Hi all,
We're back tomorrow from the winter hiatus. Victor will tell us about
his latest research. We'll start at 2pm on zoom:
https://ethz.zoom.us/j/362994444.
Best,
Joe
Hi all,
Tomorrow Oliver Knapp will tell us about his master thesis with Esteban,
entitled "Relational quantities and subsystems in symmetry-constrained
quantum theories". See below for the abstract. Join us at 2pm on Zoom:
https://ethz.zoom.us/j/362994444.
Best,
Joe
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Abstract: It is argued that only relational quantities, such as the
distance between two objects, are physically meaningful. Consequently,
unobservable quantities, such as the global position of a particle, should
be considered as artifacts of our mathematical description. In this work,
we review current approaches, based on coherent and incoherent group
averaging, aimed at identifying and describing relational quantities, and
discuss some difficulties arising within these approaches. In particular,
we study subsystems defined w.r.t. reference frames and show that
unexpected phenomena occur once we consider "unsharp" reference frames
instead of ideal ones. Furthermore, we introduce a rigorous definition of
relational subsystems based on group averaged operator algebras. However,
we prove several theorems which suggest that group averaged operators
cannot be used in quantum field theory to define a relational subsystem
structure that is compatible with the causal structure of spacetime. We
argue that such difficulties arise because approaches involving group
averaging can only capture a subset of all relational quantities contained
within a theory. Subsequently, we propose a general framework, based on
probability theory, that allows for a rigorous description of all
relational quantities. We demonstrate that within this framework many
difficulties encountered when using group averaging can be overcome.
Hi all,
Tomorrow Eliott Mamon will present his semester project with Victor,
entitled "Exploring noncontextuality of prepare-and-measure scenarios
with a numerical algorithm". See below for the abstract. Join us at
2pm on zoom: https://ethz.zoom.us/j/362994444
Best,
Joe
%%%%%
Using a numerical implementation of the unit-separability algorithm
proposed in [1], the question of existence of a noncontextual model is
assessed for various single-qubit prepare-and-measure scenarios.
Noncontextual scenarios that are not included in the Spekkens toy
model scenario are pointed out, and a trend between contextuality and
areas of randomly sampled planar scenarios is uncovered.
(Non)contextuality is also assessed for scenarios involved in
strategies of a two-agent encoding/decoding game, which exposes
contextuality as necessary and sufficient for better-than-classical
strategies at that game.
[1] Gitton, V., & Woods, M. P. (2020). Solvable criterion for the
contextuality of any prepare-and-measure scenario. arXiv preprint
arXiv:2003.06426
Hi all,
Tomorrow our guest Marco Tomamichel will tell us about some of his latest
research, specifically "Sequential quantum hypothesis testing". The paper
is available at https://arxiv.org/abs/2104.14706, and I have copied the
abstract below.
Join us at 2pm on zoom: https://ethz.zoom.us/j/362994444.
Best,
Joe
%%%%%
We consider sequential hypothesis testing between two quantum states using
adaptive and non-adaptive strategies. In this setting, samples of an
unknown state are requested sequentially and a decision to either continue
or to accept one of the two hypotheses is made after each test. Under the
constraint that the number of samples is bounded, either in expectation or
with high probability, we exhibit adaptive strategies that minimize both
types of misidentification errors. Namely, we show that these errors
decrease exponentially (in the stopping time) with decay rates given by the
measured relative entropies between the two states. Moreover, if we allow
joint measurements on multiple samples, the rates are increased to the
respective quantum relative entropies. We also fully characterize the
achievable error exponents for non-adaptive strategies and provide
numerical evidence showing that adaptive measurements are necessary to
achieve our bounds under some additional assumptions.
>From Vilasini:
Hi all,
This week we have a visitor, Ämin Baumeler from IQOQI, Vienna who will
be giving a seminar on “Logical limits on correlations and causal
structures” at 4pm on Thursday, 2nd of December at HIT E 41.1. The
abstract is as attached below. It would be a blackboard talk but I
will aim to stream it live (and also record it) at the following zoom
link, for those who would be interested in attending online.
https://ethz.zoom.us/j/66298777504
Title:
Logical limits on correlations and causal structures
Abstract:
The self-consistency principle stipulates that the only solutions to
physical laws that occur locally are those that are self-consistent.
In this talk, after a discussion of this principle, I present several
consequences. First, I show that in a classical setting a violation of
this principle not only encompasses what colloquially is known as the
grandparent paradox, but also that this paradox is equivalent to
the information paradox (with E. Tselentis, QPL 2020,
10.4204/EPTCS.340.1). Second, I present self-consistent multi-party
correlations inspired by Bell, Ardehali, and Svetlichny that violate
causal order to an unbounded degree (with A. S. Gilani and J. Rashid,
arXiv:2004.12921 [quant-ph]). Finally, I discuss some recent
characterizations of self-consistent and inconsistent causal structures
(with E. Tselentis, work in progress).
Best regards,
Vilasini
Hi all,
Tomorrow we will hear from Eloïc Vallée, who did his masters thesis in
conjunction with the Brunner group in Geneva, on "mdiQKD with
assumptions on the states overlap". See below for the abstract. We'll
start at 2pm on zoom: https://ethz.zoom.us/j/362994444.
