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I still don't understand what the hell is "quantum computing". I think my problem is my own conception of classical computing. My best approximation for what a

I still don't understand what the hell is "quantum computing". I think my problem is my own conception of classical computing. My best approximation for what a quantum computer actually is. Is that it's some sort of device to set quantum states, then run a "quantum algorithm" (i.e. a graph of "quantum logic gates"), and finally read out the "quantum bits" resulting from the algorithm. That sounds a lot more like a sensor to me. Also all the problems with noise and calibration reinforce my notion that it's more like a sensor than a computer. But to be fair I barely understand "classical computing". I think my personal notion relies too much on badly defined ideas about "the run-time" of a computer program (some software) that I have constructed from "industry" experience. To me big huge fundamental part of computing has to do with arbitrarily representing anything with a symbolic alphabet. And then automatically manipulating this symbolic alphabet to get a result with can be "mapped back" through the initial arbitrary representation so we learn something about whatever we initially chose to represent. Maybe that's not computing? but then what is that? and then what is computing?? Try as I might I cannot reconcile my understanding of classical computing with quantum computing.

Strilanc 7y ago
Here's a youtube video that might be at just the right level for you: https://www.youtube.com/watch?v=ZoT82NDpcvQ https://www.youtube.com/watch?v=ZoT82NDpcvQ . Basically it goes over the simplest quantum-vs-classical query advantage (1 instead of 2) that was ever found.
travisoneill1 7y ago
Is that different than a regular computer? To take your statement and slightly modify it: some sort of device to set electromagnetic charges, then run an "electromagnetic algorithm" (i.e. a graph of "electromagnetic logic gates"), and finally read out the "sequence of electromagnetic bits" resulting from the algorithm. That sounds to me like the computer that I am typing on now. Am I missing something here?
- outworlder 7y ago
  > Is that different than a regular computer Not very. Using their definition, logic gates, RAM, Flash or Hard Disks could all be redefined as "sensors".
fossuser 7y ago
I'd recommend Charles Petzold's Code: http://www.charlespetzold.com/code/ http://www.charlespetzold.com/code/ Which does a great job building up computing from bits all the way to a adders, CPUs, and assembly language (with the historical background around it too). I think the easiest way to get an idea for Quantum Computing and what the difference is comes from watching a specific example, I thought this example of Shor's algorithm was pretty good: https://www.youtube.com/watch?v=wUwZZaI5u0c https://www.youtube.com/watch?v=wUwZZaI5u0c
btilly 7y ago
Quick explanation. Flipping a bit in classical computing is an irreversible operation, and so has to release heat. We also have to be in one state at a time. In quantum computing, we are only allowed reversible operations, but can be in a superposition of states. There is no heat released, no upper limit on how fast the operations can go, and no upper limit on parallelism. But the flip side is that you cannot peek at the operation, and error correction is not exactly easy. The "only reversible operations" bit is a huge restriction. For example it means that "if" statements are not allowed.
svat 7y ago
If you really want to know: https://quantum.country/qcvc https://quantum.country/qcvc (previous discussion: https://news.ycombinator.com/item?id=19426573 https://news.ycombinator.com/item?id=19426573)
marcosdumay 7y ago
If you can't formally differentiate a computer from a sliding rule, you will have problems differentiating a quantum computer from a sensor too. That's normal. The difference is more on the flexibility to run different programs than on them transforming encoded data. Both a sliding rule and a sensor do transform encoded data, a computer is something that can apply any mathematical transformation into the data.
- athisntoswhry 7y ago
  Nitpick, but there are some transformations that our current computers (or our current mathematics moreso) cannot do, for example, a continuous form of the Fourier Transform. We only possess discrete formulations of it. But in nature, light passing through a prism or diffraction grating performs an equivalent operation (presumably) continuously. So as of right now, nature is still more powerful than our strongest computers.
  - nixpulvis 7y ago
    Well technically the photons only interact in "quanta" themselves anyway. And suddenly we're back at the level of quantum computing! I'm not sure what transformations you have in mind that "our current mathematics" cannot do.
grumpy8 7y ago
Overly simplified, but in classical computing, a bit is an abstraction representing a 0 or 1. In quantum computing, a bit IS the electron (or some other particle). And electrons are weird and can behave in very surprising ways. Quantum computers are controlling these electrons allowing to do things that classical computers can't. One example of such thing is Shor's algorithm which, taken from wikipedia, "is almost exponentially faster than the most efficient known classical factoring algorithm".
cdumler 7y ago
In classical computing, a bit is a binary state. The implementation isn't important. What is important is there are only two positions: true/false, on/off, electricity flowing/no electricity flowing, a wooden stick pointed to the left/right. If you treat combinations of state as binary numbers (0000=0, 0001=1, 0010=2, 0011=3, etc) and you create functions that manipulate these bits (AND, OR, NOT, etc gates) such that they generate new patterns of bits consistent with doing math. A classical computer relies on classical physics: something exactly is something. You either have a 1 or 0, because the implementation is either one thing or the other thing. Quantum computing relies on quantum effects, say the spin of an electron. The spin of the electron can be measured and when does it will either be up or down. At first it seems like it's just like a classical computer; however, it's possible to "entangle" multiple qbits together. The orientation becomes fuzzy. In this fuzzy state is possible to create quantum gates that again do math, but these results become "fuzzy." So I create the function: "Give x such that x has the reminder of 1 after dividing by some huge number?" In classical physics there is no one answer to modulus. There could be an infinite number of possible answers. Solving it with a classical computer does not mean much more than trying values and seeing what works. But, asking that question with a quantum computer does something different. Being "fuzzy," it exists across all possible states at the same time until you measure it. When you measure it, you will get a random answer but that answer will work for the equation. Say you ask the question, "Give me x where x divided by 5 has a remainder of 1." The quantum computer might first spit out 56 then next time 91 then next time 6.
- Symmetry 7y ago
  Your answer is a good one for helping to understand the problem and is true of almost all computers in active use today. But I want to point out that you can have a classical computer that isn't binary. Analog computers have a long history and are still used in some places today. Also ternary computers where bits can be 1, 0, or -1 had a role in Soviet computing and let you used Balanced Ternary[1] which is even nicer than Two's Complement. But these are classical in the sense of having one particular value in the way the rest of your answer lays out nicely. [1]https://en.wikipedia.org/wiki/Balanced_ternary https://en.wikipedia.org/wiki/Balanced_ternary
ridewinter 7y ago
As wild as it sounds, parallel universes are IMO the best way to explain a quantum computer. I posted an article about it: https://news.ycombinator.com/item?id=21337739 https://news.ycombinator.com/item?id=21337739
achenatx 7y ago
In the end a classical computer just "processes" data. It takes data in, processes it (math operations), then outputs it. The foundation is boolean math, which is then used to execute regular math. At the very most basic level a computer is made of transistors that can solve the problem of basic boolean math. Gates are AND (or NAND), OR, XOR etc. An XOR gate produces the following output from the following inputs: A B Output 0 0 0 0 1 1 1 0 1 1 1 0 an AND gate produces the following outputs: A B Output 0 0 0 0 1 0 1 0 0 1 1 1 with those gates you can make an adder. A single bit adder has the following inputs and outputs A B Output 1 Output 2 0 0 0 0 (decimal 0) 1 0 0 1 (decimal 1) 0 1 0 1 (decimal 1) 1 1 1 0 (decimal 2) Notice that Output 2 is an XOR of inputs A and B Output one is an AND of inputs A and B. So a single bit adder is simply the inputs feeding an XOR gate and an AND gate. This can be scaled up to add any number of bits. Other math operations can also be done. Transistors can be used to make memory bits that hold its state and can be set and unset. They are essentially binary gate loops that feed outputs back to inputs and rely on the delay of electron flows to reach stable states. A clock is used to sequentially count (and pull instructions from memory) and set and unset memory bits. Each cycle of the clock allows the computer to execute the next instruction (by incrementing a number that determines which instruction to fetch). From there you are right, the bits are used to encode meaning. Which could be alphabets, images, audio etc.
IIAOPSW 7y ago
A quantum computer is "a sensor" in the same sense that every laptop is "an electronics experiment". A quantum computer still fits your model because you arbitrarily represent anything with an alphabet of quantum states, automatically manipulate it with a quantum algorithm, and map back the result at the end so we learn something about whatever. I do not see the contradiction.
glandium 7y ago
Minute Physics has a really good explanation of Shor's algorithm, and by proxy, Quantum computing. https://www.youtube.com/watch?v=lvTqbM5Dq4Q https://www.youtube.com/watch?v=lvTqbM5Dq4Q And the followup: https://www.youtube.com/watch?v=FRZQ-efABeQ https://www.youtube.com/watch?v=FRZQ-efABeQ

