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Preface: I know nothing about quantum computing. What exactly is a qubit? I'm not asking what does it mean, because I know there's superpositions and all that

Preface: I know nothing about quantum computing. 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?

Analemma_ 7y ago
Just like a bit, a qubit is an abstract representation of information, which can be physically instantiated in different ways.
freeone3000 7y ago
You can do it with electron spin or photon polarization or by any number of properties, but it's the state of a fundamental particle. 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.
- drdeca 7y ago
  I thought a qubit could also be implemented with current going around a superconductor loop. Is that incorrect?
  - ben_w 7y ago
    You can implement them like that. Anything that has a quantum state that doesn’t decohere too fast will work. The hard part thus far has been that almost everything does decohere too fast.
  - Denvercoder9 7y ago
    No, that's indeed a quite popular approach.
rolltiide 7y ago
I'm in the same boat. A qubit can have more than the two states that a transistor can have, got it. 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".
- rmidthun 7y ago
  Since we're looking at Scott Aaronson, you might want to check out "Quantum Computing Since Democritus". It gives a good explanation of the math behind qubits and how they can be used. Best intro I know of.
  - rolltiide 7y ago
    thank you for that and not trying to explain it in an additional convoluted way
- rini17 7y ago
  Schrodinger's cat in unopened box is 1 qubit = it's alive and dead at the same time. When the box is opened to observe the result, the quantum state "decoheres" - decays to 1 bit result. 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 :)
  - drdeca 7y ago
buboard 7y ago
for example an electron or a polarized photon. they can take the value 0 and 1 when measured but when not measured they can be in superposition states. 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.
- AgentME 7y ago
  >>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? >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.
itcrowd 7y ago
Essentially, they use superconducting electronics to create a quantum circuit. It's not the same logic gates as a traditional computer chip and not based on the same physics. The details are difficult & messy. 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 https://arxiv.org/abs/cond-mat/0703002 edit: source [6] is more appropriate and open to access
- outworlder 7y ago
  Holy hell. > > 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.
diegoperini 7y ago
> What exactly is a qubit? 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 Why such a composite type with weird constraints you may ask? Because that's how properties of really small particles behave in the real world. So the hope is, maybe if we can build our software using this weird data type called qubit, we can implement computation on quantum hardware without abstracting every problem using a dump type like a boolean or its aliases/collections. 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.
raincom 7y ago
"[T]hose believing that a QC can manipulate or maintain huge objects "free of cost" (i.e., at unit cost) should provide a convincing explanation to this fantastic speculation. Being skeptic of this (rather over-simplified and counter-intuitive) speculation seems to be the default and natural position." (From http://www.wisdom.weizmann.ac.il/~oded/on-qc.html http://www.wisdom.weizmann.ac.il/~oded/on-qc.html )
3PS 7y ago
Short answer, a qubit is a unit vector in a 2D complex Hilbert space. Now, that doesn't actually say much about why we care or how they're useful. In practical terms, you can think of qubits as complex unit vectors along two axes, with one axis corresponding to |0⟩ (the zero qubit) and one axis corresponding to |1⟩, or the one qubit. So for example, you could have a qubit called |+⟩, which is just shorthand for (|0⟩ + |1⟩)/sqrt(2). 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.
- galaxyLogic 7y ago
  > Schrödinger's cat must be alive or dead once we open the box 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.
  - 3PS 7y ago
    So this is a little nuanced. It's true that quantum systems collapse on "observation", but that doesn't actually mean "observation by a sentient entity". It could really just be any interaction with the outside environment. (This is why qubits have to be kept incredibly well-isolated.) We don't really fully know exactly how this collapse works, and related speculation is generally classified under the measurement problem [1]. But it's true that one of the key points of Schrödinger when proposing his thought experiment was that the notion of measurement or observation was not fully defined under the Copenhagen interpretation. Side note, it's not that the cat is observing poisonous gas, but rather, that a Geiger counter is set to detect whether a radioactive atom decays or not and triggers the release of some poisonous gas if so. So, classically, Schrödinger's cat would be either alive or dead 50% of the time, not 100% dead. There are plenty of alternative ways to reconcile this classical view with quantum mechanics. Perhaps the simplest and most well-known is the many-worlds interpretation [2], which states that both events occur, just in different timelines, and we don't know what timeline we ended up in until we open the box. (Of course, it is ridiculous to speculate as to which timeline "we" end up in before the experiment is carried out, because the people in both timelines would still be "us" - this can get awkward to think about.) [1] https://en.wikipedia.org/wiki/Measurement_problem https://en.wikipedia.org/wiki/Measurement_problem [2] https://en.wikipedia.org/wiki/Many-worlds_interpretation https://en.wikipedia.org/wiki/Many-worlds_interpretation
cdumler 7y ago
TL;DR: A quantum computer is a device that uses the measurement of quantum properties to do computation. There are many ways to implement one depending on the type of entangled particles being used, from crystals to make entangled photons and superconducting mounds to entangle electrons. This is the same for binary computers, which can made from electrical devices (transistors), values with air pressure or balls falling down wooden ramps. 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.
tagrun 7y ago
Physicists here. In practice, a qubit is a two-level physical system. It can be spin state of an electron, polarization of a photon, lowest two energy states of an atom or an electron in quantum dot/well trap potential, the charge state (called charge qubit) -- whether you have 0 or 1 electrons in it, etc etc (if you have 3 levels, it's called qutrit, and for d levels qudit). This experiment uses charge qubits (a special variant which has some robustness against charge noise by design [by operating at a voltage level which is insensitive to 1st order fluctuations in the electric field, called "sweet spot"], called transmon). 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.
justinpombrio 7y ago
> 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? 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".
grumpy8 7y ago
Extremely simplified: >> 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.
- deleted 7y ago
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tzs 7y ago
> 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? (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.
sundarurfriend 7y ago
Quantum computing for the very curious [1] has been very useful to me, for getting at least a basic idea as to what exactly a quantum computer is, what a qubit is, and how they are manipulated. [1] https://quantum.country/qcvc https://quantum.country/qcvc

