What exactly is a qubit? I'm not asking what does it mean, because I know there's superpositions and all that jazz, but as in...like, in an electronic circuit, what is a qubit? Is it made out of logic gates? Which ones?
If we can make one qubit, can't we just make a bunch of them by copy and pasting circuits similar to how we used vacuum tubes in the 60s and 70s? How come our current limit is only around 54 or so?
Are qubits, and quantum computers by extension, not even electronic circuits? If so...what the hell are they?
There are several reasons it doesn't scale easily to more qubits, but you can imagine that you don't want the chip to be large (must be cooled to 25mK!) but the qubits should be spaced quite far apart so they don't influence each other. Also, it's not a very simple circuit, so the layout of the transmission lines ("wire on a chip") becomes difficult to manage. The last problem also scales badly with the number of qubits (the middle qubit becomes progressively harder to reach).
There is a paragraph in the (leaked) paper that describes their chip:
> In a superconducting circuit, conduction electrons condense into a macroscopic quantum state, such that currents and voltages behave quantum mechanically [2, 30]. Our processor uses transmon qubits [6], which can be thought of as nonlinear superconducting resonators at 5 to 7 GHz. The qubit is encoded as the two lowest quantum eigenstates of the resonant circuit. Each transmon has two controls: a microwave drive to excite the qubit, and a magnetic flux control to tune the frequency. Each qubit is connected to a linear resonator used to read out the qubit state [5].
If you want a physical picture, check out [6]: https://arxiv.org/abs/cond-mat/0703002
edit: source [6] is more appropriate and open to access
> > In a superconducting circuit, conduction electrons condense into a macroscopic quantum state, such that currents and voltages behave quantum mechanically [2, 30]. Our processor uses transmon qubits [6], which can be thought of as nonlinear superconducting resonators at 5 to 7 GHz. The qubit is encoded as the two lowest quantum eigenstates of the resonant circuit. Each transmon has two controls: a microwave drive to excite the qubit, and a magnetic flux control to tune the frequency. Each qubit is connected to a linear resonator used to read out the qubit state [5].
I understand most of the words in isolation – and I know that they are valid, even if combining them in a useful manner to understand exactly what they are describing is eluding me.
But if there was ever a paragraph that sounded like pure technobabble, this is it. Replace the technobabble found in the star trek matter transporter with quantum lingo, and it would sound very similar.
Measuring a qubit in a basis collapses it to one of the basis vectors (e.g Schrödinger's cat must be alive or dead once we open the box) with probability equal to its inner product with that basis vector. This is why we need a Hilbert space and not just any old vector space.
Finally, to answer your question about gates, a quantum gate is basically a unitary matrix, i.e. a matrix that preserves the norm of its inputs. You can feed qubits into these matrices by themselves or, more often, many at once, by using something called the tensor product of the qubits - this is where the math gets slightly more involved.
The long and short of it is that we can induce correlation patterns between qubits using these gates (aka quantum entanglement) and orchestrate circuits of interference patterns where the wrong answers cancel each other out and the right answer gets reinforced so that we measure it at the end - unfortunately, this is where my knowledge breaks down as a beginner. My apologies if I accidentally handwaved anything important but hopefully you get the gist.
And going beyond that, as Peres puts it, "Quantum phenomena do not occur in a Hilbert space. They occur in a laboratory."
I recently had a conversation about this in another thread. It seems to me, and nobody tried to convince me otherwise, that the Cat would be the Observer. Therefore it would be dead, NOT dead AND alive, as soon as it observes the poisonous gas in its box.
It's an entirely new type of computing apparatus, using the fundamental state of particles. No electron circuits, those won't work.
And it's expensive because of the above. These things are massive, have to be kept at cyrogenic temperature, and isolation gets harder the more particles you have.
>> in an electronic circuit, what is a qubit?
It's not an electronic circuit, it literally is an electron or a photon. And it's using that particle's properties to do "superposition and all that jazz".
You can't put too many next to each others because they start interacting together because that's what electrons do when they get close.
The main problem is achieving full control of these systems, which is extremely hard, because there are certain things (some random/stochastic) that you can't control at all and you have to fight+race against their influence:
- qubits are tiny, and the energy splitting between these two states are typically minuscule: this means even a small vibration from a sneeze miles away can make the qubit flip.
