Squeezed and equivalent XUV photons*

We demonstrate the generation of a train of attosecond XUV pulses that are in a superposition of wavefront states. 

Such superposition yields a high precision, self-referencing, common path XUV interferometer setup to produce pairs of spatially separated and independently controllable XUV pulses that are locked in phase and time with a temporal jitter of 3~zs. By reconstructing the interference pattern in the photon counting regime we conclude that the XUV photons are not entangled but squuzed in phase. Squeeze measurements are possible thanks to the Bessel nature of the fundamental driver, which provides strong-field-free XUV photons in the far field. In such case, one arm of the interferometer can be used as the local oscillator respect to which homodyne measurements can be done.