  In projective measurements of energy, a target system is projected to an
eigenstate of the system Hamiltonian, and the measurement outcomes provide the
information of corresponding eigen-energies. Recently, it has been shown that
such a measurement can be in principle realized without detailed knowledge of
the Hamiltonian by using probe qubits. However, in the previous approach for
the energy measurement, the necessary size of the dimension for the probe
increases as we increase the dimension of the target system, and also
individual addresibility of every qubit is required, which may not be possible
for many experimental settings with large systems. Here, we show that a single
probe qubit is sufficient to perform such a projective measurement of energy if
the target system is composed of non-interacting qubits whose resonant
frequencies are unknown. Moreover, our scheme requires only global
manipulations where every qubit is subjected to the same control fields. These
results indicate the feasibility of our energy projection protocols.
