Bad Qubits — Learn Quantum by Doing

Multiple choice

Classical coupled cluster writes the wavefunction as e^T applied to the Hartree-Fock state. Why can't e^T be implemented directly as a quantum circuit, and what does the Unitary Coupled-Cluster (UCC) ansatz do to fix it?

e^T is not Hermitian; UCC replaces it with e^(T + T^dagger) to make the exponent Hermitian and thus a valid observable
e^T is not unitary because the cluster operator T is not anti-Hermitian; UCC instead uses e^(T - T^dagger), whose exponent IS anti-Hermitian and therefore unitary
e^T cannot represent entanglement; UCC adds extra CNOT gates so the state can become entangled
e^T requires too many qubits; UCC reduces the qubit count by truncating at single excitations only