  We propose a many-body quantum engine powered by the energy difference
between the entangled ground state of the interacting system and local
separable states. Performing local energy measurements on an interacting
many-body system can produce excited states from which work can be extracted
via local feedback operations. These measurements reveal the quantum vacuum
fluctuations of the global ground state in the local basis and provide the
energy required to run the engine. The reset part of the engine cycle is
particularly simple: The interacting many-body system is coupled to a cold bath
and allowed to relax to its entangled ground state. We illustrate our proposal
on two types of many-body systems: a chain of coupled qubits and coupled
harmonic oscillator networks. These models faithfully represent fermionic and
bosonic excitations, respectively. In both cases, analytical results for the
work output and efficiency of the engine can be obtained. Generically, the work
output scales as the number of quantum systems involved, while the efficiency
limits to a constant. We prove the efficiency is controlled by the "local
entanglement gap" -- the energy difference between the global ground state and
the lowest energy eigenstate of the local Hamiltonian. In the qubit chain case,
we highlight the impact of a quantum phase transition on the engine's
performance as work and efficiency sharply increase at the critical point. In
the case of a one-dimensional oscillator chain, we show the efficiency
approaches unity as the number of coupled oscillators increases, even at finite
work output.

Quantum Physics