Quantum reversal: a general theory of coherent quantum absorbers

Mankei Tsang

doi:10.22331/q-2025-02-26-1650

Quantum reversal: a general theory of coherent quantum absorbers

Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117583
Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117551

Published:	2025-02-26, volume 9, page 1650
Editor:	Nicolai Friis
Eprint:	arXiv:2402.02502v3
Doi:	https://doi.org/10.22331/q-2025-02-26-1650
Citation:	Quantum 9, 1650 (2025).

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Abstract

The fascinating concept of coherent quantum absorber – which can absorb any photon emitted by another system while maintaining entanglement with that system – has found diverse implications in open quantum system theory and quantum metrology. This work generalizes the concept by proposing the so-called reversal conditions for the two systems, in which a "reverser" coherently reverses any effect of the other system on a field. The reversal conditions are rigorously boiled down to concise formulas involving the Petz recovery map and Kraus operators, thereby generalizing as well as streamlining the existing treatments of coherent absorbers.

Featured image: Bob, a quantum reverser, undoes any entanglement between Eve and Alice.

Popular summary

Eve had an entanglement with Alice, and Alice is now gone. How can Bob help Eve get over the heartbreak? The same problem arises in the quantum realm, where an undesirable interaction may introduce noise to a quantum device. This work proposes an apparatus, called a quantum reverser, to reverse an interaction of one quantum system with another.

For example, a gravitational-wave detector uses a laser beam (Eve) reflected by moving mirrors (Alice) to measure any gravitational wave that perturbs the mirrors, but the laser-mirror interaction also adds a noise called the measurement-backaction noise to the beam. We would like to design an apparatus (Bob) that removes the noise from the laser beam but keeps the useful signal about the gravitational wave, much like how noise-canceling headphones remove ambient noise while playing music.

Bob offers to erase all of Eve's memory, but Eve refuses—she wants to keep the good memory, just as the laser beam in a gravitational-wave detector should keep the signal. By following our quantum-reverser design, on the other hand, Bob will be able to erase only the bad memory (measurement-backaction noise in the detector example) while keeping the good memory (signal) intact.

To be sure, quantum noise cancellation is not a new concept. The key innovation of this work is to remove many assumptions made in previous proposals, to show that a quantum reverser can undo any interaction, and to derive concise formulas for the reverser design. Much work remains to be done to apply the general theory to experimental design and practical problems, such as quantum sensors.

While this work will surely be useless for curing heartbreaks in any foreseeable future, it is another step in scientists' quest to tame unwanted features of quantum mechanics, so that we can harness its full potential for information processing.

► BibTeX data

@article{Tsang2025quantumreversal,
  doi = {10.22331/q-2025-02-26-1650},
  url = {https://doi.org/10.22331/q-2025-02-26-1650},
  title = {Quantum reversal: a general theory of coherent quantum absorbers},
  author = {Tsang, Mankei},
  journal = {{Quantum}},
  issn = {2521-327X},
  publisher = {{Verein zur F{\"{o}}rderung des Open Access Publizierens in den Quantenwissenschaften}},
  volume = {9},
  pages = {1650},
  month = feb,
  year = {2025}
}

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Cited by

[1] Arthur J. Parzygnat and James Fullwood, "Time‐Symmetric Correlations for Open Quantum Systems", Annalen der Physik 537 12, e00221 (2025).

[2] Federico Girotti, Alfred Godley, and Madalin Guta, "Estimating quantum Markov chains using coherent absorber post-processing and pattern counting estimator", Quantum 9, 1835 (2025).

[3] Dayou Yang, Moulik Ketkar, Koenraad Audenaert, Susana F. Huelga, and Martin B. Plenio, "Quantum Cramér-Rao Precision Limit of Noisy Continuous Sensing", Physical Review Letters 136 7, 070802 (2026).

[4] Rocco Duvenhage, Kyle Oerder, and Keagan van den Heuvel, "Quantum detailed balance via elementary transitions", Quantum 9, 1743 (2025).

[5] Andrew Pocklington and Aashish A. Clerk, "Universal Time-Entanglement Trade-Off in Open Quantum Systems", PRX Quantum 5 4, 040305 (2024).

[6] Arthur J. Parzygnat and James Fullwood, "Time-symmetric correlations for open quantum systems", arXiv:2407.11123, (2024).

[7] Mankei Tsang, "Quantum Onsager relations", Quantum Science and Technology 10 1, 015015 (2025).

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