Analemma_ 7y ago

Just like a bit, a qubit is an abstract representation of information, which can be physically instantiated in different ways.

freeone3000 7y ago

You can do it with electron spin or photon polarization or by any number of properties, but it's the state of a fundamental particle. 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.

drdeca 7y ago
I thought a qubit could also be implemented with current going around a superconductor loop. Is that incorrect?
- ben_w 7y ago
  You can implement them like that. Anything that has a quantum state that doesn’t decohere too fast will work. The hard part thus far has been that almost everything does decohere too fast.
- Denvercoder9 7y ago
  No, that's indeed a quite popular approach.

rolltiide 7y ago

I'm in the same boat. A qubit can have more than the two states that a transistor can have, got it. 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".

rmidthun 7y ago
Since we're looking at Scott Aaronson, you might want to check out "Quantum Computing Since Democritus". It gives a good explanation of the math behind qubits and how they can be used. Best intro I know of.
- rolltiide 7y ago
  thank you for that and not trying to explain it in an additional convoluted way
rini17 7y ago
Schrodinger's cat in unopened box is 1 qubit = it's alive and dead at the same time. When the box is opened to observe the result, the quantum state "decoheres" - decays to 1 bit result. 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 :)
- drdeca 7y ago

buboard 7y ago

for example an electron or a polarized photon. they can take the value 0 and 1 when measured but when not measured they can be in superposition states. 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.

