Relativistic quantum-speed limit for Gaussian systems and prospective experimental verification

Timing and phase resolution in satellite QKD, kilometre-scale gravitational-wave detectors, and space-borne clock networks hinge on quantum–speed limits (QSLs), yet benchmarks omit relativistic effects for coherent and squeezed probes. We derive first-order relativistic corrections to the Mandelstam–Tamm and Margolus–Levitin bounds. Starting from the Foldy–Wouthuysen expansion and treating −p⁴/(8m³c²) as a harmonic-oscillator perturbation, we propagate Gaussian states to obtain closed-form QSLs and the quantum Cramér–Rao bound. Relativistic kinematics slow evolution in an amplitude- and squeezing-dependent way, increase both bounds, and introduce an ϵ²t² phase drift that weakens timing sensitivity while modestly increasing the squeeze factor. A single electron (ϵ≈1.5×10⁻¹⁰) in a 5.4 T Penning trap, read out with 149 GHz quantum-limited balanced homodyne, should reveal this drift within ∼ 15 min — within known hold times. These results benchmark relativistic corrections in continuous-variable systems and point to an accessible test of the quantum speed limit in high-velocity or strong-field regimes.