arXiv subjects

Louis Hanotel

Publications and source records attributed to Louis Hanotel.

6 recordsLinked to original sources

arXiv2025

Nonviolation of the CHSH inequality under local spin-1 measurements on two spin qutrits

In the present paper, based on the general analytical expression [arXiv:2412.03470] for the maximum of the CHSH expectation under local Alice and Bob spin-$s$ measurements in a two-qudit state of dimension $d=2s+1$, $s\geq 1/2$, we analyze whether or not, under spin-$1$ measurements in an arbitrary two-qutrit state, the CHSH inequality is violated. We find analytically for a variety of pure nonseparable two-qutrit states and also, numerically for $1,000,000$ randomly generated pure nonseparable two-qutrit states, that, under local Alice and Bob spin-$1$ measurements in each of these nonseparable states, including maximally entangled, the CHSH inequality is not violated. These results together with the spectral decomposition of a mixed state lead us to the Conjecture that, under local Alice and Bob spin-$1$ measurements, every nonseparable two-qutrit state, pure or mixed, does not violate the CHSH inequality. For a variety of pure two-qutrit states, we further find the values of their concurrence and compare them with the values of their spin-$1$ CHSH parameter, which determines violation or nonviolation by a two-qutrit state of the CHSH inequality under spin-$1$ measurements. This comparison indicates that, in contrast to spin-$\frac{1}{2}$ measurements, where the spin-$\frac{1}{2}$ CHSH parameter of a pure two-qubit state is increasing monotonically with a growth of its entanglement, for a pure two-qutrit state, this is not the case. In particular, for the two-qutrit GHZ state, which is maximally entangled, the spin-$1$ CHSH parameter is equal to $\sqrt{\frac{8}{9}}$, while, for some separable pure two-qutrit states, this parameter can be equal to unity. Moreover, for the two-qutrit Horodecki state, the spin-$1$ CHSH parameter is equal to $4\sqrt{2}/21<1$ regardless of the entanglement type of this mixed state.

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arXiv2024

High-spin measurements in an arbitrary two-qudit state

Violation of the CHSH inequality by a bipartite quantum state is now used in many quantum applications. However, the explicit analytical expression for the maximal value of the CHSH expectation under local Alice and Bob spin-$s$ measurements is still known only for $s=1/2$. In the present article, for an arbitrary state of two spin-$s$ qudits, each of dimension $d=2s+1\geq 2$, we introduce the notion of the spin-$s$ correlation matrix, which has dimension $3\times 3$ for all $s\geq \frac{1}{2}$; establish its relation to the general correlation $(d^{2}-1)\times (d^{2}-1)$ matrix of this state within the generalized Pauli representation and derive in terms of the spin-$s$ correlation matrix the explicit analytical expression for the maximal value of the CHSH expectation under local Alice and Bob spin-$s$ measurements in this state. Specifying this general expression for the two-qudit GHZ state, the nonlocal two-qudit Werner state, and some nonseparable pure two-qudit states, we find that, under local Alice and Bob high-spin ($s\geq1$) measurements in each of these nonseparable states, including the maximally entangled one, the CHSH inequality is not violated. Moreover, unlike the case of spin-$1/2$ measurements, where each pure nonseparable two-qubit state violates the CHSH inequality and the maximal value of its CHSH expectation increases monotonically with a growth of its entanglement, the situation under high-spin measurements is quite different -- for a pure two-qudit state with a higher degree of entanglement, the maximal value of the CHSH expectation turns out to be less than for a pure two-qudit state with lower entanglement and even for a separable one.

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arXiv2024

Geometric quantum discord of an arbitrary two-qudit state: the exact value and general upper bounds

The geometric quantum discord of a two-qudit state has been studied in many papers, however, its exact analytical value in the explicit form is known only for a general two-qubit state, a general qubit-qudit state and some special families of two-qudit states. Based on the general Bloch vectors formalism [J. Phys. A: Math. Theor. 54 195301 (2021)], we find the explicit exact analytical value of the geometric quantum discord for a general two-qudit state of an arbitrary dimension via the parameters of its correlation matrix and the Bloch vectors of its reduced states. This new general analytical result includes all the known exact results on the geometric quantum discord only as particular cases and proves rigorously that the lower bound on the geometric discord presented in [Phys. Rev. A 85, 024102 (2012)] constitutes its exact value for each two-qudit state. Moreover, our new general result allows us to find for an arbitrary two-qudit state, pure or mixed, the novel upper and lower bounds on its geometric quantum discord, expressed via the Hilbert space characteristics of this state.

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arXiv2023

Quantifying Bell nonlocality of a pure two-qudit state via its entanglement

For the maximal violation of all Bell inequalities by an arbitrary pure two-qudit state of any dimension, we derive a new lower bound expressed via the concurrence of this pure state. This new lower bound and the upper bound on the maximal Bell violation, found in [J. Phys. A: Math. Theor. 55, 285301 (2022)] and also expressed via the concurrence, analytically quantify Bell nonlocality of a pure two-qudit state via its entanglement, in particular, prove explicitly that entanglement of a pure two-qudit state is necessary and sufficient for its Bell nonlocality. By re-visiting the pure two-qubit case, we also find and rigorously prove the new results on the correlation properties of an arbitrary pure two-qubit state.

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arXiv2021

Symmetric Multiqudit States: Stars, Entanglement, Rotosensors

A constellation of $N=d-1$ Majorana stars represents an arbitrary pure quantum state of dimension $d$ or a permutation-symmetric state of a system consisting of $n$ qubits. We generalize the latter construction to represent in a similar way an arbitrary symmetric pure state of $k$ subsystems with $d$ levels each. For $d\geq 3$, such states are equivalent, as far as rotations are concerned, to a collection of various spin states, with definite relative complex weights. Following Majorana's lead, we introduce a multiconstellation, consisting of the Majorana constellations of the above spin states, augmented by an auxiliary, "spectator" constellation, encoding the complex weights. Examples of stellar representations of symmetric states of four qutrits, and two spin-$3/2$ systems, are presented. We revisit the Hermite and Murnaghan isomorphisms, which relate multipartite states of various spins, number of parties, and even symmetries. We show how the tools introduced can be used to analyze multipartite entanglement and to identify optimal quantum rotosensors, i.e., pure states which are maximally sensitive to rotations around a specified axis, or averaged over all axes.

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arXiv2019

When geometric phases turn topological

Geometric phases, accumulated when a quantum system traces a cycle in quantum state space, do not depend on the parametrization of the cyclic path, but do depend on the path itself. In the presence of noise that deforms the path, the phase gets affected, compromising the robustness of possible applications, e.g., in quantum computing. We show that for a special class of spin states, called anticoherent, and for paths that correspond to a sequence of rotations in physical space, the phase only depends on topological characteristics of the path, in particular, its homotopy class, and is therefore immune to noise.

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