- qubits do not live in vacuum, they are typically hosted in solid-state systems and the qubits are coupled to their hosting environment, which have their own moving parts (two-level fluctuators which lead to charge noise, phonons which also couple to electrons typically via spin-orbit coupling, spinful defects in the material which have their own dynamics, etc etc) that you can't really control. it's extremely difficult to achieve full control of a qubit in the presence of things that you can't control and random in nature.
- if the qubit is the lowest two-levels of a system with higher energy levels, one also needs to worry leakage errors to those higher states
- there are ways of suppressing the influence of such unwanted interactions (dynamically corrected gates + quantum error correction codes) given that their strength is below certain thresholds. going below those thresholds is again an enormous engineering/material science problem (extremely low temperatures, isolation from vibrations, low/high-pass filters for the classical circuitry which is used to control/drive the qubits via electric/magnetic fields, design of the device itself which typically hosts two-level fluctuators, etc etc). this problem becomes harder in general as you increase the number of qubits though.
- to do anything non-trivial, you need to have more than one-qubit and have controllable couplings between those (so you can't put them apart too far which makes it impossible to couple them). this doesn't work perfectly in practice, you can't completely control or turn off their couplings (a problem called cross-talk) which again leads to errors. so it doesn't work quite like modular classical circuit elements which you can "copy & paste" because the abstractions from the low-level, nitty-gritty physics of the underlying material fail for all these qubits.
Okay, now what can we do with that?
"crack encryption by simulating a state!" yeah but what? is that something I should be concerned about now? "hahaha no no no silly normie we'd need two thousand qubits for that, this machine only has 53!" oooookay, and you did that number in your head, how??? "we just solved the first unsolvable problem that a mere bit bound supercomputer couldn't solve, look at this math formula!" but that didn't explain "we are celebrating, are you not celebrating"
There just seems to be a lack of non-introductory but non-PhD level information. Where is the "explain it like I've been accepted into college at all".
Now imagine 53 such boxes, interconnected by quantum gates. The 53 qubits combined are in all of 2^53 states at once. The gates can be set up such that some combinations like "cat 1 alive", "cat 2 dead", etc. are much more likely result than others, after the boxes are opened. And all this computation is done in one step, whereby the classical computer must do 2^53 steps to get the same result.
To have 53 cats all undisturbed in these dead/alive states so that computation is done without errors is very technically challenging :)
If you have a collection of n qubits, the state of those corresponds to a unit vector in a 2^n dimensional space over the field of complex numbers.
You can do certain linear operations (iirc, only unitary ones. I wouldn’t claim all unitary ones either) to change the state. You can also do a measurement, which has a random outcome, following the Born rule.
So, it isn’t really just “each of them has more than 2 possible states”. That can be the case with something classical, and has been done before (there have been ternary computers. Binary computers won out. Ternary computers (or quaternary, etc. etc.) wouldn’t be particularly special.)
You can’t just think of each of the qubits as having a state always entirely independent of the rest of the qubits.
A bit is like a boolean type, has the values of true and false. Or you treat those values as 0 or 1, then gather a bunch of bits to build useful numbers.
A qubit is like a pair<number, number> such that these numbers MUST satisfy the following constraints:
pair.left^2 + pair.right^2 = 1
pair.left and pair.right can be any complex number
Remember that classical computers use a clock to flip bits over time.
A similar quantum computer would manipulate qubits instead.
A bit has the storage capacity of 2 distinct bits of information.
A qubit has the storage capacity of 2 complex numbers, which corresponds to 4 floats, which is at least 16*8 bits of information if we are conservative about our assumptions.
> Is it made out of logic gates?
Kind of. Most of the current logic gates are built with semiconductors. It means by applying different voltages/currents/flux etc to different parts of a solid material, we can alter what we measure in some other part of the same material.
A quantum logic gate uses the same principle but in order to achieve the desired speed and storage advantages, it uses an object with a measurable property that at least approximately behaves like a quantum mechanical object. Common semiconductors are too crowded of atoms in terms of their body parts to make good quantum materials. They touch to each other and are almost always exposed to air. Their running temperatures are undesirably high.
A really dark (literally without a single photon) vacuum chamber that holds a really small amount of floating matter in the middle, frozen with lasers up to 0.000...1 Kelvin would make a good example of a stable but expensive qubit. We can measure this qubit by destroying its state, i.e by applying a magnetic field and measuring the emitted photon's location, polarization or frequency. The problem is, copy pasting this device to build a circuit is really hard due to logistics and auxiliary machinery required to keep all the state stable.
