Fix bug in logic for releasing qubits after joint measurement

src/Simulation/Simulators.Tests/Circuits/CoreOperations.qs

@@ -342,6 +342,7 @@ namespace Microsoft.Quantum.Tests.CoreOperations {
 
 // Using a different namespace, so tests can be auto-discovered.
 namespace Microsoft.Quantum.Simulation.Simulators.Tests.Circuits {
+    open Microsoft.Quantum.Simulation.Simulators.Tests.Circuits.Generics;
     open Microsoft.Quantum.Intrinsic;
     open Microsoft.Quantum.Convert;
     open Microsoft.Quantum.Diagnostics;
@@ -559,6 +560,14 @@ namespace Microsoft.Quantum.Simulation.Simulators.Tests.Circuits {
         }
     }
 
+    operation ReleaseMeasureMultipleQubitCheck() : Unit {
+        using (qs = Qubit[2]) {
+            ApplyToEach(H, qs);
+            let r = Measure([PauliZ, PauliZ], qs);
+            // Should raise an exception
+        }
+    }
+
     operation BorrowingQubitCheck () : Unit
     {
         using (q = Qubit())

src/Simulation/Simulators.Tests/QuantumSimulatorTests/QubitReleaseTest.cs

@@ -33,6 +33,15 @@ public async Task MeasuredQubitReleaseTest()
             await ReleaseMeasuredQubitCheck.Run(sim);
         }
 
+        //test to check that qubits cannot be released after multiple qubit measure
+        [Fact]
+        public async Task MeasuredMultipleQubitsReleaseTest()
+        {
+            var sim = new QuantumSimulator();
+
+            await Assert.ThrowsAsync<ReleasedQubitsAreNotInZeroState>(() => ReleaseMeasureMultipleQubitCheck.Run(sim));
+        }
+
         //test to check that qubit that is released and reallocated is in state |0>
         [Fact]
         public async Task ReallocateQubitInGroundStateTest()

src/Simulation/Simulators.Tests/TestProjects/TargetedExe/Program.qs

@@ -10,6 +10,7 @@
         let arr = Default<Qubit[]>();
         using (qs = Qubit[2])  {
             Ignore(Measure([PauliX, PauliX], qs));
+            ResetAll(qs);
             return "TargetedExe";
         }
     }

src/Simulation/Simulators/QuantumSimulator/Measure.cs

@@ -31,10 +31,11 @@ public QSimMeasure(QuantumSimulator m) : base(m)
                 {
                     throw new InvalidOperationException($"Both input arrays for {this.GetType().Name} (paulis,qubits), must be of same size");
                 }
-                foreach (Qubit q in qubits)
+                if (qubits.Length == 1)
                 {
-                    //setting qubit as measured to allow for release
-                    q.IsMeasured = true;
+                    // When we are operating on a single qubit we will collapse the state, so mark
+                    // that qubit as measured.
+                    qubits[0].IsMeasured = true;
                 }
                 return Measure(Simulator.Id, (uint)paulis.Length, paulis.ToArray(), qubits.GetIds()).ToResult();
             };