December 2015

Abstracts of the QSIT Lunch Seminar, Thursday, May 12, 2016

A switchable source for extremely high magnetic field gradients

Alexander Eichler, Spin Physics and Imaging - Degen Lab, ETH Zurich

Nanoscale control over magnetic fields is an essential capability in many areas of science and technology, including magnetic data storage, spintronics, quantum control of spins, and nanoscale magnetic resonance imaging. The design of nanoscale magnetic sources has always been a compromise between attainable field strength and switching speed. While strong local fields of a few hundred mT and gradients of order 10^6 T/m can be reached by static ferromagnetic tips, dynamical fields and gradients produced by coils or microstrips are at least an order of magnitude smaller. This imposes severe restrictions on many applications such as single spin detection and manipulation. In this work, we demonstrate that the write head of a commercial hard drive is ideally suited to overcome these restrictions. By scanning a sharp diamond tip over the write pole and measuring the induced diamagnetic force with a nanomechanical transducer, we are able to image the pole magnetic field with ~10 nm spatial resolution. A gradient of 28x10^6 T/m is estimated when the tip is approached to within 5 nm of the surface, roughly five times higher compared to that of the best static tips. By design, field and gradient are switchable in ~1 ns. Further desirable features include high-vacuum compatibility, low power dissipation, and an extremely flat surface topography amenable to follow-up lithography. Recording heads thus have the potential for important advances in basic research, ranging from single nucleon magnetic resonance to the study of condensed matter under local field variations.

Practical, Reliable Error Bars in Quantum Tomography

Philippe Faist, Quantum Information Theory, ETH Zurich

Precise characterization of quantum devices is usually achieved with quantum tomography. However, most methods which are currently widely used in experiments, such as maximum likelihood estimation, lack a well-justified error analysis. Here, we propose a practical yet robust method for obtaining error bars. We do so by introducing a novel representation of the output of the tomography procedure, the quantum error bars. This representation is (i) concise, being given in terms of few parameters, (ii) intuitive, providing a fair idea of the “spread” of the error, and (iii) useful, containing the necessary information to construct confidence regions. The statements resulting from our method are formulated in terms of a figure of merit, such as the fidelity to a reference state. We present an algorithm to compute this representation and provide ready-to-use software. Our procedure is applied to actual experimental data obtained from two superconducting qubits in an entangled state, demonstrating the applicability of our method.