> How come our current limit is only around 54 or so?
That's not a fundamental limit but an engineering one, due the issues I mentioned above. The bigger the device gets, the harder to maintain its stable state. The method currently used for reaching this limit really looks like what is used in the 60s. History is repeating itself with a small twist, the running temperature is extremely low this time. Not liquid nitrogen low, but compressing atoms by sniping them from distance via laser on multiple directions low.
There is also one more problem that is unique to quantum computers. You have to measure the same qubit multiple times to be able to read those complex numbers since their values are determined statistically. You either represent a qubit with multiple qubit like devices or you use the same device to try your measurement repeatedly. Each approach comes with its own drawbacks.
> Are qubits, and quantum computers by extension, not even electronic circuits?
Even today, non quantum circuits are sometimes non electronic in some of their parts. Fiber optics, supersonic emitters/receivers and photoelectric sensors are good examples.
To this day, it is not clear whether the first consumer quantum CPU will be entirely electronic or not.
This whole explanation makes qubits sound just like a combo-pack of bits, or like something you'd find in an analog computer. They're much more powerful than that because they can be put into superpositions and entangled together while you continue to do calculations with them. Using superpositions and entanglement across qubits can let you solve some problems with lower-complexity algorithms than you could with classical computers.
It's like saying a car is defined by four tires being in close proximity, while skipping the fact that the usefulness of a car comes from the tires being connected together and steered which lets you do things that no amount of unconnected tires could do.
The fourth float would seem to be almost totally determined by the first three. If I'm visualizing correctly, there are at most 2 values it could possibly be.
This kinda sounds like translinear circuits, except a couple orders of magnitude more boutique.
>Quantum physics forbids copying a qubit to create another, but you can initialize them en masse to to be in a superposition state. Those things are too tiny to be easily manipulated so 53 is quite an accomplishment.
Quantum physics forbids copying the value of a qubit, but the poster was asking why we couldn't just make more of the device that implements a qubit. The big issue is that you want the qubits to be entangled together and it's hard to prevent decoherence as you make a larger device with more qubits.
This is a very good question! And as far as I can tell, no one has actually answered it yet.
In classical physics, which suffices to explain circuits made of vacuum tubes, the state of a system is fully captured by the states of its parts. Like, if you want to know the state of three bits, I just have to tell you what the first bit is (0 or 1), and the second bit, and the third one. Basically everything we interact with has this property: if you fully describe the state of each part of a thing, you have described the state of the whole thing.
But quantum mechanics is... weirder than this. In quantum mechanics, to describe the state of a system you have to give one complex number per classical state, such that the sum of the squares of the absolute values of the complex numbers adds up to 1. These complex numbers roughly correspond to the "probability" that the system is in that state (but not quite, it's more complicated than that).
So in quantum mechanics, to describe the state of three bits, you have to give eight complex numbers n1,n2,...,n8, one for each of the classical states of the bits 000, 001, 010, 011, 100, 101, 110, 111, where the sum of the squares of the absolutely values of n1,...,n8 add up to 1. That's a lot more information than 3 bits. (Imaging if you had 54 bits... you'd need 2^54 ~= 10^17 complex numbers to describe them.)
Technically, everything in the whole world, including you, is described by the laws of quantum mechanics. So why don't we see weird quantum effects all of the time? Quantum systems are very fragile: whenever they interact with the outside world (say a photon from the air bounces off of something in the system), the system "collapses", and then behaves as classical physics would predict. (Note that this is in accordance with the quantum prediction. The system goes from "only describable using difficult quantum mechanics" to "describable using quantum mechanics, but it'll just say the same thing as classical physics, and classical physics is simpler so you should just use that".)
So here's what a qbit is: it's just a regular bit that has been so insulated from the outside world that classical physics doesn't suffice to describe it. You won't find qbits on a regular circuit board, though, because they'll interact with the circuit board in any way whatsoever and then you're done.
And this is why making a 54-qbit quantum computer is so hard. You need to keep all of the qbits isolated, because if any of them interact with the outside world (think the air in the room, or a single photon, or the substrate that the qbits are on), then the whole system "collapses".
