4.7 Article

Robustness of Noether's Principle: Maximal Disconnects between Conservation Laws and Symmetries in Quantum Theory

Journal

PHYSICAL REVIEW X
Volume 10, Issue 4, Pages -

Publisher

AMER PHYSICAL SOC
DOI: 10.1103/PhysRevX.10.041035

Keywords

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Funding

  1. EPSRC National Quantum Technology Hub in Networked Quantum Information Technologies
  2. EPSRC through the Quantum Controlled Dynamics Centre for Doctoral Training
  3. John Templeton Foundation
  4. ARC via the Centre of Excellence in Engineered Quantum Systems Project [CE110001013]
  5. Foundation for Polish Science through IRAP project
  6. EU [2018/MAB/5, POIR.04.04.00-00-17C1/1800]
  7. Royal Society
  8. University Academic Fellowship

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To what extent does Noether's principle apply to quantum channels? Here, we quantify the degree to which imposing a symmetry constraint on quantum channels implies a conservation law and show that this relates to physically impossible transformations in quantum theory, such as time reversal and spin inversion. In this analysis, the convex structure and extremal points of the set of quantum channels symmetric under the action of a Lie group G becomes essential. It allows us to derive bounds on the deviation from conservation laws under any symmetric quantum channel in terms of the deviation from closed dynamics as measured by the unitarity of the channel epsilon. In particular, we investigate in detail the U(1) and SU(2) symmetries related to energy and angular momentum conservation laws. In the latter case, we provide fundamental limits on how much a spin- j(A) system can be used to polarize a larger spin-j(B) system, and on how much one can invert spin polarization using a rotationally symmetric operation. Finally, we also establish novel links between unitarity, complementary channels, and purity that are of independent interest.

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