  We propose a quantum information processing platform that utilizes the
ultrafast time-bin encoding of photons. This approach offers a pathway to
scalability by leveraging the inherent phase stability of collinear temporal
interferometric networks at the femtosecond-to-picosecond timescale. The
proposed architecture encodes information in ultrafast temporal bins processed
using optically induced nonlinearities and birefringent materials while keeping
photons in a single spatial mode. We demonstrate the potential for scalable
photonic quantum information processing through two independent experiments
that showcase the platform's programmability and scalability, respectively. The
scheme's programmability is demonstrated in the first experiment, where we
successfully program 362 different unitary transformations in up to 8
dimensions in a temporal circuit. In the second experiment, we show the
scalability of ultrafast time-bin encoding by building a passive optical
network, with increasing circuit depth, of up to 36 optical modes. In each
experiment, fidelities exceed 97\%, while the interferometric phase remains
passively stable for several days.