(I'm not an expert in this area, so the following may not be incomplete or limited to only certain kinds of quantum computation, or worse).
The kinds of computation that qubits can beat a classical computer on require that the qubits be entangled. If the qubits are not entangled, you can't do better than regular bits.
Briefly, if two (or more) quantum systems are entangled, and you make certain measurements that have a random outcome on one of the systems, and then measure the same property on the other system(s), there will be correlations that you would not get if the systems were not entangled. The entangled systems act is if whenever you measure one and it randomly chooses a value for the property you measured, that result is somehow communicated to the other entangled systems, and they make sure them that when they are measured they will give results with the appropriate correlation.
You might think that this could be explained if the systems had some internal variables that were set when they became entangled that determined what "random" values they would pick when later measured, but there are experiments that have shown that this is not so. The systems are truly making their random choice at the time of measurement as far as we can determine.
This happens even if after you entangled the systems, you separate them by a great distance--so far that between the time you do the measurements on the separate systems there is no time for any communication between the systems (or, rather, no time for any communication limited by the speed of light--I believe there have been experiments showing that IF there is communication, it is at more than 10000 times the speed of light). (This communication, or whatever it is, cannot be used to send messages faster than light. All it can do is make the correlations work out for entangled systems).
Anyway, the thing about entangled systems is that as soon as you make a measurement of the entangled property, you lose the entanglement. Your particle that had entangled spin, say, with another particle and that was 50/50 whether it was spin up or spin down becomes, once you actually measure spin, a non-entangled particle whose spin is a concrete value, either up or down. Measure it again, and you get the same value.
When I say "you make a measurement", I don't specifically mean you, or any other human, or any human instruments. For purposes of quantum mechanics, a measurement is anything that makes the system reveal a value. So if you have a particle with entangled spin with some other particle, and some random passing particle happens to interact with yours in a way that depends on the spin of your particle--that's a measurement and you've lost your entanglement. (Your particle might now be entangled with that random passing particle, but it is no longer entangled with the particle you intended it to be entangled with).
If you are trying to do a quantum computation on 50 qubits, you have to get them all entangled, and then you have to keep them from interacting with anything that might inadvertently do a measurements long enough for them to do their quantum computation, where you can then measure the result (which finally ends the entanglement).
This turns out to be hard, because there are a lot of things in the universe that want to effectively do a measurement. Any random particle bumping into one of yours with too much energy can do it. Some random passing electromagnetic wave can do it. The more things you need to entangled and keep entangled, the hard this is, and 50ish is the current limit.
Longer version
An observation about quantum behavior is that there are only certain properties that you can measure for the really, really small. These include things like mass (total energy), charge (intrinsic amount of electromagnetic strength), and spin (willingness to change direction in the presence of an electromagnetic field). It turns out that when you measure these properties, the measurements behave in non-intuitive ways.
The act of measuring the spin of a particle (which could be in any direction) is really the act of asking, "is this particle aligned with my detector?" The result will always be either aligned up or aligned down. It will be randomly about 50/50 up and down, also. This is not that surprising because the spin must align one direction or the other. The crazy part comes with the fact that you can entangle two particles.
Entangle particles can be sent off through different detectors and one thing will always be true: while any particular outcome is random, the detectors will always generated opposite results. The temptation is to say, "well, they were generated from the same source, so they have just opposite starting positions." Long story short, this has been proven not possible. Instead, there is some fundamental behavior is quantum mechanics that says that there are certain types of activities with entangles particles that have a correlation that is true as long as the entangle particles are not disrupted. In this case, particles sent to separate detectors will always have opposite results.
A quantum computer uses these correlation truths about measurements to do computations. A qubit is the concept of a quantum bit: an entity that represents one unit of entanglement. Just like a bit, quantum computers have many different ways to implement entanglement. You can entangle photons, electrons, and whole atoms. Each of these systems require specific implementations to achieve, like like electronic or mechanical computers.
Remember that one detail about "if they are not disrupted?" Yeah, turns out that it takes a hell of a lot to create an environment that doesn't destroy the coherence of the entanglement. You have to design something that allows you to setup the particles into starting state, be able to hold those particles in an entangled state with no disruptions and have a detector to determine the final state. Quantum mechanically, these are generally opposite goals.
