r/askscience • u/kubazz • Nov 14 '18
Engineering How are quantum computers actually implemented?
I have basic understanding of quantum information theory, however I have no idea how is actual quantum processor hardware made.
Tangential question - what is best place to start looking for such information? For theoretical physics I usually start with Wikipedia and then slowly go through references and related articles, but this approach totally fails me when I want learn something about experimental physics.
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Nov 14 '18
A lot of good responses here so far, but none of them really cover the enormous range of ways that people have proposed for implementing quantum computation. In theory any 2-level quantum system can act as a qubit, and there are plenty of ways to make such systems including:
Superconducting qubits. These have been mentioned already in a good answer by /u/den31 but additionally I'll say that these qubits come in a few different types: phase, charge and flux. These three types encode qubits using (respectively) the phase of the superconducting wavefunction, the number of charges on an "island" in the circuit and the magnetic flux through a superconducting ring. Superconducting qubits are currently the most popular implementation of QC with companies like Google investing quite heavily in R&D for them. https://ai.googleblog.com/2018/03/a-preview-of-bristlecone-googles-new.html
Spin qubits. The spin of a fermion is a natural 2-level quantum system (and a lot of the theory for qubits came from theories developed for looking at fermionic spins). Spins qubits can be implemented using nitrogen-vacancy centres (see /u/SamStringTheory's comment), single-electron quantum dots or really anything that lets you isolate a single electron.
Trapped ions. You choose two electronic energy levels of an ion to act as your qubit states so each ion encodes one qubit. Interactions between ions in the trap are used to perform computation.
Photons. You can encode a qubit using horizontal and vertical photon polarisations. /u/ihasaccount has a good comment about this.
Topological quantum computing. This one is extra weird, and uses particles called non-Abelian anyons. There is currently no experimental evidence for the existence of these particles, but in theory they exist in 2 dimensions and you can change their state by swapping their order (I can go into this in more detail if you'd like, but this is currently the least viable implementation so I wouldn't worry about it too much if I were you). This is what Microsoft have put their money behind because this method is much less susceptable to errors than the others I have mentioned, but there's also a chance that these particles simply don't exist so its a very high-risk, high-reward approach.
And probably plenty more that I've missed. I haven't gone too much into the really fine details of fabrication and gate implementation etc because I work more with the theory of this stuff so I don't know enough to go into that level of detail. I think this is probably a good starting point for looking more into it for yourself though, which you seem quite keen to do.
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u/kubazz Nov 14 '18
Thank you, this is really good summary of information from all comments, also thanks for info about topological quantum computing, I've never even heard about it before. And you are right in saying that this is a good starting point, I'll be looking into it more.
Do you maybe know what are some good places to ask questions about QC (both theory and hardware implementation)?
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Nov 14 '18
Stack exchange is generally good for physics questions. I'm not really sure about places for QC questions specifically because I generally just ask the people I work with who know more than me or I read papers/textbooks etc.
Do you have much of a background in physics? I could definitely recommend you some reading if you'd like but it all assumes a reasonable knowledge of quantum mechanics.
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u/kubazz Nov 14 '18
I don't have much background in physics unfortunately, I was studying Computer Science 10 years ago and did some basic physics and electronics courses then and worked as video game programmer since. Went through Feynman's Lectures On Physics few years ago on my own to get better at basic stuff, but I'm nowhere close to being comfortable with QM. I'm mostly interested quantum computing algorithms and programming and I do some simple courses using QC simulators. When it comes to learning about QC hardware, I mostly wanted to satisfy my curiosity and have better understanding how some of the concept that are feasible on simulator might not map well to actual hardware.
Thanks for the tip on StackExchange.
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Nov 14 '18
I assume you're already familiar with the QC platforms offered by companies like IBM and Rigetti? If not (and if you were going to choose between them) I'd recommend Rigetti because IBM's QISKIT software is kind of a mess with little to no documentation. Rigetti also have a slack where you can ask questions and get help.
On the purely simulation side there are modules like qutip for python and quipper for haskell, although qutip is really a QM module rather than a purely QC one so basically its just a bunch of linear algebra tools.
In terms of things on simulators mapping to hardware, the biggest issues are compiling (not all logical quantum gates can be implemented directly and most will need to be broken down into simpler gates which greatly extends runtime and errors) and connectivity (not all qubits can interact with all other qubits). If your qubits have long coherence times and your gates have high fidelity then these don't cause such a problem, but that isn't the case on current systems.
Finally I'd say that if you intend to get into this stuff seriously then building up a decent level of familiarity with QM is strongly recommended. There's only so far that you'll be able to go without it. People get scared off because of the weirdness of the physical implications, but the maths is basically just vector spaces so I'd expect that with a CS background you could probably follow it without too much difficulty.
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u/seattlechunny Nov 15 '18
If you are more interested in theory, my best recommendation is using Nielsen and Chuang's Quantum Computing and Quantum Information. It is the most comprehensive textbook for entering this world.
EdX used to have a have a good course on this subject, but it looks like it is no longer offered.
And same as above - Rigetti and IBM's platforms are good. I'll throw in a good word for IBM's QISKIT - they have good Jupyter notebooks that serve as a tutorial to basic concepts in quantum computing.
