[RFC] qubits with initial states

[schweitzpgi]

This is a proposal to add the ability to specify an initial state when declaring a qubit.

C++

We add new constructors to qubit and qvector to take a initializer_list or vector<complex<double>> that is $2^N$ double values in length and where $N$ is the number of qubit in the qvector object. (qubit would be restricted to 2 double values.

New constructors

  qubit::qubit(const vector<complex<double>>&);
  qubit::qubit(const initializer_list<complex<double>>&);
  qvector::qvector(const vector<complex<double>>&);
  qvector::qvector(const initializer_list<complex<double>>&);

Some examples in a CUDA Quake kernel might be the following.

  cudaq::qubit q = {0., 1.};
  cudaq::qubit q = cudaq::ket::one;
  cudaq::qubit q = {M_SQRT1_2, M_SQRT1_2};
  cudaq::qvector q = {M_SQRT1_2, 0., 0., M_SQRT1_2};
  cudaq::qvector q = complexVectorObject;

New builder method

  QuakeValue qalloc(vector<complex<double>> &)

When using the builder the builder would be called as in the following.

  std::vector<cudaq::complex> vec{M_SQRT1_2, 0., 0., M_SQRT1_2};
  auto qubits = kernel.qalloc(vec);

Python

The Python builder would be similar as in the following.

  v = [0., 1., 1., 0.]
  qubits = kernel.qalloc(v)

See #1090

[bettinaheim]

Updated #1086 and closing this as duplicate - RFC will be tracked in #1086.