  There is a growing interest in reconstructing the density matrix of
photoelectron wavepackets, in particular in complex systems where decoherence
can be introduced either by a partial measurement of the system or through
coupling with a stochastic environment. To this end, several methods to
reconstruct the density matrix, quantum state tomography protocols, have been
developed and tested on photoelectrons ejected from noble gases following
absorption of extreme ultraviolet (XUV) photons from attosecond pulses. It
remains a challenge to obtain model-free, single scan protocols that can
reconstruct the density matrix with high fidelities. Current methods require
extensive measurements or involve complex fitting of the signal. Efficient
single-scan reconstructions would be of great help to increase the number of
systems that can be studied. We propose a new and more efficient protocol that
is able to reconstruct the continuous variable density matrix of a
photoelectron in a single time delay scan. It is based on measuring the
coherences of a photoelectron created by absorption of an XUV pulse using a
broadband infrared (IR) probe that is scanned in time and a narrowband IR
reference that is temporally fixed to the XUV pulse. We illustrate its
performance for a Fano resonance in He as well as mixed states in Ar arising
from spin-orbit splitting. We show that the protocol results in excellent
fidelities and near-perfect estimation of the purity.
