MSGID: 1:317/3 6275f5fc   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    In balance: Quantum computing needs the right combination of order and   
   disorder    
    Disorder in quantum computer chips needs to be designed to perfection   
      
      
     Date:   
         May 6, 2022   
     Source:   
         University of Cologne   
     Summary:   
         Researchers have analyzed cutting-edge device structures of quantum   
         computers to demonstrate that some of them are indeed operating   
         dangerously close to a threshold of chaotic meltdown. The challenge   
         is to walk a thin line between too high, but also too low disorder   
         to safeguard device operation.   
      
      
      
   FULL STORY   
   ==========================================================================   
   Research conducted within the Cluster of Excellence 'Matter and Light for   
   Quantum Computing' (ML4Q) has analysed cutting-edge device structures of   
   quantum computers to demonstrate that some of them are indeed operating   
   dangerously close to a threshold of chaotic meltdown. The challenge   
   is to walk a thin line between too high, but also too low disorder to   
   safeguard device operation. The study 'Transmon platform for quantum   
   computing challenged by chaotic fluctuations' has been published today   
   in Nature Communications.   
      
      
   ==========================================================================   
   In the race for what may become a key future technology, tech giants like   
   IBM and Google are investing enormous resources into the development   
   of quantum computing hardware. However, current platforms are not yet   
   ready for practical applications. There remain multiple challenges,   
   among them the control of device imperfections ('disorder').   
      
   It's an old stability precaution: When large groups of people cross   
   bridges, they need to avoid marching in step to prevent the formation of   
   resonances destabilizing the construction. Perhaps counterintuitively,   
   the superconducting transmon qubit processor -- a technologically   
   advanced platform for quantum computing favoured by IBM, Google, and   
   other consortia -- relies on the same principle: intentionally introduced   
   disorder blocks the formation of resonant chaotic fluctuations, thus   
   becoming an essential part of the production of multi-qubit processors.   
      
   To understand this seemingly paradoxical point, one should think of   
   a transmon qubit as a kind of pendulum. Qubits interlinked to form a   
   computing structure define a system of coupled pendulums -- a system   
   that, like classical pendulums, can easily be excited to uncontrollably   
   large oscillations with disastrous consequences. In the quantum world,   
   such uncontrollable oscillations lead to the destruction of quantum   
   information; the computer becomes unusable.   
      
   Intentionally introduced local 'detunings' of single pendulums keep such   
   phenomena at bay.   
      
   'The transmon chip not only tolerates but actually requires effectively   
   random qubit-to-qubit device imperfections,' explained Christoph   
   Berke, final-year doctoral student in the group of Simon Trebst at the   
   University of Cologne and first author of the paper. 'In our study,   
   we ask just how reliable the "stability by randomness" principle is   
   in practice. By applying state-of-the- art diagnostics of the theory   
   of disordered systems, we were able to find that at least some of the   
   industrially pursued system architectures are dangerously close to   
   instability.'  From the point of view of fundamental quantum physics, a   
   transmon processor is a many-body quantum system with quantized energy   
   levels. State-of-the-art numerical tools allow one to compute these   
   discrete levels as a function of relevant system parameters, to obtain   
   patterns superficially resembling a tangle of cooked spaghetti. A careful   
   analysis of such structures for realistically modelled Google and IBM   
   chips was one out of several diagnostic tools applied in the paper to   
   map out a stability diagram for transmon quantum computing.   
      
   'When we compared the Google to the IBM chips, we found that in the   
   latter case qubit states may be coupled to a degree that controlled   
   gate operations may be compromised,' said Simon Trebst, head of the   
   Computational Condensed Matter Physics group at the University of   
   Cologne. In order to secure controlled gate operations, one thus needs   
   to strike the subtle balance between stabilizing qubit integrity and   
   enabling inter-qubit coupling. In the parlance of pasta preparation, one   
   needs to prepare the quantum computer processor into perfection, keeping   
   the energy states 'al dente' and avoiding their tangling by overcooking.   
      
   The study of disorder in transmon hardware was performed as part of the   
   Cluster of Excellence ML4Q in a collaborative work among the research   
   groups of Simon Trebst and Alexander Altland at the University of   
   Cologne and the group of David DiVincenzo at RWTH Aachen University and   
   Forschungszentrum Ju"lich. "This collaborative project is quite unique,"   
   says Alexander Altland from the Institute for Theoretical Physics in   
   Cologne. "Our complementary knowledge of transmon hardware, numerical   
   simulation of complex many-body systems, and quantum chaos was the perfect   
   prerequisite to understand how quantum information with disorder can be   
   protected. It also indicates how insights obtained for small reference   
   systems can be transferred to application-relevant design scales."   
   David DiVincenzo, founding director of the JARA-Institute for Quantum   
   Information at RWTH Aachen University, draws the following conclusion:   
   'Our study demonstrates how important it is for hardware developers   
   to combine device modelling with state-of-the-art quantum randomness   
   methodology and to integrate "chaos diagnostics" as a routine part of   
   qubit processor design in the superconducting platform.'   
      
   ==========================================================================   
   Story Source: Materials provided by University_of_Cologne. Note: Content   
   may be edited for style and length.   
      
      
   ==========================================================================   
   Journal Reference:   
      1. Christoph Berke, Evangelos Varvelis, Simon Trebst, Alexander   
      Altland,   
         David P. DiVincenzo. Transmon platform for quantum computing   
         challenged by chaotic fluctuations. Nature Communications, 2022;   
         13 (1) DOI: 10.1038/s41467-022-29940-y   
   ==========================================================================   
      
   Link to news story:   
   https://www.sciencedaily.com/releases/2022/05/220506113312.htm   
      
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