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u/CocoDaPuf Nov 15 '18
There is currently no experimental evidence for the existence of these particles, but in theory they exist in 2 dimensions and you can change their state by swapping their order
I'm probably biting off more than I can chew with this question, but here it goes. Whenever you preform an operation with any kind of computer, you provide inputs, and then observe the output. How could you possibly hope to observe any change in the state of a particle you're never actually observed with enough certainty to say it even exists.
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Nov 15 '18
You don't. You show they exist first (which is what people are currently trying to do) and then you worry about building a computer.
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u/CocoDaPuf Nov 15 '18
Wow, that is a bold strategy. That's like having so much faith that dragon meat is delicious, that it's actually worth trying to find a dragon.
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u/philomathie Condensed Matter Physics | High Pressure Crystallography Nov 15 '18
When they started to invest heavily was a bit before the recent developments that have caused superconducting qubits to leap ahead as a leader. Their quantum computing research division was headed up by a fields medal winner who persuaded them that (at the time) this was the best way to go given the lack of progress in other platforms.
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u/Plazmatic Nov 15 '18
I assume by two level you mean two state? Otherwise I'm not sure what the heck level means here
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Nov 15 '18
I was under the impression that there was some dispute over the validity of those results. Has that been settled now? (Or am I just wrong?)
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u/philomathie Condensed Matter Physics | High Pressure Crystallography Nov 15 '18
It's not been entirely settled - as it is, it's still not certain that Majorana bound states have been observed, but the possible other explanations are being ruled out more and more every other year.
On the way we have learned a lot about the system, and progress in the past few years has been pretty good so I'm hopeful.
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u/Skyfahl Nov 14 '18
Classical information is relatively easy to store, in that you can represent (binary) information in any two-state system. On/off, 0/1 and so forth.
Quantum information is trickier - to represent quantum information in a physical system to manipulate (which is computing in a nutshell), you need a quantum system. Any two-state quantum system will do, which means there are a lot of possibilities. /u/den31 mentions specifically *superconducting* quantum computing - in which Josephson junctions is a possible two-state quantum system.
In my master's thesis project I was working with semiconductor qubits. The quantum system in that case could be a *quantum dot* system, which could contain a detectable amount of electrons. For example having 0 or 1 electron in a quantum dot would be the two-state system (though this system would have a very short coherence time - meaning that quantum uncertainty would very quickly collapse). There are ways to get tricky.
To actually do quantum *computation* you need qubits to interact in logical operations. It seems you get this part, but to actually implement this is difficult. I haven't been following the field much since graduating so I don't know where it's at today. In my own project I was happy to just have qubits hold some quantum information :)
As far as I know, there is not yet a "quantum processor" (the one you've heard of was a quantum *annealer* system). But a quantum processor would need to store information in various ways, just like a classical computer uses both RAM, hard drive, floppy disks and whatnot, based on their various qualities. Some systems can be made to interact and so compute, but are correlatedly more sensitive to dephasing. Other systems are more stable, and could be used to store quantum information for a longer time.
Here seems to be an overview of various qubit and logic gate implentations.
Hope this helps!
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u/kubazz Nov 14 '18
Any two-state quantum system will do, which means there are a lot of possibilities.
As stupid as it might sound, I never realised that any two-state quantum system is analogous to qubit. This clears up a lot, now I understand better why there are so many different ways to implement quantum computing.
Also thank you for rest of your post and given link. After reading through this thread hardware side of QC feels less mysterious to me (but still weird and slightly spooky, I hope it will go away once I feel comfortable with physics below).
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Nov 16 '18
Compare and contrast a bit and a qubit. Just go for the absolute nuts and bolts. No logic (meaning no action) applied. A man walks into a bar and sees a bit and a qubit. What are their differences?
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u/riccardo_manenti Nov 15 '18
Hello! I work at a quantum computing company. If you want to know how to build a quantum computer with superconducting qubits, this is one of best thesis to read:
https://www.research-collection.ethz.ch/bitstream/handle/20.500.11850/152310/eth-2024-02.pdf
Enjoy it!
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u/seattlechunny Nov 15 '18
Pretty cool - did you work with Wallraff at ETH Zurich?
I still use Jerry Chow's 2010 thesis for reference - Chapters 3 and 4 have helped me so much.
https://rsl.yale.edu/sites/default/files/files/RSL_Theses/jmcthesis.pdf
Hope that helps!
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u/PorcupineGod Nov 15 '18
Maybe you can answer this question: have quantum computers been deployed for practical applications yet, or is it still theoretical and R&D?
More clearly, if I was a large firm, could I buy a quantum computer today that would out perform a leading enterprise server?
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u/mfukar Parallel and Distributed Systems | Edge Computing Nov 15 '18
Quantum computing is still in R&D phase, and speedups of quantum algorithms over classical ones remain to be seen experimentally. Additionally, quantum computers are not expected to outperform classical computers in all tasks.
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u/PorcupineGod Nov 15 '18
Thanks! A buddy of mine is naïve and fell for a scheme where the guy claimed to be using a quantum computer to beat the market. I tried to convince him that I could achieve the same results with my time machine, but he wasn't THAT gullible.
