Fig. 1: A 2 × 2 silicon spin qubit array.

a Design schematic and false colored SEM image of a device nominally identical to the one measured. In the schematic, single-electron transistor (SET) sensor positions are marked with S1 and S2, and the quantum dots are labeled clockwise with D₁ through D₄. We refer to each qubit as Q₁ through Q₄ according to the dot it primarily inhabits. The SEM image includes the micro magnet on top of the gate stack (not shown in the schematic). b Odd parity probability after driving the qubit with a microwave burst as indicated by the block diagram. The respective Chevron patterns of all qubits are arranged according to their physical position. The drive power and modulation amplitude were adjusted per qubit to achieve a Rabi frequency around 2 MHz. c Decoupled controlled phase oscillations of adjacent qubit pairs using the pulse sequence indicated in the block diagram. The Ising-like interaction U_CZ is controlled by the voltage V_Barrier on the barrier gate located between the respective plunger gates. We chose a static operating point such that the extrapolated exchange strength J_off <20 kHz and apply a pulse on the barriers large enough such that J_on >1 MHz. The large amplitudes for these barrier pulses as well as the particular fanout of gates B34, P3, and B23 often caused substantial degradation of the readout signal (see “Discussion”).