3,543 research outputs found
Tomographically reconstructed master equations for any open quantum dynamics
Memory effects in open quantum dynamics are often incorporated in the
equation of motion through a superoperator known as the memory kernel, which
encodes how past states affect future dynamics. However, the usual prescription
for determining the memory kernel requires information about the underlying
system-environment dynamics. Here, by deriving the transfer tensor method from
first principles, we show how a memory kernel master equation, for any quantum
process, can be entirely expressed in terms of a family of completely positive
dynamical maps. These can be reconstructed through quantum process tomography
on the system alone, either experimentally or numerically, and the resulting
equation of motion is equivalent to a generalised Nakajima-Zwanzig equation.
For experimental settings, we give a full prescription for the reconstruction
procedure, rendering the memory kernel operational. When simulation of an open
system is the goal, we show how our procedure yields a considerable advantage
for numerically calculating dynamics, even when the system is arbitrarily
periodically (or transiently) driven or initially correlated with its
environment. Namely, we show that the long time dynamics can be efficiently
obtained from a set of reconstructed maps over a much shorter time.Comment: 10+4 pages, 5 figure
Positivity in the presence of initial system-environment correlation
The constraints imposed by the initial system-environment correlation can
lead to nonpositive Dynamical maps. We find the conditions for positivity and
complete positivity of such dynamical maps by using the concept of an
assignment map. Any initial system-environment correlations make the assignment
map nonpositive, while the positivity of the dynamical map depends on the
interplay between the assignment map and the system-environment coupling. We
show how this interplay can reveal or hide the nonpositivity of the assignment
map. We discuss the role of this interplay in Markovian models.Comment: close to the published version. 5 pages, 1 figur
Non-Markovian memory in IBMQX4
We measure and quantify non-Markovian effects in IBM's Quantum Experience.
Specifically, we analyze the temporal correlations in a sequence of gates by
characterizing the performance of a gate conditioned on the gate that preceded
it. With this method, we estimate (i) the size of fluctuations in the
performance of a gate, i.e., errors due to non-Markovianity; (ii) the length of
the memory; and (iii) the total size of the memory. Our results strongly
indicate the presence of non-trivial non-Markovian effects in almost all gates
in the universal set. However, based on our findings, we discuss the potential
for cleaner computation by adequately accounting the non-Markovian nature of
the machine.Comment: 8 page
An introduction to operational quantum dynamics
In the summer of 2016, physicists gathered in Torun, Poland for the 48th
annual Symposium on Mathematical Physics. This Symposium was special; it
celebrated the 40th anniversary of the discovery of the
Gorini-Kossakowski-Sudarshan-Lindblad master equation, which is widely used in
quantum physics and quantum chemistry. This article forms part of a Special
Volume of the journal Open Systems & Information Dynamics arising from that
conference; and it aims to celebrate a related discovery -- also by Sudarshan
-- that of Quantum Maps (which had their 55th anniversary in the same year).
Nowadays, much like the master equation, quantum maps are ubiquitous in physics
and chemistry. Their importance in quantum information and related fields
cannot be overstated. In this manuscript, we motivate quantum maps from a
tomographic perspective, and derive their well-known representations. We then
dive into the murky world beyond these maps, where recent research has yielded
their generalisation to non-Markovian quantum processes.Comment: Submitted to Special OSID volume "40 years of GKLS
Information Systems Undergraduate Degree Project: Gaining a Better Understanding of the Final Year Project Module
The place of an individual project in the final year of Information Systems (IS) undergraduate degrees at UK universities is well established. In this paper we compare the final year project modules at four UK universities: the University of Brighton, the University of South Wales, University of West London and the University of Westminster. We find that the aims of the projects are similar, emphasising the application of the knowledge and skills from the taught element of their course in a complex development project, often including interactions with a real client. Although we show in this analysis that projects serve a similar purpose in the IS degree courses, the associated learning outcomes and the assessment practice varies across the institutions. We identify some gaps in the skills and abilities that are not being assessed. In further work we are planning to consult final year students undertaking their projects and their supervisors, in order to gain an understanding of how project assessment criteria are actually put to use
Stroke and HIV - casual or coincidental co-occurrence?
No Abstract. South African Medical Journal Vol. 96 (12) 2006: pp.1247-124
Linear Assignment Maps for Correlated System-Environment States
An assignment map is a mathematical operator that describes initial
system-environment states for open quantum systems. We reexamine the notion of
assignments, introduced by Pechukas, and show the conditions assignments can
account for correlations between the system and the environment, concluding
that assignment maps can be made linear at the expense of positivity or
consistency is more reasonable. We study the role of other conditions, such as
consistency and positivity of the map, and show the effects of relaxing these.
Finally, we establish a connection between the violation of positivity of
linear assignments and the no-broadcasting theorem.Comment: 6 pages, 1 tabl
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