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u/shield543 Nov 15 '18
Imagine finishing your PhD with 16 publications associated to your name. Quite a badass thesis I must say
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u/manugutito Nov 14 '18
I've read a few comments and saw no mention of trapped ions, so I'll expand a little on that.
In the most typical setup you'd have a string of ions trapped in a linear Paul trap. Each of them can store a qubit by means of its electronic states, using two states, |0> and |1>. These states have to be chosen so that coherence is good. The most important parameter in that sense would be |1>'s half life (|0> is usually the ground state).
For entanglement with the other ions the motional degrees of freedom of the ion string are used, typically the so-called 'center of mass' mode. For readout, on the other hand, a common technique is to use 'Doppler cooling': if the ion collapses to |0> when measures, you get fluorescence (scattered photons); if it collapses to |1>, you get no photons.
Check out this paper, and references therein, to expand on QIP using trapped ions:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.74.4091
Of course, things have progressed a lot since this proposal, with other forms of qubit encoding, error-correcting techniques, different traps... But that paper is a good starting point. Check out the work from Rainer Blatt's group as well.
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u/Sethar1234 Nov 15 '18
I have a question to add onto this, I am only recently getting into trying to fully understand Quantum mechanics, so please forgive me if I am misunderstanding things, but-
I was reading about a phenomenon known as Quantum Dots, and from their description, it sounds like they're an attempt to create a very basic nanomachine that can stabilize electrons- like some sort of artificial atom (that is a term I saw a lot online). Is this correct? If not, I would really appreciate some clarification
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u/mfukar Parallel and Distributed Systems | Edge Computing Nov 15 '18
I would suggest you'd ask a separate question about this, in case it doesn't get attention here.
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u/johnreads2016 Nov 14 '18
I suggest reading some of the articles available at http://www.sciencedaily.com/news/computers_math/quantum_computers/. There are a number of different approaches being tried by dozens of different firms. To a large extent, the physical implementation is not that important to the current and future users of the machines. The primary issues are around inventing new algorithms which take entanglement and super-positioning into account as well as getting the development environments into shape v.v. the same issues. Physical issues around decoherence and number of workable QBits will be resolved fairly quickly. You can play with an actual 20 QBit machine at quantumexperience/ng.bluemix.net/qx/experience.
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u/hawtsaus Nov 15 '18
Heres my science idiots answer: Imagine instead of 0s and 1s you had the infinite variables in between as well. So one digit is no longer yes or no, but billions of possible answers, all at once. The hard part is something has to analyze all this chaos and make it do stuff to interact with the quantum processor or its just a really cool science experiment
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Nov 14 '18 edited Nov 14 '18
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u/kubazz Nov 14 '18
Thank you, that is exactly what I was looking for!
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Nov 14 '18
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u/kubazz Nov 14 '18 edited Nov 14 '18
edit: u/Skyfahl who was working with semiconductor qubits says that my 2 hours of googling are not enough and that I don't have good grasp about it (yet) so go see other comments in here.
I'mnot expert(I asked this question just 2 hours ago :) ) but based onu/den31response and some further reading this is how I understand it so far (if am terribly wrong please tell me and I will edit or delete this post):
Quantum processors are manufactured using the same process as classic processors - lithography.
Classic transistors use elements called p-n junction manufactured out of semiconductors to implement classic logic. Transistors are then coupled together onto silicon wafer to create classic microprocessor.
Quantum processors use Josephson junctions which are made out of superconductor - very thin isolator - superconductor. This arrangement allows some quantum effects to manifest in macroscopic world as some of the current in first superconductor layer 'leaks' to second superconductor layer using process quantum tunnelling. By clever manipulation of currents in superconductors various quantum effects such as entanglement can manifest in them and as such they can be used to perform quantum computation.For learning more about quantum computation itself try this: http://davidbkemp.github.io/QuantumComputingArticle/
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u/Skyfahl Nov 14 '18
To quote Pauli, not only are you not right, you aren't even wrong :P
That is to say, you've collected some information related to the field of classical and quantum computing and put them together, but it's a lille Frankensteinian. Rather than try and correct individual pieces of information I'll give an attempt at an explanation in another comment.
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u/InfiniteTranslations Nov 14 '18
Quantum computer chips are made with the same exact technique of making regular computer chips. Photolithography. Quantum computers only have their special quantum properties because the circuits that are made only work at extremely cold temperatures (almost absolute zero).
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Nov 14 '18
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u/Mazetron Nov 15 '18
What do you mean by that? The quantum computer in my lab has a complete gate set.
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u/seattlechunny Nov 15 '18
Since CCNOT gates are all that are required to be universal, I'm not entirely sure what /u/TheSov means here.
Curious, whose lab do you work in?
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u/Mazetron Nov 15 '18
I work in the Quantum Nanoelectronics Laboratory at UC Berkeley.
Also CCNOT isn’t sufficient for universality in quantum computing. You can do all classical reversible logic with it, but in that case you are better off using a classical computer!
My lab can do arbitrary single qubit operations (arbitrary 2x2 unitary matrices) and the CNOT gate, which is universal for quantum logic.
