  An elegant method to circumvent quantum measurement backaction is the use of
quantum mechanics free subsystems (QMFS), with one approach involving the use
of two oscillators with effective masses of opposite signs. Since negative
energies, and hence masses, are a characteristic of relativistic systems a
natural question is to what extent QMFS can be realized in this context. Using
the example of a one-dimensional Dirac oscillator we investigate conditions
under which this can be achieved, and identify Zitterbewegung or virtual pair
creation as the physical mechanism that fundamentally limits the feasibility of
the scheme. We propose a tabletop implementation of a Dirac oscillator system
based on a spin-orbit coupled ultracold atomic sample that allows for a direct
observation of the corresponding analog of virtual pair creation on quantum
measurement backaction.
