Ptychography of pure quantum states
Ptychography is an imaging technique in which a localized illumination scans overlapping regions of an object and generates a set of diffraction intensities used to computationally reconstruct its complex-valued transmission function. We propose a quantum analogue of this technique designed to reconstruct $d$-dimensional pure states. A set of $n$ rank-$r$ projectors "scans" overlapping parts of an input state and the moduli of the $d$ Fourier amplitudes of each part are measured. These $nd$ outcomes are fed into an iterative phase retrieval algorithm that estimates the state. Using $d$ up to 100 and $r$ around $d/2$, we performed numerical simulations for single systems in an economic ($n=4$) and a costly ($n=d$) scenario, as well as for multiqubit systems ($n=6\log d$). This numeric study included realistic amounts of depolarization and poissonian noise, and all scenarios yielded, in general, reconstructions with infidelities below $10^{-2}$. The method is shown, therefore, to be resilient to noise and, for any $d$, requires a simple and fast postprocessing algorithm. We show that the algorithm is equivalent to an alternating gradient search, which ensures that it does not suffer from local-minima stagnation. Unlike traditional approaches to state reconstruction, the ptychographic scheme uses a single measurement basis; the diversity and redundancy in the measured data---key for its success---are provided by the overlapping projections. We illustrate the simplicity of this scheme with the paradigmatic multiport interferometer.