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

--- up 9 weeks, 4 days, 10 hours, 50 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)