Dual-species, multi-qubit logic primitives for Ca+/Sr+ trapped-ion crystals

Abstract

We demonstrate key multi-qubit quantum-logic primitives in a dual-species trapped-ion system based on $${}^{40}$$40Ca$${}^{+}$$+ and $${}^{88}$$88Sr$${}^{+}$$+ ions, using two optical qubits with quantum-logic-control frequencies in the red to near-infrared range. With all ionization, cooling, and control wavelengths in a wavelength band similar for the two species and centered in the visible, and with a favorable mass ratio for sympathetic cooling, this pair is a promising candidate for scalable quantum information processing. Same-species and dual-species two-qubit gates, based on the Mølmer–Sørensen interaction and performed in a cryogenic surface-electrode trap, are characterized via the fidelity of generated entangled states; we achieve fidelities of 98.8(2)% and 97.5(2)% in Ca$${}^{+}$$+–Ca$${}^{+}$$+ and Sr$${}^{+}$$+–Sr$${}^{+}$$+ gates, respectively. For a similar Ca$${}^{+}$$+–Sr$${}^{+}$$+ gate, we achieve a fidelity of 94.3(3)%, and carrying out a Sr$${}^{+}$$+–Sr$${}^{+}$$+ gate performed with a Ca$${}^{+}$$+ sympathetic cooling ion in a Sr$${}^{+}$$+–Ca$${}^{+}$$+–Sr$${}^{+}$$+ crystal configuration, we achieve a fidelity of 95.7(3)%. These primitives form a set of trapped-ion capabilities for logic with sympathetic cooling and ancilla readout or state transfer for general quantum computing and communication applications.