Exploring quantum foundations by NMR: Quantum correlations and measurements

Isabela Almeida Silva, Davide Girolami, R. Auccaise +6 more

Isabela A. Silva, Jefferson Gonçalves Filgueiras, R. Auccaise +7 more

T. S. Mahesh, C. S. Sudheer Kumar, Udaysinh T. Bhosale

Davide Girolami

The project concerned the study of quantum correlations (QC) in compound systems, i.e. statistical correlations more general than entanglement which are predicted by quantum mechanics but not described in any classical scenario. I aimed to understand the technical and operational properties of the measures of QC, their interplay with entanglement quantifiers and the experimental accessibility.\nIn the first part of my research path, after having acquired the conceptual and technical rudiments of the project, I provided solutions for some computational issues: I developed analytical and numerical algorithms for calculating bipartite QC in finite dimensional systems. Then, I tackled the problem of the experimental detection of QC. There is no Hermitian operator associated with entanglement measures, nor with QC ones. However, the information encoded in a density matrix is redundant to quantify them, thus the full knowledge of the state is not required to accomplish the task. I reported the first protocol to measure the QC of an unknown state by means of a limited number of measurements, without performing the tomography of the state. My proposal has been implemented experimentally in a NMR (Nuclear Magnetic Resonance) setting.\nIn the final stage of the project, I explored the foundational and operational merits of QC. I showed that the QC shared by two subsystems yield a genuinely quantum kind of uncertainty on single local observables. The result is a promising evidence of the potential exploitability of separable (unentangled) states for quantum metrology in noisy conditions.

Xinfang Nie, Ze Zhang, Xiuzhu Zhao +3 more

Out-of-time-order correlator (OTOC), been suggested as a measure of quantum information scrambling in quantum many-body systems, has received enormous attention recently. The experimental measurement of OTOC is quite challenging. The existing theoretical protocols consist in implementing time-reversal operations or using ancillary quantum systems, therefore only a few experiments have been reported. Recently, a new protocol to detect OTOC using statistical correlations between randomized measurements was put forward. In this work, we detect the OTOCs of a kicked-Ising model using this new measurement method on a 4-qubit nuclear magnetic resonance quantum simulator. In experiment, we use random Hamiltonian evolutions to generate the random operations that are required by the randomized OTOC detection protocol. Our experimental results are in good agreement with the theoretical predictions, thus confirming the feasibility of the protocols. Therefore, our work represents a step in exploring realistic quantum chaotic dynamics in complicated quantum systems.

Jitendra A. Joshi, T. S. Mahesh

Theoretical explorations have revealed that quantum batteries can exploit quantum correlations to achieve faster charging, thus promising exciting applications in future technologies. Using NMR architecture, here we experimentally investigate various aspects of quantum batteries with the help of nuclear spin systems in a star-topology configuration. We first carry out numerical analysis to study how charging a quantum battery depends on the relative purity factors of charger and battery spins. By experimentally characterizing the state of the battery spin undergoing charging, we estimate the battery energy as well as the ergotropy, the maximum amount of work that is unitarily available for extraction. The experimental results thus obtained establish the quantum advantage in charging the quantum battery. We propose using the quantum advantage, gained via quantum correlations among chargers and the battery, as a measure for estimating the size of the correlated cluster. We develop a simple iterative method to realize asymptotic charging that avoids the oscillatory behavior of charging and discharging. Finally, we introduce a load spin and realize a charger-battery-load circuit and experimentally demonstrate battery energy consumption after varying the duration of battery storage, for up to 2 min.