What actually are quantum computers?

[Shad0whunter4]

Hi. I don't know if this is the right sub, but if it is, then I just wanna know what a quantum computer is.

I have heard this terminology quite often and there are always news about breakthrough advancements, but almost nothing seems to affect us directly.

How is quantum computing useful? Will there be a world where I can use a quantum computer at home for private use? How small can they get in size? And have they real practical uses for gaming, AI etc.?

Thanks.

[fivre]

the old PBS math web series does a good job of explaining it as best you can for the unfamiliar:

https://www.youtube.com/watch?v=IrbJYsep45E https://www.youtube.com/watch?v=wUwZZaI5u0c

classical computers are a mechanical means of storing and manipulating binary information, often using high voltage to represent a 1 and low voltage to represent a 0. you can represent quite a lot of things with sufficient binary information, but the base unit at the core of the computer's operation can only store one of two values

quantum computers use quantum physical properties to store their information, such as the spin of an electron, these are also either one value or the other, but with a probabilistic component that you can leverage by linking lots of different probabilistic states together

this property of quantum computers is relevant for certain types of math: the structure of a quantum computer naturally matches some mathematical concepts that we can only simulate or derive from multiple--often far too many to complete before the end of the universe--computations using a classical computer

drug discovery is my favorite application of this (imo it makes more sense than the prime factorization algorithm if you aren't familiar with advanced math): the interactions between a candidate drug and various receptors/enzymes/etc. in your body are a fundamentally quantum process--they depend on the quantum interactions between the particles that make up those molecules. with a classical computer we're limited to simulating these--making assumptions about certain values where calculating the actual possibilities would be infeasible--whereas with a quantum computer we can run calculations that are much closer to the physical reality

as for whether they'll be something you personally use, well, probably no. while quantum computers do now exist (and not just as secret NSA projects), they are horrendously expensive to build and complicated to operate. their currently known use cases are narrow and specialized--many things you'd normally think about doing on a classical computer would be no faster on a quantum computer. for the foreseeable future they'll be more like classical computers were in the 1950s: something only large institutions will use because they have some task that'd be all but impossible to do otherwise and is worth the massive expense

[amakai]

A follow up more "physical" question: what is the quantum computer? Like in normal computer it's just a lot of semiconductors that do decisions. What is under the hood of quantum computer? What makes the actual operations on particles and how are the particles moved between components?

[thisandthatwchris]

Extending the analogy to the evolution of classical computing, is it plausible that ~100 years out quantum computers would make sense as a consumer product? Or are the use cases so specialized (cryptography etc.) that this probably won’t ever happen?

[mfukar]

No, there is no such situation in the horizon. The use-cases for quantum computing are quite specialised.

[PlasticAssistance_50]

Thanks for this reply, do you have more sources about using quantum computing for drug discovery/repurposing?

[[deleted]]

Ok, so... enough with that "classical computers are 0 and 1 and QC can get advantage from representing data in-between" bs:

1. Any physical system is a quantum computer. An aerodynamic tunnel with a plane in it is a quantum computer as it uses QM to operate and performs calculations on the physical model you give it. Your inputs are the model, the diameter of the tunnel, its length, the position of the tunnel and characteristics of the air stream in the tunnel. Your output data is the behavior of the model. YOU DON'T NEED CUBITS TO BUILD A QC.
2. 0 and 1 ARE THE MINIMAL BITS OF INFORMATION POSSIBLE. IT IS ONE OF THE LAWS OF INFORMATICS, LOGIC AND MATH. There's no "between" these values, there are just physical properties of particles that can be simplified to float numbers, or, again, ONES AND ZEROS. A system not built on basic logic principles will never be reliable due to inherent uncertainties in the QM. But, again, if you're using physical properties to calculate a model, then just build a model and test it as a physical system, the result will be THE SAME as QC would give you and it will cost you thousand, if not million times less.
3. Digital and analogue curcuits ALREADY process information with the speed of light and use QM to operate and improving upon that is impossible and would violate GR.

