Dear HQI Community,
Nearly a year ago, our friend, mentor, teacher, and hero, Roy Glauber passed away after a life dedicated to Physics. In honor of him and his life's work, we would like to invite you to celebrate his influence in the Physics community during a memorial workshop, to be held on January 15, 2020, at the Harvard Department of Physics (Jefferson Building). We will feature renowned members of the field for talks, to share fond memories of Roy, and engage in research findings discussions. Please find details about the workshop at the following website: Glauber Memorial Workshop<https://projects.iq.harvard.edu/royglaubermw2020> .
If you would like to present a poster at the poster session, to be held on January 15 from 4:30-6:30pm in Jefferson 450, please sign up HERE<https://docs.google.com/spreadsheets/d/106ajvgK06fx2DPZONMPFSmZd9gAb_UT2HGN…>.
If you have any questions about the logistics of the event, please email ann_quaicoe(a)fas.harvard.edu<mailto:ann_quaicoe@fas.harvard.edu>.
Best,
Susanne Yelin, on behalf of Glauber Memorial Organizing Committee
Dear quanta,
a reminder that Anurag is speaking today at 10:30.
> Monday, December 16, 2019
> 10:30am
> 6C-442
>
> Title: Bounds on entanglement entropy of ground states and scar states
>
> speaker: Anurag Anshu (University of Waterloo)
> abstract:
> Relevant many-body quantum states, such as gapped ground states and gibbs
> state, can be approximated as polynomials of the underlying hamiltonian.
> This simple idea has been remarkably useful; such as in the proofs of 1D
> area laws and PEPS representation of gibbs state. In this talk, we provide
> two new applications. First we show a subvolume law for frustration-free
> locally gapped spin systems in two dimensions, following the well known
> Approximate ground state projector (AGSP) approach. Our bound shows an
> entanglement entropy bound of approx L^{5/3}, for vertical and rectangular
> bipartitions of perimeter L. Second, we show that recently studied scar
> quantum states have subvolume bound on entanglement entropy on lattices of
> finite dimension, complementing a recent result of Alhambra and Wilming.
> Based on joint works with Itai Arad and David Gosset and with Alvaro
> Alhambra.
>
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Dear quanta,
Tomorrow we will meet at 11am in 6-310 and go around the room and give
updates. This will be our last group meeting for the semester.
In the afternoon we will have a talk which gives a sense of how up for
grabs the basic categories of quantum computers still are.
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)
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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)
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Dear quanta,
We will cancel group meeting tomorrow because there is a crypto seminar
that may be of interest to many of us. At 1:30pm in 6C-442, Adam will
tell us about his recent work (with me and Nicole) on macroscopic Bell
inequalities.
Next Monday we will have a talk by Jonathan Oppenheim at 10:30am in 6C-442
on his proposal for classical gravity. It should be accessible to QI
people.
The following Monday (Dec 16) will be Anurag Anshu at 10:30am speaking
about his recent paper on a subvolume law in 2D systems.
Details below.
Tomorrow's morning talk.
https://toc.csail.mit.edu/node/1360
Dhiraj Holden: No-Signaling Proofs with sqrt(log n) Provers is in PSPACE
Friday, December 6, 2019 - 10:30am to 12:00pm
Location: Hewlett, G882
Abstract: No-signaling proofs, motivated by quantum computation, have found
applications in cryptography and hardness of approximation. An important
open problem is characterizing the power of no-signaling proofs. It is
known that 2-prover no-signaling proofs are characterized by PSPACE, and
that no-signaling proofs with poly(n)-provers are characterized by EXP.
However, the power of k-prover no-signaling proofs, for 2 < k < poly(n)
remained an open problem.
We show that k-prover no-signaling proofs (with negligible soundness) for
k = √(log n) are contained in PSPACE. We prove this via two different
routes that are of independent interest. In both routes we consider a
relaxation of no-signaling called sub-no-signaling. Our main technical
contribution (which is used in both our proofs) is a reduction showing how
to convert any sub-no-signaling strategy with value at least 1 -
2^(-Omega(k^2)) into a no-signaling one with value at least 2^-O(k^2).
In the first route, we show that the classical prover reduction method for
converting k-prover games into 2-prover games carries over to the
no-signaling setting with the following loss in soundness: if a k-player
game has value less than 2^(−ck^2) (for some constant c>0), then the
corresponding 2-prover game has value at most 1−2(-dk^2) (for some constant
d>0). In the second route we show that the value of a sub-no-signaling game
can be approximated in space that is polynomial in the communication
complexity and exponential in the number of provers.
Afternoon talk
1:30pm, Fri, Dec 6, 6C-442
Adam Bene Watts
Nonlinear Bell inequality for macroscopic measurements
https://arxiv.org/abs/1911.09122
-aram
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