Hi all,
Tomorrow Anian Altherr will present his master's thesis, entitled "Quantum
metrology for non-Markovian processes". See below for the abstract. We
start at 2pm on zoom: https://ethz.zoom.us/j/362994444
Best,
Joe
---------------------
Quantum metrology is concerned with finding precision bounds in
parameter estimation of a given sample. Whereas classical strategies
allow the precision to scale at most linearly with the number of
resources used, quantum strategies can achieve a quadratic scaling.
Noise, however, in general reduces the quadratic scaling to a linear
scaling. Most work so far has dealt with assessing Markovian noise, but
in the future, assessing time-correlated noise, e.g. in deep quantum
circuits, will be of increasing interest. A general framework to assess
such non-Markovian noise has been missing. In this work, we use quantum
combs to model non-Markovian noise and express its quantum Fisher
information in terms of a semidefinite program and the conditional
min-entropy. We apply the semidefinite program to collision models,
which serve as an instance of non-Markovian noise. Numerics show that
the non-Markovian setting allows for more precise estimators than the
Markovian setting. Finally, we employ variational circuits to find an
explicit metrological setup that achieves the maximal precision.
Hi all,
Tomorrow Elias Huber will present his semester project, entitled "Maxwell's
Demon". See the abstract below. Usual time and place, 2pm at
https://ethz.zoom.us/j/362994444.
Best,
Joe
Abstract:
Maxwell's demon has been proclaimed exorcised more than once but continues
to fascinate in many ways. In this talk I will first give an introduction
to the theory of phenomenological thermodynamics presented by Dr. Philipp
Kammerlander in his PhD thesis [https://doi.org/10.3929/ethz-b-000413414].
I then talk about the role of phenomenological thermodynamics in the
discussion of Maxwell’s demon and go on to give a detailed description of
the Szilard engine. This description will be based on Landauer’s principle
and in agreement with the postulates of thermodynamics. Finally, we will
look at another thought experiment of an automated (Szilard engine-like)
demon and talk about the importance of fluctuations.
Hi all,
Tomorrow Roman Wixinger will present his semester project “Uncomputation
and Entanglement in High-level Quantum Programming Languages”, conducted
under the supervision of Prof. Christian Mendl at TU München. See below for
the abstract. We start as usual at 2pm on zoom:
https://ethz.zoom.us/j/362994444.
Best,
Joe
Quantum programming languages traditionally focus on the hardware level
and are therefore not really good at representing the intentions of the
programmer. Explicit formulation of uncomputation, which is essential
for the safe and efficient use of the qubits, makes the code
unnecessarily complex. In recent work, Vechev et al. (2020) introduced
Silq, a high-level language that allows for safe, automatic
uncomputation just using its type system. This feature makes the code
significantly shorter and more intuitive. The type system can also
ensure that any program that compiles is physical. In this project, we
compared Silq’s solution of handling uncomputation with other approaches
and give an overview of the features of quantum languages. We have also
tried to understand whether a qubit can be safely discarded by directly
looking at the entanglement.
Hi all,
Apologies for the late email. This afternoon Diane Saint Aubin will tell us
about her semester project, entitled "Lower and upper bounds on quantum key
distribution protocols". See below for the abstract. We'll start as usual
at 2pm in/at https://ethz.zoom.us/j/362994444.
Best,
Joe
Abstract:
The 3-state quantum key distribution protocol has two entanglement-based
descriptions, raising the question of which one is best used to compute the
key rate. We show that the descriptions are equivalent, and that
symmetrising the state used in the protocol can give a lower bound on the
secret key rate. In addition, we provide an upper bound for the BB84
protocol using the intrinsic information which just outperforms a recent
bound by Xing Wang. For the device independent protocol based on the CHSH
game, thresholds on the key rate are found depending on the observed
parameters. We find an upper bound for protocols using one way-classical
communication, which is much tighter than the previous two-way classical
communication bound for low values of the Bell violation.
Hi all,
Tomorrow our new postdoc Christopher Chubb will tell us about his research,
in particular his latest paper "General tensor network decoding of 2D Pauli
codes". See below for the abstract, or the full paper at
https://arxiv.org/abs/2101.04125. We start at 2pm in zoom:
https://ethz.zoom.us/j/362994444.
