Optimal quantum states for quantum information and how to prepare them

The quantum correlations in systems of indistinguishable particles are central resources for quantum technology applications. However, the efficiency of a quantum protocol may be reduced considerably by the surrounding environment that, consequently, turns the probing system into a mixed state. In this talk, we present the optimal states that achieve the maximum entanglement attainable for a multiqubit bosonic system with a fixed spectrum, and how to prepare them via a unitary transformation [1]. In particular, we study the maximum entanglement attainable for systems with two and three qubits. Along the same lines, we study multiqubit states with little or no use in quantum information applications which are absolutely separable (non-entangled) after any unitary transformation [2]. Lastly, as an application in quantum-enhanced metrology, we characterize the most susceptible mixed states to estimate an infinitesimal rotation over an arbitrary axis, called optimal quantum rotosensors [3]. These mixed states can achieve the same sensitivity as optimal pure states, and the quantum property called anticoherence comes into play.[1] Absolute separability witnesses for symmetric multiqubit states, ESE, J. Denis and J. Martin, Phys. Rev. A 109, 022430 (2024).
[2] Maximum entanglement of mixed symmetric states under unitary transformations, ESE and J. Martin, SciPost Phys. 15, 120 (2023).
[3] Quantum metrology with mixed spin states, ESE, J. Martin and C.Chryssomalakos, arXiv.2404.15548 (2024).