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There are many different ways to build quantum computers, such as superconducting qubits, quantum dots, and ion traps.

What I would like to understand is why some universities and research organizations have chosen to study trapped ion quantum computers. I understand that it is better in some respects, but what are the fundamental differences?

enter image description here

Source: Youtube video

Domain of Science

Dominic Walliman

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  • $\begingroup$ Also: quantumcomputing.stackexchange.com/questions/1390/… $\endgroup$ – Rob Apr 3 '18 at 18:54
  • $\begingroup$ It might be beneficial to reword the question to something like "why is ion trap based quantum computing so popular?", at the moment the question seems too broad. You might want to take a look at how to ask a good question and what to avoid in questions. $\endgroup$ – Kiro Apr 3 '18 at 19:39
  • $\begingroup$ out of curiosity, where is the image from? $\endgroup$ – glS Apr 3 '18 at 19:48
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    $\begingroup$ @Riz-waan I edited the question in an effort to make it less broad and not a duplicate. Feel free to revert the edit if you think it does not reflect what you wanted to know $\endgroup$ – glS Apr 3 '18 at 19:54
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    $\begingroup$ @glS Thank you, I believe the changes reflect what I want to know. Here is the YouTube video: youtu.be/90U_SmKyfGI $\endgroup$ – Riz-waan Apr 3 '18 at 22:01
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Disclosure: while I am not an experimental physicist, I am part of the NQIT project, which is aiming to develop quantum hardware which is suitable to realise scalable quantum computers. The architecture that we're investing most heavily in is optically linked ion traps.

Ions represent some of the physically best understood systems to experimental and theoretical physics, and the idea of performing quantum computation with ion traps is a very old one, as far as proposed implementations go. The main features of ion traps as a potential approach to realising quantum computation are

  • They can give rise to very stable qubits, with life times on the order of a minute or more; and
  • For well-chosen ion species, the electron state transitions are very well understood, in principle allowing for very high precision gates.
  • Furthermore, these features can be realised without the need for heavy cryogenics.

See for instance [arXiv:0805.2450, arXiv:1606.08409]. To be sure, there are obstacles to using ion traps as a basis for quantum technologies. Most importantly, you need to solve the problem of cross-talk between the ions due to collective motional excitation, either through clever quantum control or through isolation of groups of ions. If the latter, you have to find a way to regroup collections of ions in a very large trap architecture, or find a way for ions in different traps to communicate.

The scaling problems for ion traps are not easy problems. However, significant obstacles exist for all quantum architectures; while well-resourced multinational corporations have placed their bets elsewhere, it seems to me far too early to reliably predict which platform will solve their scaling problems first. To my knowledge, none of the approaches have managed to get to the point where the only problem left is engineering.

If you could solve the scaling problems for ion traps, you would likely realise computers through the unitary circuit model, the measurement-based model, or some closely related model to these, according to how you negotiated the scaling problem. Of course, this is how one might describe computation at the lowest level, i.e. to realise error correction. At higher levels, tradition would exert a significant amount of pressure to use the primitive operations to realise fault-tolerant unitary circuit model operations, unless some other computational model was somehow compelling enough to displace it for the scalable error-corrected system.

In any case — it is not really meaningful at this stage to call ion traps in general a "non-universal model", as your diagram suggests, just because some groups have aimed for quantity before versatility. This would be similar to pigeon-holing transistors as a non-universal computational platform in the 1950s, just because they were used to make radios before they were used to make computers. Even if (as the linked YouTube video suggests) the ion trap groups on the U.S. eastern seaboard are aiming first for many ions and then attempting to master versatile quantum control, other groups such as the one at Oxford are taking the opposite approach of controlling very few qubits very well, then attempting to approach the problem of scaling up. Both are possible trajectories towards solving the difficult problem of designing scalable quantum computers.

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  • $\begingroup$ Your remarks about 'universality' are interesting. Do similar remarks also apply to quantum annealing? That is, could we also consider quantum annealing to be 'universal' if we'd use it a bit differently? $\endgroup$ – Discrete lizard Apr 4 '18 at 9:19
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    $\begingroup$ @Discretelizard: I do not know enough about 'quantum annealing' to address this question, but I can address it for the purposes of the adiabatic algorithm. If we could somehow magically dispel all sources of noise for very large adiabatic computers, then they could simulate unitary circuits in polynomial time. They are therefore universal in the usual sense. (I have seen it claimed that adiabatic computation is a limiting case of quantum annealing, in which case the same holds true for quantum annealing.) The question really isn't one of computational models, but of practical engineering. $\endgroup$ – Niel de Beaudrap Apr 4 '18 at 11:35

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