dense_coding.cpp

// Qudit dense coding
// Source: ./examples/dense_coding.cpp

#include <iostream>
#include <tuple>
#include <vector>

#include "qpp/qpp.hpp"

int main() {
    using namespace qpp;

    idx d = 3; // size of the system
    std::cout << ">> Qudit dense coding, d = " << d << '\n';

    ket mes_AB = st.mes(d); // maximally entangled state resource

    // circuit that measures in the qudit Bell basis
    cmat Bell_AB =
        adjoint(gt.CTRL(gt.Xd(d), {0}, {1}, 2, d) * kron(gt.Fd(d), gt.Id(d)));

    // equal probabilities of choosing a message
    idx m_A = randidx(0, d * d - 1);
    std::vector<idx> midx = n2multiidx(m_A, {d, d});
    std::cout << ">> Alice sent: " << m_A << " -> ";
    std::cout << disp(midx, IOManipContainerOpts{}.set_sep(" ")) << '\n';

    // Alice's operation
    cmat U_A = powm(gt.Zd(d), midx[0]) * powm(adjoint(gt.Xd(d)), midx[1]);

    // Alice encodes the message
    ket psi_AB = apply(mes_AB, U_A, {0}, d);

    // Bob measures the joint system in the qudit Bell basis
    psi_AB = apply(psi_AB, Bell_AB, {0, 1}, d);

    auto measured = measure(psi_AB, gt.Id(d * d));
    std::cout << ">> Bob's measurement probabilities: ";
    std::cout << disp(std::get<PROB>(measured),
                      IOManipContainerOpts{}.set_sep(", "))
              << '\n';

    // Bob samples according to the measurement probabilities
    idx m_B = std::get<RES>(measured);
    std::cout << ">> Bob received: ";
    std::cout << m_B << " -> "
              << disp(n2multiidx(m_B, {d, d}),
                      IOManipContainerOpts{}.set_sep(" "))
              << '\n';
}