First and second quantized digital quantum simulations of bosonic systems

Abstract: We compare the basic resource requirements for first and second quantized bosonic mappings in a system consisting of $N$ particles in $M$ modes. In addition to the standard binary first quantized mapping, we investigate the unary first quantized mapping, which we show to be the most gate-efficient mapping for bosons in the general case, although less qubit-efficient than binary mappings. Our comparison focuses on the $k$-body reduced density matrix ($k$-RDM) as well as two standard bosonic Hamiltonians. The first quantized mappings use less resources for off-diagonal terms of the $k$-RDM by a factor of $ \sim N^k$, compared to the second quantized mappings. The number of gates for the first quantized binary mapping increases faster with $M$ compared to the other mappings. Nevertheless, a detailed numeric analysis reveals that the binary first quantized mapping still requires fewer gates than the binary and unary second quantized ones for realistic combinations of $N$ and $M$, while requiring exponentially fewer qubits than the unary mappings. Additionally, the number of CNOT and $R_z(φ)$ gates necessary to express the exponential of the Hamiltonian in the binary first quantized mapping is comparable to the (overall most efficient) unary first quantized one when $M = 2^n$ for both the Bose-Hubbard model and the harmonic trap with short-range interactions. This suggests that this mapping can be both qubit- and gate-efficient for practical problems.