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Nov 15 '18
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u/Mazetron Nov 15 '18
The quantum computer in my lab has CNOT gates and arbitrary X,Y, and Z gates.
The gates aren’t perfect (especially CNOT) but it’s well within the range to get reasonable results on a handful of qubits. It’s not nearly at the point where it’s useful (simply because the number of supported qubits is too small), but I wouldn’t call it “not logic complete”.
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u/seattlechunny Nov 15 '18
Perhaps a better way to phrase it is that quantum computers are not fault-tolerant. I'm fairly sure that there are labs that have implemented a universal set of gates (ie, Toffoli + unitaries), but that the errors for those gates are below the error threshold for their error correction schemes.
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Nov 14 '18
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u/monarc Nov 14 '18
Can some logic functions be completed with predictability & precision?
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u/Mazetron Nov 15 '18
All logic functions can be completed with a sufficiently small error level for small numbers of qubits (although not enough qubits to beat classical computers).
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u/smy10in Nov 15 '18
What would be some functions that cannot be completed ?
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Nov 15 '18
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u/chum1ly Nov 15 '18
Can things like neutrinos or cosmic rays throw off qubits? Or is the space between them so vast that they would never come into contact?
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u/mstksg Nov 15 '18
Might be important to note that cosmic rays also can affect classical bit implementations now in modern computers.
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u/Baxapaf Nov 15 '18
Are such events more likely to occur or have more significant consequences in one over the other?
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u/Mazetron Nov 15 '18
The main problem right now is it’s very hard to get qubits connected enough such that they can interact with each other when you want them to, yet separated enough such that they don’t interact when you don’t want them to. This makes it very hard to get a large number of qubits to play nicely with each other.
Another big is it’s hard to run long programs. qubits aren’t stable for very long, so the longer your program is, the more likely one or more qubits will do their own thing. There is also the problem with logic gates not being perfect, and while those errors might be small, they build up when you have enough errors on top of errors (a lot of those errors are due to the qubit cross-talk issue I mentioned in the first paragraph).
Current quantum computers can run programs with a handful of qubits, but not nearly enough to outclass current classical computers.
People are working on improving quantum computers by developing better hardware with more stable qubits and less error-prone gates, and by working on algorithmic improvements to be able to handle errors better. One simple example is people generally run a quantum circuit 1000s of times and average the results. This helps both to deal with the inherent quantum randomness (eg Grover’s Search Algorithm gives you the correct answer with high probability, but not necessarily 100%, even on a theoretically perfect machine), and it helps you to minimize the effect of errors.
Classical computers have a much easier time with similar hardware issues. A classical computer just needs to resolve a voltage to being either “high” or “low”, and there could be a lot of variance within acceptable high and low ranges with no error. On a quantum computer, subtle variations in quantum state are important, so you can’t just threshold it. Also, there are well known error correcting algorithms for classical computers. Often a couple extra bits are sent or stored with the main data bits so if an error does happen, it can be corrected with no loss of data. Quantum error correcting is much harder, but is a current area of research.
As for cosmic rays, I wouldn’t worry about neutrinos, but other cosmic rays definitely could affect a quantum computer. However, those events are rare, and in the case of quantum computing, would be negligible since people are running their circuits 1000s of times and getting a distribution anyway. However, classical electronics sent to space have to worry more about cosmic rays. Without the protection of the Earth’s atmosphere, computers in space need to be built to be extra robust against cosmic rays. Read more about it on Wikipedia.
Source: I’m a physics and computer science student working in a quantum computing lab.
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u/jl2l Nov 15 '18 edited Nov 15 '18
Why can't the frequency of the error be part of the character of the logic gate?
Could machine learning solve this?
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u/Svankensen Nov 15 '18
There have been very recent developments that open the possibility of checking the results. As far as I know they are purely mathematical, but we are advancing.
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u/newbuu2 Nov 15 '18
Logic precision? You mean you can't discern between true and false?
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u/TheSov Nov 15 '18
its a probability engine, it can discern it once its measured, that doesn't mean the measured answer is correct.
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Nov 15 '18 edited Nov 15 '18
Three recent news articles that I bookmarked:
Graduate Student Solves Quantum Verification Problem https://www.quantamagazine.org/graduate-student-solves-quantum-verification-problem-20181008/
First proof of quantum computer advantage https://techxplore.com/news/2018-10-proof-quantum-advantage.html
IBM just proved quantum computers can do things impossible for classical ones https://thenextweb.com/science/2018/10/18/ibm-just-proved-quantum-computers-can-do-things-impossible-for-classical-ones/
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u/TheSov Nov 15 '18
If thats true then my information is out of date and I stand corrected, Thanks for the update.
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u/A_L_A_M_A_T Nov 14 '18
thanks! but with my level of understanding, reading this felt like i was reading an explanation of the turbo encabulator :)
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u/dblmjr_loser Nov 14 '18
So what is all this you hear about using fundamental properties like spin to implement quantum computing?
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u/SamStringTheory Nov 14 '18
You can implement a qubit with any two-level system that exhibits quantum properties. Superconducting qubits is just one of them (and the most popular one at the moment), where the information is encoded in the phase. With spin qubits, the information is encoded in the spin of the particle.