Best,
Joe
Abstract:
In this work we develop a general tensor network decoder for 2D codes.
Specifically, we propose a decoder which approximates maximally likelihood
decoding for 2D stabiliser and subsystem codes subject to Pauli noise. For
a code consisting of $n$ qubits our decoder has a runtime of $O(n\log
n+n\chi^3)$, where $\chi$ is an approximation parameter. We numerically
demonstrate the power of this decoder by studying four classes of codes
under three noise models, namely regular surface codes, irregular surface
codes, subsystem surface codes and colour codes, under bit-flip, phase-flip
and depolarising noise. We show that the thresholds yielded by our decoder
are state-of-the-art, and numerically consistent with optimal thresholds
where available, suggesting that the tensor network decoder well
approximates optimal decoding in all these cases. Novel to our decoder is
an efficient and effective approximate contraction scheme for arbitrary 2D
tensor networks, which may be of independent interest.
Hi all,
Tomorrow David Ittah will tell us about his master thesis on "Multi-level
IR for Quantum Program Optimization", which was supervised by Torsten
Hoefler from the CS department and Thomas Häner from Microsoft (formerly
Troyer group). See below for the abstract. The zoom link is
https://ethz.zoom.us/j/362994444.
Best,
Joe
Abstract:
Intermediate representations (IR) have traditionally provided numerous
benefits to compilation systems, in particular in the domain of static
program analysis and optimization. However, the quantum programming
landscape has yet to produce similarly powerful IRs, instead focusing on
the development of embedded domain-specific languages (eDSL). As these only
feature rudimentary IRs in the form of data structures for quantum
circuits, or simple quantum assembly (QASM) languages, they lack meaningful
integration with their classical host compilation infrastructure. As a
remedy, we propose a novel quantum IR design based on exposing SSA-like
quantum dataflow in the IR alongside classical dataflow. Our language- and
hardware-agnostic IR is designed to enable classical-quantum
co-optimization, and with the intent to maximize the reuse of existing
compilation infrastructure. An implementation in the MLIR compiler
framework demonstrates that ∼99.8% of savings identified by ProjectQ’s
run-time optimizations on Shor’s algorithm can be exploited by static
optimizations in our IR. A resource estimation routine to count the number
of rotation gates in Shor’s algorithm is shown to run 5-6 orders of
magnitudes faster on application-scale input sizes than on existing systems
in ProjectQ and Qiskit.
Hi all,
Tomorrow at 2pm Severin Meng will tell us about his master thesis, entitled
"Thermodynamic Properties of Passive States". See below for the abstract.
The meeting link is https://ethz.zoom.us/j/362994444.
Best,
Joe
Abstract:
A passive state is defined such that no unitary transformation can lower
its average energy. Within the set of passive states one finds the
well-known thermal states, which are described by a quantum analogue of
classical temperature. Interesting quantities like the efficiency, cooling
performance or even possibility of a state transition within settings that
use a thermal state resource are characterised by the state's temperature
and Hamiltonian.
Non-thermal passive states appear naturally, for example at the end of a
thermodynamic process like cooling, and they may thus be used as an initial
state in a different thermodynamic scenario. There is no unique temperature
of a non-thermal passive state that characterises its thermodynamic
performance. We investigate which properties of passive states determine
their thermodynamic behaviour. We approach this task by exploring different
thermodynamic settings that involve passive states. The considered settings
include an autonomous heat engine and refrigerator as well as state
transformations and more abstract work extraction protocols.
We find different parameters for the various scenarios that describe the
thermodynamics of the passive state. The set of relevant parameters
includes the temperatures of the thermal state of the same energy as well
as the thermal state of the same entropy, the asymptotic activation energy,
the free energy and entropy as well as specific combinations of energy gaps
and virtual temperatures within the passive state.
Hi all,
Tomorrow Henrik will kick off the QIT Seminar in the new year. He will tell
us about the connection between entropy and reversible catalysis, from
arXiv:2012.05573 <https://arxiv.org/abs/2012.05573> . The talk starts at
2pm on zoom: https://ethz.zoom.us/j/362994444.
Best,
Joe