Is this true? I thought that in the 40s-50s there was some argument over whether practical computers could really be built given that vacuum tubes were so unreliable. Von Neumann wrote gave a series of lectures in 1952 (eventually transcribed into an article) showing how it could be done: http://arep.med.harvard.edu/gmc/Von_Neumann_1956ro.pdf , and the argument became more or less moot once transistors arrived).
There's perhaps an analogy here to be made with quantum error correction, but that seems to be a lot harder...
An acquaintance flew Concorde a lot in the 80's, the reason - "I could get to Heathrow in the morning, fly to New York, do a meeting, fly back and get home the next morning". Another friend is a exec at a bank now and goes on month long odysseys to the USA, Asia and Australia. The contrast in perspective and commitment is striking, I don't think that Concorde would matter at all to these folks.
I'd be really interested to hear what Gil Kalai has to say on this particular subject; he's generally pretty fair minded while being of a QC skeptic. He has a response up [1] but I haven't read through it yet.
[1] https://gilkalai.wordpress.com/2019/09/23/quantum-computers-...
Except that you'd only be able to pay with physical cash.
If you're willing to admit handwaving and future-tech plans, then you can be excited now. It's estimated that a few million physical qubits (corresponding to few thousand logical qubits) will be necessary to crack an RSA key. It's at least a decade away, maybe several, but few experts believe there are hard barriers to number of qubits or minimal cost.
"Thing is ridiculous, that could never work because of reasons A, B and C."
"What are you talking about, here is project that does thing, it functions perfectly."
Nothing like reality proving someones baseless naysaying wrong immediately.
>Under some people’s definitions of “quantum computational supremacy,” they do! The key difference with Google’s effort is that they have a fully programmable device—one that you can program with an arbitrary sequence of nearest-neighbor 2-qubit gates, just by sending the appropriate signals from your classical computer.
>In other words, it’s no longer open to the QC skeptics to sneer that, sure, there are quantum systems that are hard to simulate classically, but that’s just because nature is hard to simulate, and you don’t get to arbitrarily redefine whatever random chemical you find in the wild to be a “computer for simulating itself.” Under any sane definition, the superconducting devices that Google, IBM, and others are now building are indeed “computers.”
This is the core of it to me. It's a question of 'some people’s definitions of "quantum computational supremacy."' Many people say that the definition here is a crappy one, and sure this shows a form of it, but not the kind to justify the hype. Sure, it's fully programmable, but not so programmable as to do anything that anyone cares about (even a teeny tiny few-bit version of something people care about) better than we can otherwise.
To appeal back to his analogy to the wright brothers, it's like they carved a frisbee from a stick while working towards airplanes. It's amazing that they carved a log into a neat shape you can throw further than another log, and the hype train is saying that it's fully carve-able, so it counts as "airplane-supremacy," but that's a crappy definition and not what we're waiting for.
This proves the underlying principle of a quantum speedup is a physical reality. It might be something people already take for granted if they're interested in factoring, but it's a crucial prerequisite that was not yet irrefutable.
Q: Why can the D-wave not be used to illustrate “quantum supremacy” in a similar way?
(As I understand it the D-wave can sample from the solutions to ”ising model-like” problems, which I assume would be extremely difficult for a classical computer to do (but probably possible to verify).)
As for Ising model, D-wave didn't outperform classical algorithm after classical algorithm was tuned (it was a new problem, so existing classical algorithm wasn't the best possible), see https://arxiv.org/abs/1401.1084, also there are reasons to suspect why Ising model will not provide any quantum speedup, see https://arxiv.org/abs/1411.5693.
To answer your question specifically: DWave does not allow the same level of control over qubits.
Quantum supremacy in any reasonable sense of the word supremacy is a long way off.
1. https://hal.inria.fr/file/index/docid/451604/filename/smalli...
> [quantum computing supremacy] term refers to the use of a quantum computer to solve some well-defined set of problems that would take orders of magnitude longer to solve with any currently known algorithms running on existing classical computers
.. and continues to explain why this setup does exactly that.
That is still clearly quantum supremacy, even if Shor's algorithm is not yet implementable.
Uh, there are birds. Literally everyone thought useful air travel was possible, and not only possible, but so easy that a Darwinian process was able to produce it, not once, but literally thousands of times, in thousands of ways.
----
But looking at the actual "experiment", I don't count that as computation in any meaningful sense, and morally equivalent to the following:
Set up a digital camera and point it at a scene. Take a picture. Now take millions of additional pictures, without moving the camera.
