Dear quanta,
We will have two talks next week. We will also have a group meeting Friday at 11 where we go around the room and give updates. This will be our last group meeting for the year.
The talks are
1. Mon, 10:30, Jonathan Oppenheim
A post-quantum theory of classical gravity?
2. Fri, 1:30, Raghavendra Srinivas
Laser-free trapped-ion quantum logic using an oscillating magnetic field gradient at radio frequency
Here is more detail.
1.
Monday, December 9, 2019
10:30am
6C-442
title: A post-quantum theory of classical gravity?
speaker: Jonathan Oppenheim (University College London)
abstract:
We present a consistent theory of classical systems coupled to quantum ones. The dynamics is linear in the density matrix, completely positive and trace-preserving. We apply this to construct a theory of classical gravity coupled to quantum field theory. The theory doesn't suffer the pathologies of semi-classical gravity and reduces to Einstein's equations in the appropriate limit. The assumption that gravity is classical necessarily modifies the dynamical laws of quantum mechanics -- the theory must be fundamentally information destroying involving finite sized and stochastic jumps in space-time and in the quantum field. Nonetheless the quantum state of the system can remain pure conditioned on the classical degrees of freedom. The measurement postulate of quantum mechanics is not needed since the interaction of the quantum degrees of freedom with classical space-time necessarily causes collapse of the wave-function. The theory can be regarded as fundamental, or as an effective theory of quantum field theory in curved space where backreaction is consistently accounted for.
2.
Friday, Dec 13, 2019
1:30pm
6C-442
Laser-free trapped-ion quantum logic using an oscillating magnetic field gradient at radio frequency
Raghavendra Srinivas (NIST & U.C. Boulder)
Trapped-ion quantum logic is usually performed using laser-induced coupling of the ions’ internal spin states to their motion. Laser-free spin-motion coupling methods, which eliminate photon scattering errors and offer benefits for scalability, have been proposed and demonstrated using static magnetic field gradients or magnetic field gradients oscillating at GHz frequencies [1-4]. We present a new method of spin-motion coupling for trapped ions without lasers, instead using microwaves and a magnetic field gradient oscillating at radio frequency [5]. We demonstrate and characterize this method with trapped Mg+ ions, cooling a single mode of motion to close to its ground state, and generating entangled states of two ions. This implementation offers important technical advantages over other laser-free techniques, while also enabling laser-free entangling gates with reduced sensitivity to qubit frequency errors [6]. These experiments are performed in a surface-electrode trap that incorporates current-carrying electrodes to generate the microwave fields and the oscillating magnetic field gradient. Currently, we achieve a Bell-state fidelity of 0.996(2) with ground-state-cooled ions and 0.991(3) for ions cooled to the Doppler limit (nbar = 2).
[1] Mintert and Wunderlich PRL 87, 257904 (2001)
[2] Weidt et al. PRL 117, 220501 (2016)
[3] Ospelkaus et al. Nature 476, 181 (2011)
[4] Harty et al. PRL 117, 140501 (2016)
[5] Srinivas et al. PRL 122, 163201 (2019)
[6] Sutherland et al. NJP 21, 033033 (2019)