Extracting order from a quantum measurement finally shown experimentally


Date:
September 8, 2020
Source:
University of Copenhagen
Summary:
In physics, it is essential to be able to show a theoretical
assumption in actual, physical experiments. For more than a
hundred years, physicists have been aware of the link between
the concepts of disorder in a system, and information obtained by
measurement. However, a clean experimental assessment of this link
in common monitored systems, that is systems which are continuously
measured over time, was missing so far.



FULL STORY ==========================================================================
In physics, it is essential to be able to show a theoretical assumption
in actual, physical experiments. For more than a hundred years, physicists
have been aware of the link between the concepts of disorder in a system,
and information obtained by measurement. However, a clean experimental assessment of this link in common monitored systems, that is systems
which are continuously measured over time, was missing so far.


==========================================================================
But now, using a "quantum drum," a vibrating, mechanical membrane,
researchers have realized an experimental setup that shows the physical interplay between the disorder and the outcomes of a measurement. A collaboration of experimentalists from the Niels Bohr Institute,
University of Copenhagen and theorists at Queen's University Belfast,
and the University of Sao Palo, could show how to extract order from
this largely disordered system, providing a general tool to engineer
the state of the system, essential for future quantum technologies,
like quantum computers. The result is now published in as an Editors' Suggestion in Physical Review Letters.

Measurements will always introduce a level of disturbance of any system it measures. In the ordinary, physical world, this is usually not relevant, because it is perfectly possible for us to measure, say, the length of
a table without noticing that disturbance. But on the quantum scale, the consequences of the disturbance made by measurements are huge. These large disturbances increase the entropy, or disorder, of the underlying system,
and apparently preclude to extract any order from the measurement. But
before explaining how the recent experiment realized this, the concepts
of entropy and thermodynamics need a few words.

Breaking an egg is thermodynamics The law of thermodynamics covers
extremely complicated processes. The classic example is that if an egg
falls off of a table, it breaks on the floor. In the collision, heat is produced -- among many other physical processes -- and if you imagine
you could control all of these complicated processes, there is nothing
in the physical laws that say you can't reverse the process. In other
words, the egg could actually assemble itself and fly up to the table
surface again, if we could control the behavior of every single atom,
and reverse the process. It is theoretically possible. You can also think
of an egg as an ordered system, and if it breaks, it becomes extremely disordered. Physicists say that the entropy, the amount of disorder,
has increased. The laws of thermodynamics tell us that the disorder
will in fact always increase, not the other way round: So eggs do not
generally jump off floors, assemble and land on tables in the real world.

Correct quantum system readouts are essential -- and notoriously
difficult to obtain If we turn to quantum mechanics, the world looks
rather different, and yet the same. If we continuously measure the
displacement of a mechanical, moving system like the "membrane-drum" (illustration 1) with a precision only limited by the quantum laws,
this measurement disturbs the movement profoundly. So you will end
up measuring a displacement which is disturbed during the measurement
process itself, and the readout of the original displacement will be
spoiled - - unless you can measure the introduced disorder as well. In
this case, you can use the information about the disorder to reduce
the entropy produced by the measurement and generate order from it -- comparable to controlling the disorder in the shattered egg-system. But
this time we have the information on the displacement as well, so
we have learnt something about the entire system along the way, and,
crucially, we have access to the original vibration of the membrane,
i.e. the correct readout. Alessio Belenchia, the study's senior author,
and his colleagues from Belfast and Sao Paolo have established a powerful formal framework for this kind of analysis.

A generalized framework for understanding entropy in quantum systems "The connection between thermodynamics and quantum measurements has been known
for more than a century. However, an experimental assessment of this link
was missing so far, in the context of continuous measurements. That is
exactly what we have done with this study. It is absolutely essential that
we understand how measurements produce entropy and disorder in quantum
systems, and how we use it in order to have control over the readouts
we shall have in the future from, say, a quantum system like a quantum computer. If we are not able to control the disturbances, we basically
won't be able to understand the readouts -- and the quantum computer
readouts will be illegible, and useless, of course," says Massimiliano
Rossi, PhD student and first author on the scientific article.

"This framework is important in order to create a generalized basic
foundation for our understanding of entropy producing systems on the
quantum scale. That's basically where this study fits into the grander
scale of things in physics."

========================================================================== Story Source: Materials provided by University_of_Copenhagen. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Massimiliano Rossi, Luca Mancino, Gabriel T. Landi, Mauro
Paternostro,
Albert Schliesser, Alessio Belenchia. Experimental Assessment
of Entropy Production in a Continuously Measured Mechanical
Resonator. Physical Review Letters, 2020; 125 (8) DOI:
10.1103/PhysRevLett.125.080601 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200908101634.htm

--- up 2 weeks, 1 day, 6 hours, 50 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1337:3/111)