Measure the values at each pixel. See how they correspond to "amplitudes"? That's our computation. <= (This is the part of the article that should raise eyebrows...)
Article (smugly): How about you simulate (render) the scene using an actual computer? Measure the amplitudes of the resulting millions of images. OMG, that took you so long!!! Loser.
Are we being punk'd here?
The digital camera is the quantum computer. The "scene" is the random initial state C. The scene is then translated into a renderable scene for the classical computing version, and rendered with an unbiased physically based renderer, which produces the same result.
I fail to see how any of this is even remotely exciting, much less interesting. A camera is not a computer, no matter how many measurements you make with it, nor how "random" the scene you are taking pictures of is.
And yes, simulating reality takes more cycles than just measuring whatever happened with a camera. Photos are "faster" to get than the equivalently rendered scene—news at 11!
Again: Are we being punk'd here?
Imagine a bunch of magnets. Imagine forcing them into a "frustrated" configuration; maybe you have them all on a grid, and you have servomotors that can rotate them to face any way you like. The servomotors are strong, so counteracting the magnetic forces is easy for them. You design an appropriately frustrated configuration, and then release all of the magnets at once. What configuration do they rotate into?
A quantum annealer is conceptually similar. Each qubit, on a regular, patterned graph, has connections to its neighbours. You can leave these alone (no corellation) or tune them up to +/- 1, corresponding to "must be the same as this other qubit" and "must be different than this other qubit". You can also bias each individual qubit to be a 0 or a 1.
Then, you let it go, and it anneals, and you observe the result. Your goal is to get to the _lowest energy state_ possible: the least possible frustration remaining.
In our magnet example, it would be as few magnets as possible wanting to move - if you poked them with your finger they'd want to go back into their current state. You could imagine that your magnets might not get down to their absolute lowest energy state; maybe it would take too much energy to flip from their starting state to that lower state. In a quantum system, because of tunneling, the system can reach these lower ground states. Rather than being in a fixed position the way our magnets were, qubits are in a quantum superposition, so they can reach a lower energy state without having to climb up that energy hill. Or so we're lead to believe by the numbers, anyway; I'm not a physicist.
Now, if you can map some useful computational question onto the original configuration of qubits that is answered by the ending position, you've got yourself a useful quantum computer. This is the hard part! The key is to use optimization algorithms where a lower energy state = a more optimized result. If you can do this, there's a ton of employment waiting for you.
Then, if you want "quantum supremacy", it's matter of providing more optimized answers in less time, particularly as the problem scales up in complexity. There does indeed appear to be a crossover point coming in a decade or so, at least for the small class of real-world problems that the Ising Hamiltonian works for.
Yes, I agree. But this has not been demonstrated. What's being demonstrated (apparently) is that measuring a quote-unquote "quantum computer" doing whatever it does naturally is easier than simulating said quantum computer classically. Well, yeah. Duh.
That's the same thing as "rendering" a scene with an unbiased renderer vs. setting up that same scene in reality and using a camera. No one in their right mind would point to the camera and say they'd create a next-gen, heretofor impossibly fast unbiased rendering algorithm.
Technically, the digital camera is "computing" the same result—but no one would call it that, and IMO, the same is true of what is being discussed in the FAQ. It's literally NOT COMPUTATION, which brings us back to your line:
> Now, if you can map some useful computational question onto the original configuration of qubits that is answered by the ending position, you've got yourself a useful quantum computer. This is the hard part!
It's not only the hard part, it's the only part that matters. Until then, you have AT BEST a "quantum camera". Potentially useful, perhaps—but it's not a computer, or computation.
Anyway, thanks for responding. Much better than drive-by downvoters probably hoping to get PhDs in this stuff.
Also, my intent is not to belittle Google's engineering effort. In the same way that I wouldn't belittle Sony for making 24mmx36mm backlit CMOS sensors. It's impressive! Good for them. But it's not computation, and it definitely doesn't establish some kind of "quantum computing supremacy" (since no meaningful computation is being done). When they stop handwaving about mapping actual computation problems to the scene they've set up and are measuring, then I'll get excited. Maybe it's doable, maybe not. But a quantum camera, AT BEST, is one step along the path...
I saw this happen with string theory first hand, and my experience with string theory was a major factor that led to changing paths to pure mathematics and computer science.
