July 2018

Abstracts of the QSIT Lunch Seminar, Thursday, July 5, 2018

Watching the precession of a single nuclear spin by weak measurements

Kristian Cujia – Spin Physics and Imaging (Degen group), ETH Zurich

Nuclear magnetic resonance (NMR) spectroscopy is a powerful technique for analyzing the structure and function of molecules, and for performing three-dimensional imaging of the spin density. At the heart of NMR spectrometers is the detection of electromagnetic radiation, in the form of a free induction decay (FID) signal, generated by nuclei precessing around an applied magnetic field. While conventional NMR requires signals from 1e12 or more nuclei, recent advances in sensitive magnetometry have dramatically lowered this number to a level where few or even individual nuclear spins can be detected. It is natural to ask whether continuous FID detection can still be applied at the single spin level, or whether quantum back-action modifies or even suppresses the NMR response. Here we report on tracking of single nuclear spin precession using periodic weak measurements. Our experimental system consists of a 13C nuclear spin in diamond that is weakly interacting with the electronic spin of a nearby nitrogen-vacancy center, acting as an optically readable meter qubit. We observe and minimize two important effects of quantum back-action: Measurement-induced decoherence and frequency synchronization with the sampling clock. We use weak measurements to demonstrate nanoscale NMR spectroscopy with a simultaneous enhancement of the signal-to-noise ratio (SNR), frequency bandwidth and spectral resolution. Our method may provide the optimum route for performing single-molecule NMR at atomic resolution.

Quantum computing for quantum chemistry simulations

Panagiotis Kl. Barkoutsos – IBM Research - Zurich

Quantum computing is developing into a new computational paradigm that offers the unique opportunity to tackle classically hard problems with exponential speed up. One of the fields that can directly benefit from this development is quantum chemistry. In this case, the exponentially hard part of the calculations consists in the sampling of the molecular Hilbert space in search for the minimal energy eigenstate, which however can be efficiently mapped into a quantum circuit. Recently a general methodology for the generation of hardware specific trial wave functions was introduced by Kandala et. al in [1]. Here we mainly focus on the investigation of classically inspired wave function ansatzes (like the Unitary Coupled Cluster expansion) that preserve important constraints as the total number of electrons and the spin multiplicity. These developments will be presented within the framework of the IBM Quantum Software Information Kit (QISKit) using the higher level library QISKit Algorithms and Circuits for QUantum Applications (QISKit ACQUA) [2].

Co-author:
Ivano Tavernelli  – IBM Research - Zurich

References:
[1] A. Kandala, A. Mezzacapo, K. Temme, M. Takita, M. Brink, J. M. Chow, and J. M. Gambetta. Hardware-efficient variational quantum eigensolver for small molecules and quantum magnets. Nature, 549(7671):242-246, Sept. 2017.
[2] QISKit Open Source Quantum Information Software Kit - https://www.qiskit.org/.