Issue 1 | Physical Review Research, published 19 October 2026

Reading out a quantum process directly, with help from an effective “time loop”

Our understanding

Slide 1: What does a quantum device do? Read it off, entry by entry. Measured with photons. A grid of 16 cells stands for the table that describes the device; a beam points at one highlighted cell.Slide 2: Testing a quantum device is a big puzzle. Many lines run from a device to a cloud of measurement points, and an arrow leads to a table that is computed from them.Slide 3: This team reads each entry directly. A circuit with a system line and a helper line, linked before and after a process box; the helper’s record is erased and one entry is read out.Slide 4: In the maths, this looks like a time loop. A time arrow with the steps link, process, link and erase, and a dashed loop from erase back to the first link, marked only in the maths.Slide 5: Tested with photons: fidelity above 99 %. Average fidelity above 99 % for test processes with 2 and 4 dimensions, every setting fixed. Schematic: a 4 by 4 grid labelled 2 dimensions and a 16 by 16 grid labelled 4 dimensions, each marked above 99 %.Slide 6: Simpler control, not fewer measurements. For general processes the number of settings stays the same; how it behaves beyond four dimensions is open. Two rows of five dials: the usual way with rotated settings, this method with fixed settings, same number of settings. Link to anyon.news.
Slide 1 of 6

To find out what a quantum device does, physicists usually run many measurements and then put the answer together like a puzzle. This team reads each piece of the answer directly. A helper particle is connected to the system twice, and then its record of what happened is wiped. In the mathematics this works like a loop in time, but nothing actually travels back in time. With particles of light, it worked with more than 99 % accuracy on small examples.

The details

Schematic: left, conventional tomography with many settings reconstructs the process matrix indirectly; right, the system passes through the process while coupled step by step to an ancilla, an erasure step removes distinguishing information, and one matrix element is read out directly.
Schematic drawn for this newsletter (anyon, CC BY 4.0), illustrating Yang et al., Phys. Rev. Research (2026), DOI 10.1103/bskn-3gy3. Not a figure from the paper.

What they did

To find out what a quantum device does to a quantum state, physicists usually run many measurements and then reconstruct a large table of numbers, the process matrix, indirectly. The authors read each entry of that table directly: they couple the system step by step to a helper system (an ancilla) and use quantum erasure, which removes the information that would otherwise distinguish the ancilla’s states. In the pseudo-density-matrix framework they use, this sequence acts as an effective “erasure-based closed timelike curve”, the mathematical analogue of a loop in time. (Our inference: this is a description within a formalism, not a claim that anything travels backwards in time in the lab.) The protocol combines addressing of computational-basis states with fixed superposition preparations and projections, so no basis rotations are needed. They demonstrated it with photons on two- and four-dimensional processes with average fidelities above 99 %.

Why it matters

When the computational-basis states of a system are easy to address, fixed settings can simplify the control needed to characterise it, which gets harder as the dimension grows.

What is still open

The authors say that the number of measurement settings is not reduced for general processes. In which real systems does the simpler control outweigh that, and how does the method behave as the dimension grows beyond four?

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