 2

From a7497adc150812fe5db20026f74efc3fb7ae8847 Mon Sep 17 00:00:00 2001
From: Damien Nguyen <damien1@huawei.com>
Date: Mon, 20 Apr 2020 13:44:46 +0200
Subject: [PATCH 18/19] Fix error in conf.py

---
 docs/conf.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/docs/conf.py b/docs/conf.py
index 169414e6c..4083653fa 100755
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -1,3 +1,4 @@
+
 #!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 #
@@ -28,7 +29,6 @@
 import projectq.setups.default
 import projectq.setups.grid
 import projectq.setups.ibm
-import projectq.setups.ibm16
 import projectq.setups.linear
 import projectq.setups.restrictedgateset
 import projectq.setups.decompositions

From c787e1ade4b2a6e0861ee867aba78abc231235c5 Mon Sep 17 00:00:00 2001
From: Damien Nguyen <damien1@huawei.com>
Date: Mon, 20 Apr 2020 13:58:49 +0200
Subject: [PATCH 19/19] Increase test coverage

---
 projectq/libs/hist/_histogram.py      |  2 +-
 projectq/libs/hist/_histogram_test.py | 33 ++++++++++++++++++++++++++-
 2 files changed, 33 insertions(+), 2 deletions(-)

diff --git a/projectq/libs/hist/_histogram.py b/projectq/libs/hist/_histogram.py
index 0ad980892..77b0d2c20 100644
--- a/projectq/libs/hist/_histogram.py
+++ b/projectq/libs/hist/_histogram.py
@@ -32,7 +32,7 @@ def histogram(backend, qureg):
         fig: The histogram as figure
         axes: The axes of the histogram
         probabilities (dict): A dictionary mapping outcomes as string
-        to their probabilities
+            to their probabilities
 
     Note:
         Don't forget to call eng.flush() before using this function.
diff --git a/projectq/libs/hist/_histogram_test.py b/projectq/libs/hist/_histogram_test.py
index f4a0ab4ef..1f38573b0 100644
--- a/projectq/libs/hist/_histogram_test.py
+++ b/projectq/libs/hist/_histogram_test.py
@@ -17,7 +17,8 @@
 import matplotlib.pyplot as plt
 
 from projectq import MainEngine
-from projectq.ops import H, C, X, Measure, All
+from projectq.ops import H, C, X, Measure, All, AllocateQubitGate, FlushGate
+from projectq.cengines import DummyEngine, BasicEngine
 from projectq.backends import Simulator
 from projectq.libs.hist import histogram
 
@@ -30,6 +31,36 @@ def matplotlib_setup():
     matplotlib.use(old_backend)
 
 
+def test_invalid_backend(matplotlib_setup):
+    eng = MainEngine(backend=DummyEngine())
+    qubit = eng.allocate_qubit()
+    eng.flush()
+
+    with pytest.raises(RuntimeError):
+        histogram(eng.backend, qubit)
+
+
+def test_backend_get_probabilities_method(matplotlib_setup):
+    class MyBackend(BasicEngine):
+        def get_probabilities(self, qureg):
+            return {'000': 0.5, '111': 0.5}
+
+        def is_available(self, cmd):
+            return True
+
+        def receive(self, command_list):
+            for cmd in command_list:
+                if not isinstance(cmd.gate, FlushGate):
+                    assert isinstance(cmd.gate, AllocateQubitGate)
+
+    eng = MainEngine(backend=MyBackend(), verbose=True)
+    qureg = eng.allocate_qureg(3)
+    eng.flush()
+    _, _, prob = histogram(eng.backend, qureg)
+    assert prob['000'] == 0.5
+    assert prob['111'] == 0.5
+
+
 def test_qubit(matplotlib_setup):
     sim = Simulator()
     eng = MainEngine(sim)