Depth Counter

The Depth Counter is a part of the quantum computer Trace Simulator. It is used to gather counts of the depth of every operation invoked in a quantum program. All operations from Microsoft.Quantum.Primitive are expressed in terms of single qubit rotations, T gates, single qubit Clifford gates, CNOT gates and measurements of multi-qubit Pauli observables. Users can set the depth for each of the primitive operations.

By default, all operations have depth 0 except the T gate which has depth 1. This means that by default, only the T depth of operations is computed (which is often desirable). Collected statistics are aggregated over all the edges of the operations call graph.

Let us now compute the T depth of the CCNOT operation. We will use the following Q# driver code:

open Microsoft.Quantum.Primitive;
operation CCNOTDriver() : () {
    body {
        using( qubits = Qubit[3] ) {
            CCNOT(qubits[0],qubits[1],qubits[2]);
            T(qubits[0]);
        } 
    }
}

Using Depth Counter within a C# Program

To check that CCNOT has T depth 5 and CCNOTDriver has T depth 6 we can use the following C# code:

using Microsoft.Quantum.Simulation.Simulators.QCTraceSimulators;
using System.Diagnostics;
var config = new QCTraceSimulatorConfiguration();
config.useDepthCounter = true;
var sim = new QCTraceSimulator(config);
var res = CCNOTDriver.Run(sim).Result;

double tDepth = sim.GetMetric<Primitive.CCNOT, CCNOTDriver>(DepthCounter.Metrics.Depth);
double tDepthAll = sim.GetMetric<CCNOTDriver>(DepthCounter.Metrics.Depth);

The first part of the program executes CCNOTDriver. In the second part, we use the method QCTraceSimulator.GetMetric to get the T depth of CCNOT and CCNOTDriver:

double tDepth = sim.GetMetric<Primitive.CCNOT, CCNOTDriver>(DepthCounter.Metrics.Depth);
double tDepthAll = sim.GetMetric<CCNOTDriver>(DepthCounter.Metrics.Depth);

Finally, to output all the statistics collected by Depth Counter in CSV format we can use the following:

string csvSummary = sim.ToCSV()[MetricsCountersNames.depthCounter];

See also