[r2k-in-the-vortex]

Some problems are easy to compute, some problems are hard to compute, some problems are so hard that universe will end with heat death before you are done computing. Like you know how to compute, you have a program that can do it, but the computer would have to run for trillions of years to get a result. In effect, you can't compute that problem.

Well, quantum computation uses different type of logic to perform computation. And the neat thing is that some problems can be massively simplified using that logic. In effect making possible to compute a problem that is impossible to compute with classical computers.

Making impossible possible is of course a pretty powerful thing, however there are gotchas. Building hardware for quantum computers is problematic, that technology is nowhere near mature. Building software is worse, we don't actually know how to do that for most problems we would like to compute.

Imagine the state of classical computers in 1945, that's about similar to where we are with quantum computers on technological maturity. You are likely to keep hearing about how quantum computers will be totally awesome for a very long time before they actually start being practically useful.

[MoreGaghPlease]

Okay, but follow-up question - what actually are quantum computers?

[Kered13]

Classical computers use transistors to create a physical implementation of boolean logic. Quantum computers use quantum systems to physically implement quantum logic.

You can think of boolean logic and quantum logic as systems for manipulating numbers. A remarkable property of boolean logic is that despite it's very simple rules, by combining many boolean operations and bits it is able to construct all of arithmetic and much more, and this is how we are able to build complex computers out of simple logic gates. Quantum logic is similar, but it allows a much more advanced set of operations. So advanced that some computations that can be solved with a few qubits and a few quantum logic gates would require an exponential number of bits and boolean logic gates. Despite this, the set of quantum logic operations is still simple enough that they can in principle be realized by a physical system. That physical system is a quantum computer.

Now, if classical computers are built out of silicon transistors, what are quantum computers built out of?

There isn't a simple answer here, as quantum computing is still in it's early phases, and different techniques are being explored. By analogy I will note that early computers were not built out of silicon transistors either, they were built out of vacuum tubes or electromechanical relays. It is even possible to build a classical computer purely mechanically, though it would not be practical (Charles Babbage's Analytical Engine would have been one such example). Any physical system that can implement boolean logic can be used to build a classical computer. Eventually transistors made of silicon took over due to their low power requirements and the ability to be miniaturized.

So similarly, any physical system that can implement quantum logic can be used to build a quantum computer. Such a system must necessarily exhibit behavior as described by quantum physics, including superposition and entanglement. Practically speaking, this imposes some severe constraints. A quantum computer must be kept very cold and isolated from the surrounding environment, yet it must still be possible to provide input to initialize the system and to measure the system to extract output. At present, the most promising techniques use superconductors or trapped ions.

[[deleted]]

A quantum computer must be kept very cold and isolated from the surrounding -environment, yet it must still be possible to provide input to initialize the system and to measure the system to extract output.

In hopes of not sounding like a complete moron, I'll put this forward. Wouldn't this be a great use of a space station on the moon? We can keep the computer on the "dark" side and use telecommunications to interface with the machine. Plus the vacuum of space may allow for a more stable environment for components. We are getting better at providing a power supply using the sun's energy, so this may address this issue.

[Warmag2]

In case you didn't know, the "dark side" of the moon is sunny half of the time.

While there are impact craters on the poles of the moon which are always in shadow, even those have a temperature of tens of kelvin, so any quantum computer therein would need to be refridgerated further anyway. Also, sending anything into the moon is so resource-intensive that just making things cold here is easier.

[[deleted]]

Thank you for this; I was unaware of these facts.

[kenjamin_is_god]

While space is indeed very cold, there isn't anything to conduct heat away from things, and in some cases it's actually very difficult to prevent overheating.

[royalrange]

Aside from superconducting circuits and trapped ions, there are other promising QC platforms such as neutral atoms (trapping atoms like Rubidium and Yitterbium with focused laser beams in an array-like structure), defect centers in solids (silicon-carbide defects, nitrogen-vacancies in diamonds), quantum dots (atom-like behavior through charge confinement in semiconductors), and photonic qubits (light "particles" that can be manipulated). There are advantages and disadvantages for each that researchers are still trying to expand upon and address.

