How monitoring quantum Otto engine affects its performance
Minimizing the measurement effects preserves coherence across engine
cycles and improves the power output and reliability

Date:
November 9, 2021
Source:
Institute for Basic Science
Summary:
How monitoring quantum Otto engine affects its
performance. Minimizing the measurement effects preserves coherence
across engine cycles and improves the power output and reliability.



FULL STORY ========================================================================== Covering a broad spectrum of different modes of operations of engines with
a working substance having just two quantum states, the researchers found
that only for idealized cycles that perform infinitely slowly it makes
no difference which monitoring scheme is applied. But all engines that
run in finite time and hence are of practical interest work considerably
better for their power output and reliability when they are monitored
according to the repeated contact scheme.


========================================================================== confined to a piston, which undergoes four subsequent strokes: it is
first compressed, then heated up, expanded, and finally cooled down to
its initial temperature.

Today, with significant advancements in nano-fabrication, the
quantum revolution is upon us, bringing quantum heat engines into the limelight. Like their classical counterparts, quantum heat engines
could be operated in various protocols which might be continuous or
cyclic. Unlike classical engine which uses a macroscopic amount of
the working substance, the working substance of a quantum engine
has pronounced quantum features. The most prominent of these is
the discreteness of the possible energies it can take. Even more
outlandish from the classical point of view is the fact that a
quantum system may stay in two or more of its allowed energies at the
same time. This property, which has no classical analog, is known as `coherence'. Otherwise, a quantum Otto engine is also characterized by
four strokes like its classical counterpart.

Determining the quantum Otto engine's performance metrics, such as power
output or efficiency is the key to improving design and tailoring better working substances. A direct diagnosis of such metrics requires measuring
the energies of the engine at the beginning and end of each stroke. While
a classical engine is only negligibly affected by measurements, in quantum engines the act of the measurement itself causes a bizarre measurement
effect in which the engine's quantum state is severely affected via
quantum mechanics. Most importantly, any coherence in the system at the
end of the cycle would be completely removed by the measurement effect.

It has long been believed that these strange measurement-induced effects
are irrelevant for the understanding of quantum engines and hence have
been neglected in traditional quantum thermodynamics. Moreover, not
much thought has been put into the design of monitoring protocols that
yield a reliable diagnosis of the engine's performance while minimally
altering it.

However, novel breakthrough research performed at the Center for
Theoretical Physics of Complex Systems within the Institute for Basic
Science, South Korea, may change this rigid perspective. The researchers investigated the impact of different measurement-based diagnostic schemes
on the performance of a quantum Otto engine. In addition, they discovered
a minimally invasive measurement method that preserves coherence across
the cycles.

The researchers utilized the so-called `repeated contacts scheme', where
they record the engine's states using an ancillary probe, and measurements
of the probe are performed only at the end of the engine's working
cycles. This bypasses the need to measure the engine repeatedly after
each stroke and avoids undesirable measurement-induced quantum effects
such as the removal of any coherence that was built up during the cycle.

The preservation of coherence throughout the engine's lifetime
enhanced critical performance metrics like the maximum power output and reliability, making the engine more capable and dependable. According
to Prof. Thingna, "this is the first example in which the influence of
an experimenter, who wants to know whether the engine does what it is
designed to do, has been properly considered." Covering a broad spectrum
of different modes of operations of engines with a working substance
having just two quantum states, the researchers found that only for
idealized cycles that perform infinitely slowly it makes no difference
which monitoring scheme is applied. But all engines that run in finite
time and hence are of practical interest work considerably better for
their power output and reliability when they are monitored according to
the repeated contact scheme.

Overall, the researchers concluded that the nature of the measurement techniques can bring theory closer to experimental data. Hence, it is
vital to take these factors into account when monitoring and testing
quantum heat engines. This research was published in the Physical Review
X Quantum.

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


========================================================================== Journal Reference:
1. Jeongrak Son, Peter Talkner, Juzar Thingna. Monitoring Quantum Otto
Engines. PRX Quantum, 2021; 2 (4) DOI: 10.1103/PRXQuantum.2.040328 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211109120316.htm

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