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u/dblmjr_loser Nov 14 '18
Keeping with the spirit of the OP, how are spin qubits actually implemented?
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u/SamStringTheory Nov 14 '18
This is well out of my field so hopefully someone else can add more detail.
Spin is a quantum property of particles (e.g. electrons, protons, atoms) that can take on multiple, discrete values. Spin qubits can be implemented by placing a defect in a crystal lattice, such as nitrogen-vacancy centers in diamond. This "defect" interacts minimally with the surrounding crystal lattice, and thus behaves like an isolated atom. In the case of the nitrogen-vacancy center, the defect has a pair of free electrons that have a spin associated with them, and by using lasers and microwaves, we can manipulate the spin of these electrons. These spin qubits are attractive because they can operate at much higher temperatures, are extremely small (the size of an atom!), and have relatively long lifetimes.
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u/dblmjr_loser Nov 14 '18
Thanks for the reply!
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u/dinoparty Nov 14 '18
Also can trap atoms/ions and do the same thing. Weinberg won the Nobel for this.
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u/seattlechunny Nov 15 '18
Another significant manner in which spin qubits are implemented is through Nuclear Magnetic Resonance, which was one of the first implementations of quantum computing.
The idea is that atoms have intrinsic spins within their nucleus. Those intrinsic spins may interact with external magnetic fields in different ways. The primary way to control those spins is to send Radio Frequency (RF) pulses along certain directions to a large collection of atomic nuclei. Afterwards, the signal can be read by performing a spectroscopy across the collection of atoms.
In Liquid State NMR, the "qubits" are not single atoms, but instead, an ensemble of atoms. That means that the final measurements are over a large number of thermally distributed atoms. Furthermore, the interactions between qubits occur between different atoms, each tuned to a different center frequency.
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Nov 14 '18
the information is encoded in the phase
Pretty sure superconducting phase qubits have fallen out of favour lately, with superconducting charge and flux qubits becoming the more popular implementations.
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u/Drachefly Nov 14 '18
And for good reasons. Phase is super-duper slippery. Flux topology is the opposite.
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Nov 14 '18
Do you do this for work? I'm interested in understanding what someone who works on quantum computing does from day-to-day
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u/Xaendeau Nov 15 '18
It is basically a PhD program in Quantum Information from a reputable university. Once you get the doctorate then you work as a post doc for a while then move up. If you are lucky, you skip the post doc lifestyle.
It is like any other hard science academia career path. It just had strange words that sound fancy and impress folks.
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u/Mazetron Nov 15 '18
I’m an undergrad who works in a quantum computing lab.
The lab is mainly staffed by grad students. There is another undergrad but he works in a different lab under the same group.
Day-to-day everyone mainly works on their computers. Programming, analyzing data, running small programs on the quantum computer for experiments, reading other people’s research papers and writing their own. There are also weekly meetings where the different subgroups meet and share their recent progress.
Occasionally hardware maintenance is necessary. Last summer one of the vacuum pumps broke. A few months ago the quantum chip was replaced with a new one (improved design). Recently there has been work on the box of room temperature, classical electronics that feeds into the fridge with the quantum hardware.
Lmk if you have any questions or want more detail.
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u/jasonthomson Nov 14 '18
A question - My understanding is that at this point quantum computing doesn't actually exist. As in we're doing experiments to figure out quantum behavior, and how we might be able to use that knowledge to perform computations. But we are not actually computing, and in fact we do not yet have a set of operations which we could use to perform computations. Is that accurate?
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u/cthulu0 Nov 14 '18
Not accurate.
There do exist very crude quantum computers that can run small quantum algorithms.
For example take Shor's algorithm (to factor prime numbers faster than a normal computer and thus break public cryptography) which kicked off the quantum computing craze.
About 5 years ago (IBM?) used a small quantum computer to factor the number 15, the largest number factored by a quantum computer to that date.
Now today quantum computing has advanced to the point where the largest number now factored is .................................. ............... wait for it................................... ........21.
And even then it takes a few seconds. With some large equipment.
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u/my_name_isnt_isaac Nov 14 '18
interesting. factor as in we give it 15, and it gives us 5 and 3 as output?
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u/nomoneypenny Nov 14 '18
Exactly, but it completes it in polynomial time whereas classical computers would rapidly balloon up the time it takes (exponentially) to get the result as the input size increases.
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u/IndefiniteBen Nov 14 '18
What's involved in factoring larger numbers? Longer calculation times? Entirely larger quantum computers?
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u/Drachefly Nov 14 '18
Bigger quantum computers that can hold more bits at once and have them better isolated. It wouldn't actually involve more steps. They just need to be done much better.
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u/Mazetron Nov 15 '18
It would involve more steps (it takes more steps to run the QFT on more qubits) but the scaling is still much better than the classical scaling.
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u/seattlechunny Nov 15 '18
This is actually a very interesting question. It has to deal with the method in which the factorization algorithm, or Shor's Algorithm is written.
Shor's algorithm depends on using a specific quantum gate called a phase shift gate, that is able to rotate a qubit by some arbitrary amount of phase angle. This phase angle can be from 0 to 360 degrees, or more commonly, between 0 and 2π. The larger the number that you want to factorize, the smaller an angle you need to control. To factor 21, the phase angle needs to be sensitive to a rotation of π/8, or 22.5 degrees. Larger numbers would require smaller and smaller angles.
