Causal inequalities witness non-stabilizerness
Abstract
Stabilizer operations describe a fragment of quantum theory that is known to be efficiently classically simulable, thanks to the Gottesman-Knill theorem. For this reason, nonstabilizer resources such as magic states are necessary for universal quantum computation. Interestingly, the operational and axiomatic approaches to the resource theory of magic differ: the set of free operations in the former, namely, stabilizer operations (SO), is strictly smaller than that in the latter, namely, completely stabilizer preserving operations (CSPO). A simple example showing the separation is given by a three-qubit stabilizer product basis whose states cannot be perfectly discriminated using SO, but which do admit perfect discrimination using CSPO. Such an ensemble of states is said to exhibit nonstabilizerness without magic (NSWM). Here we obtain a principled understanding of this phenomenon, proving necessary and sufficient conditions for its existence. We first derive a simple criterion to decide whether, given a stabilizer basis, its states can be perfectly discriminated using stabilizer operations alone. We then consider the case where the stabilizer basis contains only product states and use its link with process functions---classical models of paradox-free causal loops---to prove the following: the states in a stabilizer product basis require nonstabilizerness for perfect discrimination if and only if the corresponding process function violates a causal inequality. This provides a new operational meaning to causal inequality violations as witnesses of nonstabilizerness, a form of computational nonclassicality.
Related
- Sub-extensive non-stabilizerness in the Dyck-Fredkin spin chain
- A Universal Budget for Entanglement and Nonlocal Non-Stabilizerness
- Optimized discrete Wigner representations and non-stabilizerness in qubit systems
- Non-stabilizerness and entanglement in (2+1)-dimensional SU(2) lattice gauge theory using tensor networks
- Tsirelson's nonclassicality witness under dissipative dynamics
BibTeX
@misc{vaglini2026causal,
title = {{Causal inequalities witness non-stabilizerness}},
author = {Leonardo Vaglini and Nasra Daher Ahmed and Ravi Kunjwal},
howpublished = {HAL},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.40223},
eprint = {2609.40223},
archivePrefix = {arXiv},
url = {https://hal.science/hal-05791419v1},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Causal inequalities witness non-stabilizerness
AU - Leonardo Vaglini
AU - Nasra Daher Ahmed
AU - Ravi Kunjwal
PY - 2026
DA - 2026/09/30
PB - HAL
DO - 10.48550/arxiv.2609.40223
UR - https://hal.science/hal-05791419v1
AB - Stabilizer operations describe a fragment of quantum theory that is known to be efficiently classically simulable, thanks to the Gottesman-Knill theorem. For this reason, nonstabilizer resources such as magic states are necessary for universal quantum computation. Interestingly, the operational and axiomatic approaches to the resource theory of magic differ: the set of free operations in the former, namely, stabilizer operations (SO), is strictly smaller than that in the latter, namely, completely stabilizer preserving operations (CSPO). A simple example showing the separation is given by a three-qubit stabilizer product basis whose states cannot be perfectly discriminated using SO, but which do admit perfect discrimination using CSPO. Such an ensemble of states is said to exhibit nonstabilizerness without magic (NSWM). Here we obtain a principled understanding of this phenomenon, proving necessary and sufficient conditions for its existence. We first derive a simple criterion to decide whether, given a stabilizer basis, its states can be perfectly discriminated using stabilizer operations alone. We then consider the case where the stabilizer basis contains only product states and use its link with process functions---classical models of paradox-free causal loops---to prove the following: the states in a stabilizer product basis require nonstabilizerness for perfect discrimination if and only if the corresponding process function violates a causal inequality. This provides a new operational meaning to causal inequality violations as witnesses of nonstabilizerness, a form of computational nonclassicality.
ER -