# Quantum Trace Simulator

The Microsoft quantum computer trace simulator executes a quantum program without actually simulating the state of a quantum computer. For this reason, the trace simulator can execute quantum programs that use thousands of qubits. It is useful for two main purposes:

• Debugging classical code that is part of a quantum program.
• Estimating the resources required to run a given instance of a quantum program on a quantum computer.

The trace simulator relies on additional information provided by the user when measurements must be performed. See Section Providing the probability of measurement outcomes for more details on this.

## Providing the Probability of Measurement Outcomes

There are two kinds of measurements that appear in quantum algorithms. The first kind plays an auxiliary role where the user usually knows the probability of the outcomes. In this case the user can write AssertProb from the Microsoft.Quantum.Intrinsic namespace to express this knowledge. The following example illustrates this:

operation TeleportQubit(source : Qubit, target : Qubit) : Unit {
using (qubit = Qubit()) {
H(qubit);
CNOT(qubit, target);
CNOT(source, qubit);
H(source);

AssertProb([PauliZ], [source], Zero, 0.5, "Outcomes must be equally likely", 1e-5);
AssertProb([PauliZ], [q], Zero, 0.5, "Outcomes must be equally likely", 1e-5);

if (M(source) == One)  { Z(target); X(source); }
if (M(q) == One) { X(target); X(q); }
}
}


When the trace simulator executes AssertProb it will record that measuring PauliZ on source and q should be given an outcome of Zero with probability 0.5. When the simulator executes M later, it will find the recorded values of the outcome probabilities and M will return Zero or One with probability 0.5. When the same code is executed on a simulator that keeps track of the quantum state, such a simulator will check that the provided probabilities in AssertProb are correct.

## Running your Program with the Quantum Computer Trace Simulator

The quantum computer trace simulator may be viewed as just another target machine. The C# driver program for using it is very similar to the one for any other quantum Simulator:

using Microsoft.Quantum.Simulation.Core;
using Microsoft.Quantum.Simulation.Simulators;
using Microsoft.Quantum.Simulation.Simulators.QCTraceSimulators;

namespace Quantum.MyProgram
{
class Driver
{
static void Main(string[] args)
{
QCTraceSimulator sim = new QCTraceSimulator();
var res = MyQuantumProgram.Run().Result;
System.Console.WriteLine("Press any key to continue...");
}
}
}


Note that if there is at least one measurement not annotated using AssertProb, the simulator will throw UnconstrainedMeasurementException from the Microsoft.Quantum.Simulation.Simulators.QCTraceSimulators namespace. See the API documentation on UnconstrainedMeasurementException for more details.

In addition to running quantum programs, the trace simulator comes with five components for detecting bugs in programs and performing quantum program resource estimates:

Each of these components may be enabled by setting appropriate flags in QCTraceSimulatorConfiguration. More details about using each of these components are provided in the corresponding reference files. See the API documentation on QCTraceSimulatorConfiguration for specific details.