Best,
Joe
%%%%%%%%
Title: mdiQKD with assumptions on the states overlap
Abstract:
With the upcoming generation of quantum computers, secure
communications will essentially depend on quantum key distribution
(QKD) algorithms. One of the most promising algorithm is the
measurement-device-independent QKD (mdiQKD), first proposed by Lo,
Curty and Qi in 2012. We explore a modified mdiQKD protocol with a
relaxed assumption on the initial states. The assumption relies on the
overlap between the states and not on the exact characterization of
the states. The protocol is a tripartite prepare-and-measure based
protocol. Two parties prepare states and send them to a connecting
node, which performs a measurement on the joint states. And no
assumption is made on the node's measurement. The protocol is
therefore immune to attack on the third party. A secret key can be
established by verifying the honesty of the measurement at the node
through observed statistics. A positive key rate is found for
different situations and demonstrates the protocol viability.
Hi all,
Tomorrow Zhenning Liu will tell us about his master thesis with Andru,
entitled "Depth-efficient proofs of quantumness". See below for the
abstract.
We start as usual at 2pm on zoom: https://ethz.zoom.us/j/362994444.
Best,
Joe
%%%%%%
Abstract: A proof of quantumness is a type of challenge-response protocol
in which a classical verifier can efficiently certify the quantum advantage
of an untrusted prover. That is, a quantum prover can correctly answer the
verifier's challenges and be accepted, while any polynomial-time classical
prover will be rejected with high probability, based on plausible
computational assumptions. To answer the verifier's challenges, existing
proofs of quantumness typically require the quantum prover to perform a
combination of polynomial-size quantum circuits and measurements.
In this project, we give two proofs of quantumness constructions in which
the prover need only perform constant-depth quantum circuits (and
measurements) together with log-depth classical computation. Our first
construction is a generic compiler that allows us to translate all existing
proofs of quantumness into constant quantum depth versions. Our second
construction is based around the learning with rounding problem and yields
circuits with shorter depth and requiring fewer qubits than the generic
construction. In addition, the second construction also has some robustness
against noise.
Hi all,
Tomorrow we will hear from our guest Raffaele Salvia, from SNS in Pisa, on
his recent work "The classical capacity of quantum channels and the problem
of energy preservation". See below for the abstract. We start at 2pm on
Zoom: https://ethz.zoom.us/j/362994444.
Best,
Joe
%%%%%%
Finding the classical capacity of quantum channels is one of the main open
problems in Quantum Information Theory; it has been solved only for some
special classes of channels. This long-standing problem reveals a
surprising connection with the optimal output entropy problem, which arises
in the theory of quantum energy storage. Through the examples of the qubit
amplitude-damping channel and of continuous variable Gaussian channels, I
will show how this relationship can shed new light on both of the problems.
Hi all,
Tomorrow Lorenzo Laneve will tell us about his semester project with Lídia,
entitled "Impossibility of composable Oblivious Transfer in relativistic
quantum cryptography". See below for the abstract. We start at 2pm on zoom:
https://ethz.zoom.us/j/362994444.
Best,
Joe
%%%%
https://arxiv.org/abs/2106.11200
Abstract: We study the cryptographic primitive Oblivious Transfer; a
composable construction of this resource would allow arbitrary multi-party
computation to be carried out in a secure way, i.e. to compute functions in
a distributed way while keeping inputs from different parties private.
First we review a framework that allows us to analyze composability of
classical and quantum cryptographic protocols in special relativity:
Abstract Cryptography implemented with Causal Boxes. We then (1) explore
and formalize different versions of oblivious transfer found in the
literature, (2) prove that their equivalence holds also in relativistic
quantum settings, (3) show that it is impossible to composably construct
any of these versions of oblivious transfer from only classical or quantum
communication among distrusting agents in relativistic settings, (4) prove
that the impossibility also extends to multi-party computation, and (5)
provide a mutual construction between oblivious transfer and bit commitment.
Hi all,
Tomorrow we will hear from Davide Materia on his master's thesis with Ivano
Tavernelli at IBM, entitled "DFT functionals trained with Quantum
Computers". See below for the abstract. We start at 2pm on zoom:
https://ethz.zoom.us/j/362994444.
Best,
Joe
%%%%%%
Abstract: In this thesis, we developed different methods for the training
and the optimization of a Density Functional Theory (DFT) exchange and
correlation functionals, vxc, which relies on the minimization of the
electronic density difference obtained by subtracting the approximated DFT
density with a reference one evaluated on a quantum computer. The
determination of accurate and universal DFT vxc functional represents an
unsolved problem in computational chemistry and despite many attempts
(based on theoretical as well as heuristic approaches) a general solution
that is applicable in all physical contexts is still lacking. The new
generation of DFT functionals are build combining different energy
contributions and energy corrections (such as exchange, correlation but
also van der Waals energy terms with different short and long range
behaviors) weighted by a number of parameters that are usually fitted to
minimize the errors on a set of properties (mainly relative energies) in a
training set of molecules and solid state systems.
In this work, motivated by the central role of the density in the
Hohenberg-Kohn theorem, we investigate the possibility to use directly the
electronic density of molecular systems to optimize the DFT functionals,
hoping for a greater universality of the new functional resulting from this
optimization.