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PID: hpt/lnx 1.9.0-cur 2019-01-08
TID: hpt/lnx 1.9.0-cur 2019-01-08
 Absolute zero in the quantum computer 
 Erasing data perfectly and reaching the lowest possible temperature -
- those two things seem to be completely different, but they are closely
intertwined 

  Date:
      April 4, 2023
  Source:
      Vienna University of Technology
  Summary:
      Absolute zero cannot be reached -- unless you have an infinite
      amount of energy or an infinite amount of time. Scientists in Vienna
      (Austria) studying the connection between thermodynamics and quantum
      physics have now found out that there is a third option: Infinite
      complexity. It turns out that reaching absolute zero is in a way
      equivalent to perfectly erasing information in a quantum computer,
      for which an infinetly complex quantum computer would be required.


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FULL STORY
==========================================================================
The absolute lowest temperature possible is -273.15 degrees Celsius. It
is never possible to cool any object exactly to this temperature -- one
can only approach absolute zero. This is the third law of thermodynamics.


==========================================================================
A research team at TU Wien (Vienna) has now investigated the question:
How can this law be reconciled with the rules of quantum physics? They
succeeded in developing a "quantum version" of the third law of
thermodynamics: Theoretically, absolute zero is attainable. But for
any conceivable recipe for it, you need three ingredients: Energy, time
and complexity. And only if you have an infinite amount of one of these
ingredients can you reach absolute zero.

Information and thermodynamics: an apparent contradiction When quantum
particles reach absolute zero, their state is precisely known: They are
guaranteed to be in the state with the lowest energy. The particles then
no longer contain any information about what state they were in before.

Everything that may have happened to the particle before is perfectly
erased.

From a quantum physics point of view, cooling and deleting information
are thus closely related.

At this point, two important physical theories meet: Information theory
and thermodynamics. But the two seem to contradict each other: "From
information theory, we know the so-called Landauer principle. It says
that a very specific minimum amount of energy is required to delete
one bit of information," explains Prof. Marcus Huber from the Atomic
Institute of TU Wien.

Thermodynamics, however, says that you need an infinite amount of
energy to cool anything down exactly to absolute zero. But if deleting
information and cooling to absolute zero are the same thing -- how does
that fit together?  Energy, time and complexity The roots of the problem
lie in the fact that thermodynamics was formulated in the 19th century
for classical objects -- for steam engines, refrigerators or glowing
pieces of coal. At that time, people had no idea about quantum theory.

If we want to understand the thermodynamics of individual particles, we
first have to analyse how thermodynamics and quantum physics interact --
and that is exactly what Marcus Huber and his team did.

"We quickly realised that you don't necessarily have to use infinite
energy to reach absolute zero," says Marcus Huber. "It is also possible
with finite energy -- but then you need an infinitely long time to do
it." Up to this point, the considerations are still compatible with
classical thermodynamics as we know it from textbooks. But then the team
came across an additional detail of crucial importance: "We found that
quantum systems can be defined that allow the absolute ground state to
be reached even at finite energy and in finite time -- none of us had
expected that," says Marcus Huber. "But these special quantum systems
have another important property: they are infinitely complex." So you
would need infinitely precise control over infinitely many details of
the quantum system - - then you could cool a quantum object to absolute
zero in finite time with finite energy. In practice, of course, this is
just as unattainable as infinitely high energy or infinitely long time.

Erasing data in the quantum computer "So if you want to perfectly erase
quantum information in a quantum computer, and in the process transfer
a qubit to a perfectly pure ground state, then theoretically you would
need an infinitely complex quantum computer that can perfectly control an
infinite number of particles," says Marcus Huber. In practice, however,
perfection is not necessary -- no machine is ever perfect.

It is enough for a quantum computer to do its job fairly well. So the
new results are not an obstacle in principle to the development of
quantum computers.

In practical applications of quantum technologies, temperature plays
a key role today -- the higher the temperature, the easier it is for
quantum states to break and become unusable for any technical use. "This
is precisely why it is so important to better understand the connection
between quantum theory and thermodynamics," says Marcus Huber. "There
is a lot of interesting progress in this area at the moment. It is
slowly becoming possible to see how these two important parts of physics
intertwine."
    * RELATED_TOPICS
          o Matter_&_Energy
                # Physics # Quantum_Physics # Quantum_Computing #
                Spintronics
          o Computers_&_Math
                # Quantum_Computers # Computers_and_Internet # Hacking
                # Encryption
    * RELATED_TERMS
          o Absolute_zero o Electron_configuration o Quantum_entanglement
          o Quantum_computer o Bose-Einstein_condensate o Quantum_number
          o John_von_Neumann o Quantum_mechanics

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


==========================================================================
Journal Reference:
   1. Philip Taranto, Faraj Bakhshinezhad, Andreas Bluhm, Ralph Silva,
   Nicolai
      Friis, Maximilian P.E. Lock, Giuseppe Vitagliano, Felix
      C. Binder, Tiago Debarba, Emanuel Schwarzhans, Fabien Clivaz,
      Marcus Huber. Landauer Versus Nernst: What is the True Cost
      of Cooling a Quantum System? PRX Quantum, 2023; 4 (1) DOI:
      10.1103/PRXQuantum.4.010332
==========================================================================

Link to news story:
https://www.sciencedaily.com/releases/2023/04/230404114303.htm

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