As someone who has spent a lot of time researching QC, I do not believe it will ever be possible to build a practical quantum computer. We have been over this so many times on this site. Here is a good link from a serious professional who takes the same position [1]
In fact, my personal opinion is that quantum computers are functionally, a hoax, the main purpose of which is to generate hype, secure research grants, ensure career stability for academics, and give science reporters something to write about to get clicks while deceiving the public.
[1] https://www.quantamagazine.org/gil-kalais-argument-against-q...
You're implying that Scott Aaronson is being deceptive.
I think that needs better evidence. The story you linked to is two years old and it's about QC skeptic Gil Kalai.
Scott addresses him directly in his post:
> If quantum supremacy was achieved, what would it mean for the QC skeptics?
> I wouldn’t want to be them right now! They could of course retreat to the position that of course quantum supremacy is possible (who ever claimed that it wasn’t??), that the real issue has always been quantum error-correction. And indeed, some of them have consistently maintained that position all along. But others, including my good friend Gil Kalai, are on record, right here on this blog predicting that even quantum supremacy can never be achieved for fundamental reasons. I won’t let them wiggle out of it now.
There is no way to argue against this kind of goal post moving style of debate because even if another 20 years go by and QC still don't exist on a practical level, the goal posts will just keep getting moved.
I am extremely confident that this will continue until the public gets bored of hearing about it.
Yet I have no trouble believing in this quantum supremacy at the current scale.
The way I see it is a quantum computer is "running" an algorithm like Toshiba's "simulated bifurcation". It's running it at a frequency much higher than current computers.
If I had to take a guess it is like running classically at the Bohr frequency of an electron ~10^16 Hz. So it's kind of like trying every combination with some heuristics. Currently we are tapping into this 6 additional order of magnitudes of computation, which allows a brute force search to appear instant. But once we have used this 10^6 speed factor the quantum computer will miss a lot of the solutions and you will have to increase the sampling time exponentially to find your solution. We are currently in the exponential part of the S curve.
You've got to understand that those machines are expensive to build, so putting a cap on the performance won't help you raise funds. But these machines are beautiful. They are some marvel of engineering. There are plenty of things to discover. They need to get built.
They will bring plenty of useful techniques in the how to manipulate and build very small things, and more importantly how to compute without releasing so much heat. But those advances will probably get locked into a few private companies by monopolizing the few big names that can lend the credibility required to raise funds.
This seems more comparable to the moon landing or LHC Higgs-Boson discovery. Challenging to pull off once, and the next steps seem to increase exponentially in man-power and cost.
Does it only do certain kinds of simulations well, i.e. simulations of quantum systems, or can that be generalized? Can it be applied to economic, environmental, traffic, etc as well?
Our current understanding is that quantum computers won't offer a speedup for the simulation of nonquantum systems. The only simulations they'll be faster for are systems for which quantum effects are important.
Of course it's possible that someone will discover an algorithm that gives quantum computers an exponential speedup in the simulation of any system. But I think that's pretty unlikely because it would imply that quantum computers were exponentially faster than classical computers computers for every problem, because you could just use your quantum computer to simulate a classical one.
The speedup from grover's algorithm is essentially universal it's just not an exponential speedup. So for example in your non-quantum simulation task, you want to search for an input to a cellular automata that makes it spell your name after 5000 timestemps. You can use grover's algorithm to find that input with a sqrt() the number of simulation runs that a classical machine would need (and perhaps fewer, since there are likely multiple solutions).
To me that's one of the most interesting things about quantum computation: There is a very broad class of things where it offers a modest speedup (sqrt) and a narrow (but useful!) class of things where it offers an exponential speedup. But if you imagine almost any kind of 'superpowered quantum computation'-- something that is a little more powerful than the laws of physics as we know them allow-- you almost always get an exponential speedup for everything (or even more absurd results, like every problem being solvable in constant time). QC has this interesting property of being just strictly more powerful than classical computing, but without being magic-instant-computing.
[Which is also why many of the common incorrect descriptions of quantum computing are sad, stuff like "testing all values in paralle"-- if it worked like that description it would be magic instant computing.]