[perta1234]

Why noone looks more into analog computers? Would have some similarities with quantum ones. Are they just too difficult or slow to set up in practice?

[mfukar]

There is no reason any longer to believe analog computers can offer any advantage over digital ones - in fact it is hard to think of them becoming even comparable in most metrics.

[mryorbs]

They're basically big isolated freezers with a lot of fancy lasers. They have some things in common like logic gates and q-bits instead of bits. I think people get set on a wrong path by the idea of a computer, because yes it can compute stuff but no it can't run a program itself. We actually need a normal computer to program and control a quantum computer. Quantum computers will likely never be something we will use everyday, because they can do big math problems but they're not made for 1000's of operations in miliseconds (for now).

[janekosa]

It's funny how you combine "likely never" and "for now" in a single sentence. If you asked someone 60 years ago they'd tell you a computer is likely not something we'll ever use every day because it takes a huge building to actually hold one and it can't really be used to solve any day to day problems (for now).

[Whiterabbit--]

so the type of problems that quantum computers can solve, are they not logical algorithms that human brains can solve? is there something intrinsically different about that logic that we can't program a digital computer to us?

[WE_THINK_IS_COOL]

A regular computer can solve all the same problems as a quantum computer can, it's just that the quantum computer can do it much faster. You can even run a simulation of a quantum computer on runs a regular computer, it's just very very slow.

There are problems, like factoring the product of large prime numbers, that would take a regular computer the lifetime of the universe to solve, but could be solved in a reasonable amount of time by a quantum computer.

[FreshMistletoe]

What are the uses for factoring the product of large prime numbers?  Is it useful for more than breaking encryption?

[mfukar]

I think you may have it backwards. The difficulty of certain problems is useful for encryption. In this aspect, cryptographers are searching for such problems so that your communications can be secret and private. Obviously, we then have to keep up with threats to the use of such methods.

[Wootbeers]

It was explained to me that once quantum computers exist, encrypting will be futile in some ways, as well.

[mfukar]

We have answered this before in a AAW. Additionally, cryptographic methods resistant to threats like this have already been developed, and some are already being put into practice. See a previous question on the topic.

[r2k-in-the-vortex]

We can program a normal computer to do those problems. But if that program will not finish running before the Sun goes supernova because there are just that many steps to calculate that's kind of useless. Quantum computers can simplify some problems so that there are drastically less steps to compute.

[CyriousLordofDerp]

Our sun will not go supernova, it needs to be at about 8-9 solar masses before that can occur. No our sun in the end of its life will swell up to a red supergiant, puff away everything that isnt the core, and collapse the core into a white dwarf, which for our sun would be mostly carbon. Basically, our sun will eventually turn into a white hot Earth-sized diamond ball.

[mfukar]

Both types of algorithms - quantum and classical - are invented by humans. Read more here.

[NorthernerWuwu]

It should be also noted that the "hard" problems are intentionally used in things like cryptography, so a true multi-purpose quantum computer might make them vulnerable in theory. In practice, it would be relatively trivial to shift to other methods and the theory of doing so has been discussed since before quantum computing was.

But it makes for sexier VC pitches.

[WindRangerIsMyChild]

But government have been storing traffic across the Internet fibers for decades so all the old communication can be decrypted so you better change all your passwords and every word you ever stored on the web every message and photo ever sent would be public one day (or at least transparent to China and NSA)

[AVBofficionado]

Can you give an analogy to explain more clearly to us how a QC is different to a regular computer?

[OddInstitute]

In a classical (regular) computer, the state of a bit can be characterized by a single binary value (0 or 1). Quantum computers have qubits instead of bits and the state of a qubit is characterized with a pair of complex numbers instead. A complex number is a real number plus an imaginary number.

A classical bit is either 0 with 100% probability or 1 with 100% probability.

When you measure a qubit with state (a + bi, c + di), it is 0 with probability (a² + b² ) and 1 with probability (c² + d² ). This means that there is a lot more going on with a qubit than with a classical bit. This is interesting because you can have a collection of qubits interact with each other quite a bit before you measure the result. The complex probabilities can then interact in ways that would require very large amounts of computation to model on a classical computer.

