  We describe a laser-plasma platform for photon-photon collision experiments
to measure fundamental quantum electrodynamic processes such as the linear
Breit-Wheeler process with real photons. The platform has been developed using
the Gemini laser facility at the Rutherford Appleton Laboratory. A laser
wakefield accelerator and a bremsstrahlung convertor are used to generate a
collimated beam of photons with energies of hundreds of MeV, that collide with
keV x-ray photons generated by a laser heated plasma target. To detect the
pairs generated by the photon-photon collisions, a magnetic transport system
has been developed which directs the pairs onto scintillation-based and hybrid
silicon pixel single particle detectors. We present commissioning results from
an experimental campaign using this laser-plasma platform for photon-photon
physics, demonstrating successful generation of both photon sources,
characterisation of the magnetic transport system and calibration of the single
particle detectors, and discuss the feasibility of this platform for the
observation of the Breit-Wheeler process. The design of the platform will also
serve as the basis for the investigation of strong-field quantum electrodynamic
processes such as the nonlinear Breit-Wheeler and the Trident process, or
eventually, photon-photon scattering.
