Default constructor of `QubitRegister` not working properly

[snsunx]

I'm trying to simulate a quantum system with Intel QS and need to declare a QubitRegister before initializing it with the number of qubits. However, the default constructor of QubitRegister does not seem to work properly. After including <qureg.hpp>, simply doing

iqs::QubitRegister<ComplexDP> psi;

will throw an error. The error occurs at the checking statement if(GlobalSize()) assert(GlobalSize() * 2UL == new_num_amplitudes);, but after disabling this checking statement other errors come up in num_qubits and in using state vs state_storage.

I'd like to propose the following changes: we should comment out Line 60 of qureg_init.cpp and change Line 38-39 from

Resize(1UL);
state_storage[0] = {1., 0.};

to

Resize(2UL);
state[0] = {1., 0.};

Changing these three lines and rebuilding the library worked for me. These changes also seem consistent with the rest of the QubitRegister class.

[giangiac]

Hi @snsunx ,

Thanks for looking into the issue and suggesting a fix.
Indeed QubitRegister objects were thought to be initialized with a specific number of qubits.
There is a method to resize the register, but it is not up to date with the rest of the implementation.

I will look into your suggested fix (by the way, have you tested it with MPI enabled?).

As part of the IQS development, we worked on a different approach.
We worked on a method to resize the state register by either adding extra qubits in the |00...0> state, or by removing qubits after projecting them to |00...0><00...0|.
It is MPI-ready and we should be able to open the corresponding PR soon.

[snsunx]

I tested it with MPI enabled and also wrote a unit test in unit_test/include/one_qubit_register_test.hpp. I will open a PR soon.