Many QC hardware designs are being tried at the present time. Eventually, one of those will prevail over all the others. Luckily, as a writer of QC software, I don’t need to worry too much about which hardware design will win out. That’s because my software is to a large extent “platform independent”. By this I mean that, because it outputs a sequence of one and 2-qubit gates, my software is compatible with any QC hardware design that implements the “gate-circuit model”, and that includes most QC hardware designs that experimentalists are currently trying to build.
Even though as a QC programmer I don’t need to worry too much about the details of QC hardware design, I still enjoy learning about it.
Natural and artificial atoms for quantum computation
by Iulia Buluta, Sahel Ashhab, Franco Nori
This excellent paper describes the current state of the art of QC hardware for implementing the gate-circuit model.
(The paper says nothing about topological QCs. For info about topological QCs, visit the website of Michael Freedman’s group, “Microsoft Station Q”, at UCSB.)
(The paper says nothing about adiabatic QCs. For info about adiabatic QCs, visit D-wave’s website. )
The paper is easy to understand (even if you are not an experimentalist), informative, well-written, and short (just 5 pages of actual text. The paper contains a total of 21 pages, but pages 6-21 are references and pictures.) Since the paper is short, I recommend that you read the whole thing yourself, but here is a preview to pique your interest.
As its title suggests, the paper groups QC experimental approaches into two kinds: those that use natural atoms, and those that use artificial atoms. As the paper itself warns the reader, the fact that one particular experimental approach is at the present time more advanced than another, doesn’t necessarily mean that it is simpler, or better than the other, or that it will go farther in the future. It might just mean that it has been around longer.
The paper is structured as follows:
- Natural Atoms:
- Artificial Atoms:
Sec.4-Spins in Semiconductors (quantum dots and NV centers in diamond)
The references section at the end is a wonderful list, a treasure trove of experimental QC papers. For convenience, I cut and pasted just the references section into a simple text document (see here). I also created a text document with the references grouped by chapters (see here).
So how does one compare the current state of the art of various QC experimental approaches? You’ll have to read the paper in order to find out! The paper has informative tables that compare the same characteristics (about 30 of them) for each of 4 approaches (neutral atoms, ions, superconducting circuits, and spins in semiconductors). Out of those 30 characteristics, 4 of them are:
T1 (relaxation time) is the average time that the system takes for its excited state to decay to the ground state.
T2 (decoherence time) represents the average time over which the qubit energy-level difference does not vary.
Q1 (quality factor) represents the number of one-qubit quantum gates that can be realized within the time T1.
Q2 (quality factor) represents the number of two-qubit quantum gates that can be realized within the time T1.
In the final “Prospects” section, the paper concludes that:
In both natural and artificial atoms, almost all the basic requirements for realizing QC  have been demonstrated (i.e., (i) a scalable system with well-characterized qubits; (ii) initialization of the qubits; (iii) reasonably long decoherence times; (iv) a universal set of quantum gates; (v) measurement of the qubits)… The current challenges are to attain increased controllability (and minimize decoherence) and scale the existing systems to tens and hundreds of qubits and many-gate operations.
Qubit Mixology (i.e., 2-qubit gates)
I am particularly interested in how advanced each of the 4 approaches is in producing good 2-qubit gates. So I cut and pasted those sentences in the paper that address this issue: (To find out what are the references being alluded to by number, look in here, or in a pdf version of the paper.)
- NEUTRAL ATOMS
The effective spin-spin interaction between two atoms in a double-well potential was used to demonstrate a two-qubit SWAP gate . Furthermore, with polar molecules  or Rydberg atoms [21, 22] dipole-dipole interactions could be exploited for realizing two-qubit gates. Very recently, a CNOT gate using Rydberg blockade interactions has been demonstrated .
…quantum algorithms [36, 37],…
- SUPERCONDUCTING CIRCUITS
…one can now realize simple algorithms ,…
- SPINS IN SEMICONDUCTORS
While Rabi oscillations have already been observed [60, 61], two-qubit gates have not yet been demonstrated (although, a SWAP gate between logical states has been realized ). However, long coherence times [63, 64] have been measured for both quantum dots (~microsec) and NV centers (20 ms) . Moreover, for NV centers the entanglement between the electron and nuclear spins has also been shown .
Websites of Some Key Players
From the references of the paper, you can get a good idea of which are the most productive QC experimental groups in the world. Here are the websites of some of those groups:
Hey, Where is China and India?
I was looking up SOME of the references in the Buluta/Ashhab/Nori review paper. And before I knew it, I ended up finding web links for ALL of the references. What a nerd!* So others don’t have to redo all this google searching, I’ve posted the results on my website. Just drag and drop any url from this text document
into your search engine’s url input box, and presto! Whenever there is an arXiv version of the paper, I give a link to that. Otherwise, I give a less suitable link.
*This turned out to be simpler to do than I thought. I used Google Scholar (just the basic interface, not the advanced one). I pasted the title of each reference into the search box of Google Scholar. If there is an arXiv version of the paper, Google Scholar always gives it as the top entry of the search results; then you just drag and drop the title of that top entry to an open text editor window, and voilà, you get on your text editor window a perfectly formed url for the paper .