Universal Control and Quantum Simulations in a Ge/SiGe Singlet-Triplet Qubit Array

Elizaveta Morozova

Semiconductor quantum dots offer significant potential for quantum computing and simulations, with advantages like individual qubit control and intrinsic Coulomb interactions. While progress has been made with single-spin qubits in silicon and germanium, singlet-triplet qubits simplify control, using baseband voltages for full electrical manipulation.

We present a 2x4 quantum dot ladder in Ge/SiGe, where we form four singlet-triplet qubits with controllable interactions between neighboring spins. Using randomized benchmarking we achieve single-qubit gate fidelities of 99.49(8)-99.84(1)% and gate set tomography results of 98.97(9)-99.61(5)%. Additionally, a two-qubit SWAP-like gate delivers Bell-state fidelities of 73(1)% to 90(1)%, with concurrences from 21(4)% to 64(4)%. We further demonstrate entanglement distribution across the array with a remote Bell-state fidelity of 75(2)% and concurrence of 22(4)%.

Expanding on this, we explore quantum phase transitions in the system. By varying the ratio of exchange interaction along the rungs of the ladder to the Zeeman energy we observe a transition from a polarized to an unpolarized ground state, with preliminary evidence of an intermediate phase as the exchange interaction along the rails increases. We also study triplet propagation by tuning singlet-triplet energies, observing quantum walk behavior when triplets are presented in the system.

These results put germanium singlet-triplet qubits as a strong candidate for quantum technologies, showcasing both high-fidelity control and insights into quantum magnetism.