  Multiqubit entanglement is extremely important to perform truly secure
quantum optical communication and computing operations. However, the efficient
generation of long-range entanglement over extended time periods between
multiple qubits randomly distributed in a photonic system remains an
outstanding challenge. This constraint is mainly due to the detrimental effects
of decoherence and dephasing. To alleviate this issue, we present engineered
epsilon-near-zero (ENZ) nanostructures that can maximize the coherence of
light-matter interactions at room temperature. We investigate a practical ENZ
plasmonic waveguide system which simultaneously achieves multiqubit
entanglement in elongated distances, extended time periods, and, even more
importantly, independent of the emitters positions. More specifically, we
present efficient transient entanglement between three and four optical qubits
mediated by ENZ with results that can be easily generalized to an arbitrary
number of emitters. The entanglement between multiple qubits is characterized
by computing the negativity metric applied for the first time to the proposed
nanophotonic ENZ configuration. The ENZ response is found to be substantially
advantageous to boost the coherence between multiple emitters compared to
alternative plasmonic waveguide schemes. Finally, the superradiance collective
emission response at the ENZ resonance is utilized to design a new high
fidelity two-qubit quantum phase gate that can be used in various emerging
quantum computing applications.