AgentME 7y ago
>>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? >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.

itcrowd 7y ago

Essentially, they use superconducting electronics to create a quantum circuit. It's not the same logic gates as a traditional computer chip and not based on the same physics. The details are difficult & messy. 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 https://arxiv.org/abs/cond-mat/0703002 edit: source [6] is more appropriate and open to access

outworlder 7y ago
Holy hell. > > 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.

diegoperini 7y ago

> What exactly is a qubit? 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 Why such a composite type with weird constraints you may ask? Because that's how properties of really small particles behave in the real world. So the hope is, maybe if we can build our software using this weird data type called qubit, we can implement computation on quantum hardware without abstracting every problem using a dump type like a boolean or its aliases/collections. 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.

raincom 7y ago

"[T]hose believing that a QC can manipulate or maintain huge objects "free of cost" (i.e., at unit cost) should provide a convincing explanation to this fantastic speculation. Being skeptic of this (rather over-simplified and counter-intuitive) speculation seems to be the default and natural position." (From http://www.wisdom.weizmann.ac.il/~oded/on-qc.html http://www.wisdom.weizmann.ac.il/~oded/on-qc.html )

3PS 7y ago

Short answer, a qubit is a unit vector in a 2D complex Hilbert space. Now, that doesn't actually say much about why we care or how they're useful. In practical terms, you can think of qubits as complex unit vectors along two axes, with one axis corresponding to |0⟩ (the zero qubit) and one axis corresponding to |1⟩, or the one qubit. So for example, you could have a qubit called |+⟩, which is just shorthand for (|0⟩ + |1⟩)/sqrt(2). 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.

galaxyLogic 7y ago
> Schrödinger's cat must be alive or dead once we open the box 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.
- 3PS 7y ago
  So this is a little nuanced. It's true that quantum systems collapse on "observation", but that doesn't actually mean "observation by a sentient entity". It could really just be any interaction with the outside environment. (This is why qubits have to be kept incredibly well-isolated.) We don't really fully know exactly how this collapse works, and related speculation is generally classified under the measurement problem [1]. But it's true that one of the key points of Schrödinger when proposing his thought experiment was that the notion of measurement or observation was not fully defined under the Copenhagen interpretation. Side note, it's not that the cat is observing poisonous gas, but rather, that a Geiger counter is set to detect whether a radioactive atom decays or not and triggers the release of some poisonous gas if so. So, classically, Schrödinger's cat would be either alive or dead 50% of the time, not 100% dead. There are plenty of alternative ways to reconcile this classical view with quantum mechanics. Perhaps the simplest and most well-known is the many-worlds interpretation [2], which states that both events occur, just in different timelines, and we don't know what timeline we ended up in until we open the box. (Of course, it is ridiculous to speculate as to which timeline "we" end up in before the experiment is carried out, because the people in both timelines would still be "us" - this can get awkward to think about.) [1] https://en.wikipedia.org/wiki/Measurement_problem https://en.wikipedia.org/wiki/Measurement_problem [2] https://en.wikipedia.org/wiki/Many-worlds_interpretation https://en.wikipedia.org/wiki/Many-worlds_interpretation

cdumler 7y ago

TL;DR: A quantum computer is a device that uses the measurement of quantum properties to do computation. There are many ways to implement one depending on the type of entangled particles being used, from crystals to make entangled photons and superconducting mounds to entangle electrons. This is the same for binary computers, which can made from electrical devices (transistors), values with air pressure or balls falling down wooden ramps. 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.

tagrun 7y ago

Physicists here. In practice, a qubit is a two-level physical system. It can be spin state of an electron, polarization of a photon, lowest two energy states of an atom or an electron in quantum dot/well trap potential, the charge state (called charge qubit) -- whether you have 0 or 1 electrons in it, etc etc (if you have 3 levels, it's called qutrit, and for d levels qudit). This experiment uses charge qubits (a special variant which has some robustness against charge noise by design [by operating at a voltage level which is insensitive to 1st order fluctuations in the electric field, called "sweet spot"], called transmon). 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.

justinpombrio 7y ago

> 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? 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".

grumpy8 7y ago

Extremely simplified: >> 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.

deleted 7y ago
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tzs 7y ago

> 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? (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.

sundarurfriend 7y ago

Quantum computing for the very curious [1] has been very useful to me, for getting at least a basic idea as to what exactly a quantum computer is, what a qubit is, and how they are manipulated. [1] https://quantum.country/qcvc https://quantum.country/qcvc