MSGID: 1:317/3 642cf96b   
   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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