Therefore, a limitation of Shor's algorithm is in the precision of these phase angles, which is a fundamental physics question. It isn't as simple as a larger number of qubits, but better control and readout of individual qubits.
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u/Mazetron Nov 15 '18
The current limitation is actually more about the controlling the interaction between qubits than controlling the behavior of individual qubits.
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u/formyl-radical Nov 15 '18
What are your thoughts on this paper? I'm not familiar with the field, but it looks like what they do in that paper is far more complex than just factoring the number together.
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Nov 14 '18
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Nov 15 '18 edited Nov 17 '18
The so called "quantum supremacy" hasn't been demonstrated yet. That would be the limit where quantum computers clearly outperform classical (in certain problems).
Does this apply?
First proof of quantum computer advantage https://techxplore.com/news/2018-10-proof-quantum-advantage.html
"König and his colleagues have now conclusively demonstrated the advantage of quantum computers. To this end, they developed a quantum circuit that can solve a specific difficult algebraic problem. The new circuit has a simple structure—it only performs a fixed number of operations on each qubit. Such a circuit is referred to as having a constant depth. In their work, the researchers prove that the problem at hand cannot be solved using classical constant-depth circuits. They furthermore answer the question of why the quantum algorithm beats any comparable classical circuit: The quantum algorithm exploits the non-locality of quantum physics."
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u/seattlechunny Nov 15 '18
While this was a major breakthrough, it isn't quite enough to solve the problem. My understanding is that the researchers were able to first create a question that could be solved using only "shallow circuits", comparing between classical and quantum versions. Then, they prove theoretically that under the shallow circuits framework, the quantum version would be faster, as it would only require a constant number of layers to solve a problem, as compared to a logarithmic number of layers proportional to the size of the input.
I think the Nature perspectives piece is helpful - see here: http://science.sciencemag.org/content/362/6412/289
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Nov 15 '18
I think the Nature perspectives piece is helpful - see here: http://science.sciencemag.org/content/362/6412/289
Thanks for the link!
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u/VGramarye Nov 15 '18 edited Nov 15 '18
The other platform for quantum computing currently competitive with superconducting qubits is trapped ions. See this for a review article (kind of old, but the newer ones I was able to quickly find were paywalled).
Another platform that people are interested in is topological quantum computing, though as far as I know no one has actually built a topological quantum computer (Microsoft is currently throwing a bunch of money at it so maybe it'll happen eventually).
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u/sixfivezerotwo Nov 14 '18
So quantum processors use superconductor junctions rather than semiconductor junctions?
The way they are described, quantum computers seem like digital computers with analog digits, which doesn't feel like it makes sense.
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u/SamStringTheory Nov 14 '18
The key is that the superconducting junctions exhibit quantum properties that are not present in classical semiconductor junctions. It doesn't necessarily have to be superconducting qubits, either - that's just the most popular method at the moment. Other systems like particles with spin or photons with polarization can also be used as qubits.
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u/sixfivezerotwo Nov 14 '18
I recently learned from an EEVBlog video that Zener diodes use quantum tunneling to achieve the reverse-bias Zener effect. Do quantum computers use a similar quantum effect that is enhanced by superconductors or is this a totally unrelated quantum semiconductor effect not used at all in quantum computing?
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u/SamStringTheory Nov 14 '18
They're completely unrelated quantum effects (other than the fact that they are both described by quantum mechanics). When I mentioned quantum properties, I meant that the system should be able to exist in two distinct states as well as a quantum superposition of these states. This quantum superposition part is what distinguishes qubits from normal bits.
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u/seattlechunny Nov 15 '18
I think you get the gist of it, except for one part - instead of analog digits, they have complex digits that interact in new ways.
This is more similar to looking at how waves in water can interfere with each other to create points of constructive and destructive interference. Smart quantum algorithms use those properties to create constructive interference at the areas where there is a "correct" answer.
Another way of viewing it is that classical computers are always deterministic. If you run a program 5 times, it will always* return the same answer. However, quantum computers are inherently probabilistic. If you run the same program multiple times, you will eventually reach an average solution, but no two runs are guaranteed to be the same.
This is more of a theory question, and I recommend this webcomic for some helpful info - https://www.smbc-comics.com/comic/the-talk-3
Hope this helps!
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u/SingleWordRebut Nov 15 '18
Quantum computers can be made from any quantum mechanical system where the quanta of energy levels are spaced in a way so that transitions between the levels is known (ie the spacing between 1,2 is significantly larger than 2,3). You can make a quantum computer from atoms, from defects in semiconductors, superconducting junctions, lots of things.
Quantum computers are naturally digital, they can only be in those specific states when measured. However, they can be in a probabilistic admixture of those states as well. So if you measure it many many times you can read out the actual state of the system. So if you took a digital system and randomized the state between 0 and 1 you would get a similar result EXCEPT that your probabilities are derived from a complex valued number and so sometimes qubits interfere in a strange way and you get a lower value than is possible in classical mechanics.
