Pre- and post-selection (PPS) paradoxes are striking demonstrations of quantum nonclassicality. Logical PPS paradoxes, where inferences made with the Aharonov-Bergmann-Lebowitz (ABL) rule are exactly 0 or 1, are linked to contextuality. Non-logical paradoxes lack this strong signature. In this work, we analyse more general, non-logical PPS scenarios involving mixed pre- and post-selected states. We show that two such scenarios, the $N$ -box and quantum pigeonhole paradoxes, require coherence of both pre- and post-selected states in the basis of the intermediate measurement. This is done by showing each paradox holds if and only if there is weak-value anomaly for a single, paradox-specific operator, together with the fact that weak-value anomaly requires coherence. This clarifies the role of different notions of nonclassicality in these scenarios, highlighting the required quantitative departures from (strictly) classical explanations provided by incoherent sub-theories of quantum theory.Pre- and post-selection (PPS) paradoxes are striking demonstrations of quantum nonclassicality. Logical PPS paradoxes, where inferences made with the Aharonov-Bergmann-Lebowitz (ABL) rule are exactly 0 or 1, are linked to contextuality. Non-logical paradoxes lack this strong signature. In this work, we analyse more general, non-logical PPS scenarios involving mixed pre- and post-selected states. We show that two such scenarios, the $N$-box and quantum pigeonhole paradoxes, require coherence of both pre- and post-selected states in the basis of the intermediate measurement. This is done by showing each paradox holds if and only if there is weak-value anomaly for a single, paradox-specific operator, together with the fact that weak-value anomaly requires coherence. This clarifies the role of different notions of nonclassicality in these scenarios, highlighting the required quantitative departures from (strictly) classical explanations provided by incoherent sub-theories of quantum theory.

quant-pharXiv:2607.24282Jul 28, 20260 citations

Security of a practical quantum secure direct communication system under weak-randomness basis selection attack

Fu-Le Hao, Cheng Zhang, Shi‐Pu Gu +3 more

Quantum secure direct communication (QSDC) enables the user to directly transmit secret messages through quantum channels without pre-shared keys. However, the imperfection of devices brings a security loophole to QSDC. The weak-random basis selection attack is a strong attack mode, where the eavesdropper can manipulate users’ basis selection probability distribution, and thus reduce the error rate caused by her eavesdropping. In this paper, we propose the first, to the best of our knowledge, general quantitative security analysis model for the practical QSDC system under the weak-randomness basis selection attack. We characterize the imperfect basis selection by the parameter ε . The simulation results show that when ε increases from 0 to 0.1, the maximal secure communication distance reduces from 34 km to 24 km. When ε > 0.32, the maximal secure communication distance reduces to 0. Our work indicates the importance of inspecting the QSDC system’s security loopholes from the weak-randomness basis selection attack and contributes to enhancing its practical security under real experimental conditions.Quantum secure direct communication (QSDC) enables the user to directly transmit secret messages through quantum channels without pre-shared keys. However, the imperfection of devices brings a security loophole to QSDC. The weak-random basis selection attack is a strong attack mode, where the eavesdropper can manipulate users’ basis selection probability distribution, and thus reduce the error rate caused by her eavesdropping. In this paper, we propose the first, to the best of our knowledge, general quantitative security analysis model for the practical QSDC system under the weak-randomness basis selection attack. We characterize the imperfect basis selection by the parameter ε . The simulation results show that when ε increases from 0 to 0.1, the maximal secure communication distance reduces from 34 km to 24 km. When ε > 0.32, the maximal secure communication distance reduces to 0. Our work indicates the importance of inspecting the QSDC system’s security loopholes from the weak-randomness basis selection attack and contributes to enhancing its practical security under real experimental conditions.

Optics LettersMar 17, 20260 citations

Average versus maximal coherence

Shunlong Luo, Yuan Sun

Physics Letters AJun 21, 201933 citations

Robust Coherence

ScienceOct 20, 20000 citations

Coherence generating power of quantum unitary maps

Paolo Zanardi A, Georgios Styliaris, Lorenzo Campos Venuti

Given a preferred orthonormal basis $B$ in the Hilbert space of a quantum system we define a measure of the coherence generating power of a unitary map with respect to $B$ . We give its explicit analytical form and provide an operational protocol to directly detect it. This measure vanishes if and only if the unitary map transforms all the incoherent states into themselves and is non-increasing under pre- or post-processing by incoherent operations. We characterize the set of unitaries with maximal coherence generating power and study the properties of our measure when the unitary is drawn at random from the Haar distribution. For large state-space dimension a random unitary has, with overwhelming probability, nearly maximal coherence generating power with respect to any basis. Finally, extensions to general unital completely positive maps and the relation to the concept of asymmetry are discussed.Given a preferred orthonormal basis $B$ in the Hilbert space of a quantum system we define a measure of the coherence generating power of a unitary map with respect to $B$. We give its explicit analytical form and provide an operational protocol to directly detect it. This measure vanishes if and only if the unitary map transforms all the incoherent states into themselves and is non-increasing under pre- or post-processing by incoherent operations. We characterize the set of unitaries with maximal coherence generating power and study the properties of our measure when the unitary is drawn at random from the Haar distribution. For large state-space dimension a random unitary has, with overwhelming probability, nearly maximal coherence generating power with respect to any basis. Finally, extensions to general unital completely positive maps and the relation to the concept of asymmetry are discussed.

Oct 2, 20160 citations

Coherence as a resource for

N

-box and quantum pigeonhole paradoxes

N

-box and quantum pigeonhole paradoxes, require coherence of both pre- and post-selected states in the basis of the intermediate measurement. This is done by showing each paradox holds if and only if there is weak-value anomaly for a single, paradox-specific operator, together with the fact that weak-value anomaly requires coherence. This clarifies the role of different notions of nonclassicality in these scenarios, highlighting the required quantitative departures from (strictly) classical explanations provided by incoherent sub-theories of quantum theory.

Given a preferred orthonormal basis

B

in the Hilbert space of a quantum system we define a measure of the coherence generating power of a unitary map with respect to

B

. We give its explicit analytical form and provide an operational protocol to directly detect it. This measure vanishes if and only if the unitary map transforms all the incoherent states into themselves and is non-increasing under pre- or post-processing by incoherent operations. We characterize the set of unitaries with maximal coherence generating power and study the properties of our measure when the unitary is drawn at random from the Haar distribution. For large state-space dimension a random unitary has, with overwhelming probability, nearly maximal coherence generating power with respect to any basis. Finally, extensions to general unital completely positive maps and the relation to the concept of asymmetry are discussed.