Robust two-qubit gates using pulsed dynamical decoupling

Abstract

We present the experimental implementation of a two-qubit phase gate,
using a radio frequency (RF) controlled trapped-ion quantum processor. The RFdriven
gate is generated by a pulsed dynamical decoupling sequence applied to the
ions’ carrier transitions only. It allows for a tunable phase shift with high-fidelity
results. The conditional phase shift is measured using a Ramsey-type measurement
with an inferred fringe contrast of up to 99+1
−2%. We also prepare a Bell state using
this laser-free gate. The phase gate is robust against common sources of error. We
investigate the effect of the excitation of the center-of-mass (COM) mode, errors in the
axial trap frequency, pulse area errors and errors in sequence timing. The contrast of
the phase gate is not significantly reduced up to a COM mode excitation < 20 phonons,
trap frequency errors of +10%, and pulse area errors of -8%. The phase shift is
not significantly affected up to < 10 phonons and pulse area errors of -2%. Both,
contrast and phase shift are robust to timing errors up to -30% and +15%. The gate
implementation is resource efficient, since only a single driving field is required per
ion. Furthermore, it holds the potential for fast gate speeds (gate times on the order
of 100 μs) by using two axial motional modes of a two-ion crystal through improved
setups.