1 - cat having its ears tickled
2 - cat watching something move under a sheet that might be a mouse
3 - cat waiting for tin of cat food to be opened
4 - cat wanting to get through a door
etc etc up to 'N'
So a “challenger” generates a random number C between 1 and N, and the challenger then sends C to me and my cat, and I apply the appropriate 'input' to my cat, tickling her ears or openeing a tin of food or whatever, to get her into the correct mode. We then measure her behaviour and then fire up our cluster of computers and run the simulation and then wait for a few months for the numbers to get crunched to verify if the cats behaviour was within expected probability distribution for C.
He has exceeded my expectations with this post, which cuts through all the hype to communicate exactly what the results of this experiment mean for the field. It's worth reading and sharing.
"Enormity" implies moral reprobation, so it's a poor way of describing the significance of a computational discovery.
I question this. All of this.
Here is Gil Kalai's post from yesterday: "Quantum computers: amazing progress (Google & IBM), and extraordinary but probably false supremacy claims (Google)." https://gilkalai.wordpress.com/2019/09/23/quantum-computers-...
> Scott is correct that inability to achieve quantum supremacy is quite central to my argument (since 2014), so naturally I don’t expect that the recent claims by the Google team will stand. Of course, if these claims (or any other quantum supremacy claim) are correct then this would defeat my theory. It goes without saying that the claims are so fantastic that also responsible believers in quantum computers should examine these specific claims (like an alleged NP=! P proof) carefully and skeptically. Also, it would be nice to hear some details about the precise claims and methodology of the Google team.
(Seriously though, why the hell did those vcrs have such a large green board??)
"I’m trying to understand the chain of inference from Google’s leaked result of quantum supremacy to theoretical computer-science “hardness” of the computation.
Computing the exact probabilities of a random quantum circuit is proven hard, but computing the exact probability of a random algorithm is also an open problem, so what you really care about is approximation of computing the probability (up to epsilon), or even weaker, just sampling from a probability (also up to epsilon under some metric of comparing distributions).
Their computer implements Random Circuit Sampling, and their cited “theoretical” hardness results are your paper from 2017, of QUATH => HOG, and as far as I understood from your paper, proving that approximation / sampling from a (random) quantum computer/circuit is hard is still an open problem (Am I wrong here? I’m not up to date with everything), and a difficult one. But you did make a compelling argument that even if QUATH was solvable, it will lead to new insights.
Their actual benchmark uses cross-entropy benchmarking, called xeb in their paper, and defined as 2^n* [P(x_i)] _i-1 (Can’t type brackets). I could not find any ‘theoretical hardness’ paper at all using this benchmark, some results talk about different XEB using log but they prove these are not strong enough and can be reached classically.
Are there any hardness results for their benchmark? I wonder why they would use that instead of the proven HOG, even for smaller input sizes, I would see more value in a benchmark which has theoretical roots. I feel intuitively like their benchmark is much more similar to the log variation of XEB than to HOG, but didn’t think it through completely.
As for their gates, I understand they are not general random quantum gates, but instead they have a variation of iSWAP and controlled phase CZ. The hardness results that do exist also don’t address the limited gates, in how it changes the distribution of random circuits. Are those two gates at least universal, so that we have hope that this could be proved? Their statement was “but reliably generating a target unitary remains an active area of research”, so I assume they aren’t universal. I would love to see some heuristic argument as to why those two in particular are hard, especially given results like Gottesman–Knill theorem, it seems like some surprising gates do have classical simulation. iSWAP is just swap and phase gates, and CZ is phase gate only on the 11 state. Doesn’t feel like it could be universal to me but maybe I’m wrong.
I probably missed some things, so I’d love it if you could point to papers filling the gaps between their results and a real theoretical statement. I don’t have the intuition to tell which gaps are important and which are not. I also don’t know which papers/results already exist and I can’t really search as I am not an academic, and don’t have full access to many papers, and you probably know the state of the art results. "
- CZ is a perfect entangler. No, iSWAP and CZ do not form a universal set. They typically combine virtual Z rotations (which has a continuous angle parameter) with an additional one-qubit gate, such as a X or Y gate (or their square root, which they mention in the paper) which gives a universal set. Although they don't make use of continuous Z rotations in that particular experiment, the device is capable of doing that, and is a (noisy) universal quantum computer.
- https://www.ibm.com/blogs/research/2017/10/quantum-computing...
- https://www.newscientist.com/article/2151032-googles-quantum...
So technically Google cannot claim to reach quantum supremancy with only 54 qubits as described in the paper.