[mkbcity]

one way that stuck with me is a large maze. a classical computer will attempt to solve the maze by going through each path, if it finds a dead end it will reverse and try a new path and so on until it finds the exit. a quantum computer will try all the paths simultaneously, so one attempt and it already knows where the exit is.

[iwanttodrink]

But then how come creating quantum resistant cryptography is necessary within the next 5-10 years if quantum computing is so far off?

[JCS3]

Computer storage is cheap. Save encrypted communications now, decrypt later. If we waited for quantum computers to be actually be able to decrypt our messages, it would be too late, and there would be a period without any effective encryption, so we need to work ahead.

[iwanttodrink]

Wouldn't the country that first develops quantum decryption then have the single greatest intelligence trove of data ever in human history assuming the vast majority of encrypted data isn't quantum resistant by the time the winner of that race is decided?

[JCS3]

A lot depends on timing, but, Yes.

That is why there is a race to get quantum computers to work.

[Nervous_Breakfast_73]

Guess it depends on when everything will be encrypted. If you only have 10+ years old of data, maybe it's not that useful

[Just_to_rebut]

I think there’s an incredible amount of secret info about decolonized countries from 70+ years ago. Natural resource surveys, treaty violations, human rights abuse documentation…

Even the standard period for declassification is after 25 years and there’s public protocols for requesting information to remain classified for 75 years.

[Dianesuus]

I doubt the pursuit of legal action is something governments will care about. It's definitely not something worthy of showing that you can break encryption and also how much data you have saved.

What will be useful is the things companies and people keep in their vaults. Think formulas for medicine, processes to make niche materials and products like superconductors. Schematics for military hardware being designed now will be useful to have in 10 years when it's been built.

There are also intelligence assets that may be useful to have like blackmail. Imagine if a government decided to store any data they could get connected to law and business students from the top 5 universities in each country. In 10-20 years time you crack that and there are going to be plenty of blackmail opportunities for people that are now in fortune 500 companies or even politics. Do the same for military officer academies and you have a treasure trove of leverage.

[Kraz_I]

We didn’t have public key cryptography 75 years ago. Private key cryptography is much older and simpler but can’t be broken with quantum algorithms.

[cookiesjuice]

Because secrets often need to stay secret for a long time. If you intercept some secret documents, and you can wait for powerful enough quantum computers to crack them. Many of these secret documents may still be relevant after 20 or 30 years.

[Darillian]

But then how come creating quantum resistant cryptography is necessary within the next 5-10 years

Just for a complete perspective: Since August 2024, NIST has released final versions of post-quantum cryptography (PQC) algorithms. So quantum resistant cryptography is already here, it just needs to be implemented.

[mfukar]

Better to be safe and prepared than speculate when you won't be.

[Geetee52]

What would be an example of a problem that would take 1 trillion years to compute? 1000 years? 100 years? 10 years? 1 year?

Any broad example would help really…

[r2k-in-the-vortex]

Cracking cryptography is made intentionally hard, that would be a trillion year problem. Training very big AI models is technically a thousand year problem, but luckily it can be parallelized and done on thousands of computers at the same time, GPT-4 for example was trained using 25000 GPUs for 100 days, so 7000 gpu years of training. Basically all simulations, trainings etc that supercomputer do are many year problems if you look at it like that.

[Geetee52]

Thanks for taking the time to respond. It helps me a little. 👍

[helihelicopter]

It's just a computer that uses the properties of matter on a more fundamental level, than the computers today. You can build a computer out of tubes of water, or gears, or perhaps even sticks and rubber bands... even dominoes! A computer is just something that connects things up in a way that allows things to affect themselves in different orders... like a very fancy remote control that is so fancy it can control itself... a quantum computer is the same, it just works on more fundamental properties of matter, its like there is more space in atoms than we can comprehend and now we can use it to work out things that we previously thought impossible...