Demonstration of a Wavelength-Insensitive Entangling Gate for Group-2 Atomic Ions

Entanglement generation in trapped-ion systems has thus far relied on two distinct but related two-qubit geometric phase gate techniques: Mølmer-Sørensen (MS) and light-shift (LS) gates. In both schemes, normal modes of ion motion are employed as a “quantum bus” whereby an internal-state-dependent force excites ion motion to induce entanglement between internal (i.e. spin) and external (i.e. motion) degrees of freedom. We have recently proposed a variant of the LS scheme where the qubit levels are separated by an optical frequency [1]. Some advantages of this optical transition dipole force (OTDF) gate include: a broad range of feasible entangling laser wavelengths (including visible and infrared wavelengths), two-qubit photon scattering error <10^-4 in some wavelength regimes, and straightforward extension to multispecies co-trapped group-2 ions. We report an experimental demonstration of the OTDF gate using a co-trapped pair of ⁴⁰Ca⁺ ions in a cryogenic surface-electrode ion trap. We measure a two-qubit entanglement infidelity of 8(4)×10^-4 obtained directly from Bell state parity analysis without subtraction of state preparation, measurement, or one-qubit gate errors. To our knowledge, this represents the highest laser-based two-qubit entanglement fidelity yet reported in a surface trap, and it establishes the OTDF scheme as competitive with typical LS and MS schemes.

This work was done in collaboration with Los Alamos National Laboratory.

[1] B. C. Sawyer and K. R. Brown, PRA 103, 022427 (2021)