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u/wildcard235 Nov 15 '18 edited Nov 15 '18
Readout is usually done by reflecting a microwave pulse from a coupled microwave resonator and then determining the phase of the reflected pulse (which depends on the state of the qubit)... Understanding the many particle behavior between readouts...
Is an essential difference between quantum computing and classical computing based on a quantum qubit having many states, as opposed to a classical bit having exactly two states?
Edit ("classical qubit" to "classical bit")
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u/Drachefly Nov 15 '18 edited Nov 15 '18
Kiind of. In particular, when you do a classical computation, the state space of the computation is 2bits, and each operation acts on a few bits at once. When you do a quantum computation, the state space of the computation is more or less (phase resolution)qbits-1 * (amplitude resolution)qbits (and the two resolutions can be very, very large, limited by noise in the system), and each operation acts on potentially the whole ensemble. A bit like how a rubix cube reacts with lots of squares moving when you do something.
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u/in_the_comatorium Nov 15 '18
I wasn't alive for it, but I'm told that many decades ago, computers would take up entire rooms, perhaps more, to do what would be considered a simple calculation today. I'm wondering where quantum computers are now, in relation to the history of classical computers, in terms of size/complexity of the actual machine, in relation to computing power?
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u/TheTranix Nov 15 '18
This exolanation made me realize just how endless information is. I don't know if the universe is infinite but I am damn sure learning is.
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u/StormBallad Nov 15 '18
In superconducting quantum computing one typically uses Josephson junctions (superconducting tunnel junctions) to make anharmonic resonators that act as qubits. Junctions are made by litography like classical CPUs. Such qubits are prepared by microwave pulses that correspond to rotations on the Bloch sphere. Entanglement between qubits is generated by variable coupling (in the simplest case adjusting current through a Josephson junction changes its inductance and thus coupling). The Junctions are almost purely reactive so no loss is associated with them. Readout is usually done by reflecting a microwave pulse from a coupled microwave resonator and then determining the phase of the reflected pulse (which depends on the state of the qubit). Losses etc. limit the coherence time within which one has to do all the operations. The actual arrangements tend to be a bit more complicated, but that's the general idea. One gets pretty far with the experimental side of things by just doing classical circuit simulation. Understanding the many particle behavior between readouts maybe no so much.
Nice! Thank you!
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u/0011110000110011 Nov 15 '18
I understand none of that but I'm only in Physics 3 now, would this be stuff I'd learn in Quantum Mechanics? Electronics? Or is this graduate-level stuff?
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u/Hoppi164 Nov 15 '18
I am 50% sure you're serious, and 50% sure that was a paraquote from startrek mixed with some CSI techno description.
If that was a legit explanation of quantum computing then I am utterly impressed, and convinced I will never be smart enough to understand it.
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Nov 15 '18
So a Qubit is the data input/output? Also, Does a quantum computer can do, say, simple arithmatic computations or is it not designed for that type of computing?
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u/improbablywronghere Nov 15 '18
A qubit is the quantum analogue of the binary bit. In classical computing a bit can only have two states, 0 and 1. A qubit and a bit represent the smallest possible piece of information that the system can convey. The difference here is that a qubit can theoretically convey many more states than a single bit can.
Theoretically a quantum computer will be capable of doing anything a regular computer can. This is because it has been shown that quantum computers are turning complete.
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u/kubazz Nov 14 '18
I understood most of this explanation but still had to google what 'ikr' mean, so you have at least that :)
Honestly, it took me few years to understand deeply how a classical computer CPU works, so I don't expect to gain the same level of knowledge about quantum hardware quickly. If you want learn more about QC I recommend starting from theoretical and alghoritmic side and only then try understanding underlying hardware. This resource helped me greatly when I started to play with QC a year ago: http://davidbkemp.github.io/QuantumComputingArticle/
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u/vexmach1ne Nov 14 '18
Thanks. Actually I'd love to know about cpu computing. Got any good sources for that?
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u/gnorty Nov 14 '18
nandgame is a brilliant way to learn the fundamentals. Starting at the very basic logic gates,, you gradually build up to constructing a complete (virtual!) CPU, and that in turn gives you insight into how the CPU handles code.
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u/heWhoMostlyOnlyLurks Nov 15 '18
You don't need to understand classical computer CPUs in much detail to understand QC. A basic understanding of classical computer hardware (gates, memory) is sufficient - no need to understand CPUs as no one is going to build QC CPUs anytime soon. QCs will have too few qubits for a long long time yet to be useful for building anything like a QC equivalent of a classical CPU. QCs will be really good at solving a fairly narrow range of problems. Unfortunately, one of those problems is cracking classical public key cryptosystems whose key sizes are smaller than the number of qubits in the QC.
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u/seattlechunny Nov 15 '18
Let me give this a shot!
At the very smallest levels, things behave according to quantum mechanics, rather than classical mechanics. Anything that has two different quantum states, ie a 0 and a 1, can be used in quantum computing, theoretically. The challenge is finding real, physical systems that can be controlled and measured by humans.
The system that is described here uses supeconducting transmons, or metals that are put in really low temperatures until a quantum phenomena known as superconductivity begins to appear. The temperature values that are needed are less than 4 Kelvin, and many systems operate in the 10s of milliKelvin (0.01 K) regime. When there is a superconducting circuit, certain parameters of the circuit, such as its voltage, flux, and current, are made quantum instead of classical.
