Fig. 2: Schematic diagram of time-bin GHZ state generation.

a The experimental sequence consists of nuclear spin narrowing and spin initialization, which is then followed by the GHZ protocol with a series of π/2, π-pulses (blue) and single photon generations (red). The last part of the sequence (enclosed in the box) is repeated n − 1 times to generate an n qubit GHZ state. The duration of the full sequence is 1.8 μs. Nuclear spin narrowing consists of two overlapped 1.1 μs Raman pulse and a 1.2 μs pump pulse. The longer pump pulse optically drives the \(\left\vert \downarrow \right\rangle \to \left\vert \downarrow \uparrow \Downarrow \right\rangle\) transition and initializes the spin state to \(\left\vert \! \! \uparrow \! \right\rangle\) before the GHZ state generation. b Time bin interferometer (TBI) setup: A picosecond pulsed laser (blue) propagates through the long and short arm of TBI to excite a QD emitting a single photon in the early and late time bin, respectively. The single photon is collected through the same TBI but in the opposite direction. Each photonic qubit has three peaks in the output that are separated by 12 ns, which is determined by the path-length difference between the long and short arms of the TBI.