Experimental assessment of geometric quantum speed limits in an open quantum system

The quantum speed limit sets a lower bound on the evolution time for quantum systems undergoing general physical processes. Here, using an ensemble of chloroform molecules, we study the speed of evolution of a qubit subject to decoherence. In this solution, the carbon nuclear spin encodes the two-level system, while the hydrogen spin plays the role of an environment for the latter. By adding a paramagnetic salt, we control the system-reservoir interaction as the hydrogen spin relaxation rates change, and we probe the speed of qubit evolution. We address geometric QSLs based on two distinguishability measures of quantum states, quantum Fisher information (QFI) and Wigner-Yanase skew information (WY) metrics. For high concentrations of the salt, the system undergoes a Markovian dynamics, and the tighter QSL is set by the WY metric. For low concentrations, we observe crossovers between QSLs related to the QFI and WY metrics, while the system exhibits non-Markovian dynamics. The QSLs are sensitive to even small fluctuations in spin magnetization, from low to high concentrations. Our results find applications in quantum computing and optimal control

Effects of clays on spin-spin relaxation: a route for non-invasive total clay content quantification

Clay minerals are important components of sandstone rocks, due to their significant role on petrophysical properties like porosity and permeability. These minerals have a particular impact on Nuclear Magnetic Resonance measurements since the iron contained on clays generates internal gradients which directly affect the transverse relaxation. Here, we apply a methodology recently developed to a set of 20 sandstones, with varying clay content and mineralogy, in order to estimate the total clay content by using the effect of internal gradients on transverse relaxation. Based on these measurements, we propose a geochemical rock typing from quantities determined by our measurements, namely the total clay content and porosity

Quantum discord determines the interferometric power of quantum states

Quantum metrology exploits quantum mechanical laws to improve the precision in estimating technologically relevant parameters such as phase, frequency, or magnetic fields. Probe states are usually tailored to the particular dynamics whose parameters are being estimated. Here we consider a novel framework where quantum estimation is performed in an interferometric configuration, using bipartite probe states prepared when only the spectrum of the generating Hamiltonian is known. We introduce a figure of merit for the scheme, given by the worst-case precision over all suitable Hamiltonians, and prove that it amounts exactly to a computable measure of discord-type quantum correlations for the input probe. We complement our theoretical results with a metrology experiment, realized in a highly controllable room-temperature nuclear magnetic resonance setup, which provides a proof-of-concept demonstration for the usefulness of discord in sensing applications. Discordant probes are shown to guarantee a nonzero phase sensitivity for all the chosen generating Hamiltonians, while classically correlated probes are unable to accomplish the estimation in a worst-case setting. This work establishes a rigorous and direct operational interpretation for general quantum correlations, shedding light on their potential for quantum technology