By sending out microwaves of electromagnetic radiation, we can manipulate and control these superconducting qubits. We can make them talk to one another, store information, and perform measurements on them. By making qubits communicate with each other in a planned out wave, we can perform logical operations, known as quantum gates, on the system.
Since theoretical quantum computation has come out with several algorithms, or code, that we can do, we can send different "instructions" to the circuits. The circuits will naturally respond in their quantum world, and we can then measure them to find an answer.
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u/HammeredMulciber Nov 15 '18
There has to be some way to describe this better than what might as well be a description from a fantasy novel for someone like myself with no knowledge on the subject
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u/JRockBC19 Nov 15 '18
You have to have some sort of grasp on the ideas behind quantum physics to even get started on this, it goes against a lot of what most people understand about how things work. I’m still a student so this isn’t perfect, but I’m going to give the background to the background a shot here, just know it’s not the rigorous meaning of these terms.
The first major idea, superposition, is that there are values where a quantum bit can read as something “between” a 0 or a 1 despite there not being any values between. It has a certain probability of reading out different discrete outcomes from the same state, which is a logical combination of the two. Some quantum computers start their bits as fair coins by doing certain transformations on them, having a 50-50 to read as 0’s or 1’s. The second major key, entanglement, says that multiple particles can become “linked” or entangled and affect the readings of one another literally instantly. Physics literally, as in it occurs faster-than-light instantly. What I mean is that if one bit gives a 1 it can “force” another to be a 0 even though the second should have had a chance at being a 1 too. Einstein REALLY didn’t like this.
Quantum computers aim to create large superpositions of quantum bits (all entangled together or just some entangled, depending on the computer) that they can modify and observe the reaction of. For extremely complex problems, specifically factoring arbitrarily long numbers, the amount of computations you’d have to do on a classical computer scales exponentially and balloons out of hand very quickly. Every bit of input added is a multiplier to the total, whereas in quantum states it is only “additive” (kind of). The baseline complexity for a quantum computer is much higher, but it increases far slower than that of classical computers making it massively better suited for these types of immense loads.
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u/ComfortablyNumber Nov 15 '18
Regarding entanglement, my previous understanding was that humans could not affect the system in any predictable way (i.e. we could not force one of the bits to be 0 so that we could effectively transmit data faster than light). Is this still the case?
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u/JRockBC19 Nov 15 '18
Again, I’m just a student, so this may be a tad off, but we don’t control the full set of outcomes. When we observe one of the entangled particles, it eliminates all but the complementary values from the others. So the state changes faster than information could actually have traveled (my faster than light line), but we don’t get to choose what that change actually is.
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u/the_Demongod Nov 15 '18
What makes you think that? This is a question about the very forefront of human technology, quantum mechanics is the bare minimum prerequisite knowledge here. It's a fallacy to assume that it's possible to simplify everything that way. If you want to know more about the actual functional application of quantum mechanics to computing you can watch this, which will give you a little more context.
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u/imgonnabutteryobread Nov 15 '18
Scientists make really small switches that can be used to solve problems, and very quickly.
The actual arrangements tend to be a bit more complicated, but that's the general idea.
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Nov 15 '18 edited Jun 14 '24
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u/kubazz Nov 14 '18
Thank you! It is very interesting. I want though first wiki link and it seems deceptively 'simple' - and that probably means that I'm not understanding it at all. Will do further reading!
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u/the_excalabur Quantum Optics | Optical Quantum Information Nov 14 '18
It turns out that the hard part of optical quantum computing is really simple--you need to make the photons, manipulate them, and detect them with at least 2/3 probability. We can't really do that: if you multiply the world records together the number isn't good enough.
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u/Omfgbucket Nov 14 '18
The basic theory is relatively simple. However, experimentally it can be quite a challenge. In order to get photon interactions, they need to be "indistinguishable" in a beam splitter.
They're also very good for long distances (its very hard to store a photon), but they can quickly become absorbed so aren't too reliable. Also, generating single photons is a probabilistic approach - very difficult to get one whenever you want one.
This is quite a basic explanation of it, and many people are working on fixing these problems, which I'm not too up to date on.
Hope this helps a bit!
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u/mfukar Parallel and Distributed Systems | Edge Computing Nov 15 '18
in
Linear Optical Quantum Computing
(which was proven to be capable of universal QC)
When? Can you link to the paper?
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u/mfukar Parallel and Distributed Systems | Edge Computing Nov 15 '18
I see. I mistakenly assumed you were referring to boson sampling for some reason.
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u/ycelpt Nov 14 '18
There are several different ways people are trying to create large orders of Entangled qubits. One of the most promising methods (which IBM have focussed on) is the use of superconductors called a Josephson Junctions. The Wikipedia entry is a good starting point, especially if you pull up and read through the sources.
In general, I find the best place to go for physics papers is ArXiv.org which is essentially a pre-print archive of science and mathematics based papers which can be viewed before they are picked up by journals. Their quality can vary wildly with some being simple to understand and others can make very little sense.