Computing Expectation Values

CUDA-Q provides generic library functions enabling one to compute expectation values of quantum spin operators with respect to a parameterized CUDA-Q kernel. Let’s take a look at an example of this:

C++

// Compile and run with:
// ```
// nvq++ expectation_values.cpp -o d2.x && ./d2.x
// ```

#include <cudaq.h>
#include <cudaq/algorithm.h>

// The example here shows a simple use case for the `cudaq::observe`
// function in computing expected values of provided spin_ops.

struct ansatz {
  auto operator()(double theta) __qpu__ {
    cudaq::qvector q(2);
    x(q[0]);
    ry(theta, q[1]);
    x<cudaq::ctrl>(q[1], q[0]);
  }
};

int main() {

  // Build up your spin op algebraically
  using namespace cudaq::spin;
  cudaq::spin_op h = 5.907 - 2.1433 * x(0) * x(1) - 2.1433 * y(0) * y(1) +
                     .21829 * z(0) - 6.125 * z(1);

  // Observe takes the kernel, the spin_op, and the concrete
  // parameters for the kernel
  double energy = cudaq::observe(ansatz{}, h, .59);
  printf("Energy is %lf\n", energy);
  return 0;
}

Here we define a parameterized CUDA-Q kernel, a callable type named ansatz that takes as input a single angle theta. This angle becomes the argument of a single ry rotation.

In host code, we define a Hamiltonian operator via the CUDA-Q spin_op type. CUDA-Q provides a generic function cudaq::observe. This function takes as input three terms. The first two terms are a parameterized kernel and the spin_op whose expectation value we wish to compute. The last term contains the runtime parameters at which we evaluate the parameterized kernel.

The return type of this function is an cudaq::observe_result which contains all the data from the execution, but is trivially convertible to a double, resulting in the expectation value we are interested in.

To compile and execute this code, we run the following:

nvq++ expectation_values.cpp -o exp_vals.x
./exp_vals.x

Parallelizing across Multiple Processors

multi-processor platforms page.

One typical use case of multi-processor platforms is to distribute the expectation value computations of a multi-term Hamiltonian across multiple virtual QPUs.

The following shows an example using the nvidia-mqpu platform:

Python

import cudaq
from cudaq import spin

cudaq.set_target("nvidia", option="mqpu")
target = cudaq.get_target()
num_qpus = target.num_qpus()
print("Number of QPUs:", num_qpus)


# Define spin ansatz.
@cudaq.kernel
def kernel(angle: float):
    qvector = cudaq.qvector(2)
    x(qvector[0])
    ry(angle, qvector[1])
    x.ctrl(qvector[1], qvector[0])


# Define spin Hamiltonian.
hamiltonian = 5.907 - 2.1433 * spin.x(0) * spin.x(1) - 2.1433 * spin.y(
    0) * spin.y(1) + .21829 * spin.z(0) - 6.125 * spin.z(1)

exp_val = cudaq.observe(kernel,
                        hamiltonian,
                        0.59,
                        execution=cudaq.parallel.thread).expectation()
print("Expectation value: ", exp_val)

C++

  using namespace cudaq::spin;
  cudaq::spin_op h = 5.907 - 2.1433 * x(0) * x(1) - 2.1433 * y(0) * y(1) +
                     .21829 * z(0) - 6.125 * z(1);

  // Get the quantum_platform singleton
  auto &platform = cudaq::get_platform();

  // Query the number of QPUs in the system
  auto num_qpus = platform.num_qpus();
  printf("Number of QPUs: %zu\n", num_qpus);

  auto ansatz = [](double theta) __qpu__ {
    cudaq::qubit q, r;
    x(q);
    ry(theta, r);
    x<cudaq::ctrl>(r, q);
  };

  double result = cudaq::observe<cudaq::parallel::thread>(ansatz, h, 0.59);
  printf("Expectation value: %lf\n", result);

One can then target the nvidia-mqpu platform by executing the following commands:

nvq++ observe_mqpu.cpp -target nvidia-mqpu
./a.out

In the above code snippets, since the Hamiltonian contains four non-identity terms, there are four quantum circuits that need to be executed in order to compute the expectation value of that Hamiltonian and given the quantum state prepared by the ansatz kernel. When the nvidia-mqpu platform is selected, these circuits will be distributed across all available QPUs. The final expectation value result is computed from all QPU execution results.