Strilanc 7y ago

Here's a youtube video that might be at just the right level for you: https://www.youtube.com/watch?v=ZoT82NDpcvQ https://www.youtube.com/watch?v=ZoT82NDpcvQ . Basically it goes over the simplest quantum-vs-classical query advantage (1 instead of 2) that was ever found.

travisoneill1 7y ago

Is that different than a regular computer? To take your statement and slightly modify it: some sort of device to set electromagnetic charges, then run an "electromagnetic algorithm" (i.e. a graph of "electromagnetic logic gates"), and finally read out the "sequence of electromagnetic bits" resulting from the algorithm. That sounds to me like the computer that I am typing on now. Am I missing something here?

outworlder 7y ago
> Is that different than a regular computer Not very. Using their definition, logic gates, RAM, Flash or Hard Disks could all be redefined as "sensors".

fossuser 7y ago

I'd recommend Charles Petzold's Code: http://www.charlespetzold.com/code/ http://www.charlespetzold.com/code/ Which does a great job building up computing from bits all the way to a adders, CPUs, and assembly language (with the historical background around it too). I think the easiest way to get an idea for Quantum Computing and what the difference is comes from watching a specific example, I thought this example of Shor's algorithm was pretty good: https://www.youtube.com/watch?v=wUwZZaI5u0c https://www.youtube.com/watch?v=wUwZZaI5u0c

btilly 7y ago

Quick explanation. Flipping a bit in classical computing is an irreversible operation, and so has to release heat. We also have to be in one state at a time. In quantum computing, we are only allowed reversible operations, but can be in a superposition of states. There is no heat released, no upper limit on how fast the operations can go, and no upper limit on parallelism. But the flip side is that you cannot peek at the operation, and error correction is not exactly easy. The "only reversible operations" bit is a huge restriction. For example it means that "if" statements are not allowed.

svat 7y ago

If you really want to know: https://quantum.country/qcvc https://quantum.country/qcvc (previous discussion: https://news.ycombinator.com/item?id=19426573 https://news.ycombinator.com/item?id=19426573)

marcosdumay 7y ago

If you can't formally differentiate a computer from a sliding rule, you will have problems differentiating a quantum computer from a sensor too. That's normal. The difference is more on the flexibility to run different programs than on them transforming encoded data. Both a sliding rule and a sensor do transform encoded data, a computer is something that can apply any mathematical transformation into the data.

athisntoswhry 7y ago
Nitpick, but there are some transformations that our current computers (or our current mathematics moreso) cannot do, for example, a continuous form of the Fourier Transform. We only possess discrete formulations of it. But in nature, light passing through a prism or diffraction grating performs an equivalent operation (presumably) continuously. So as of right now, nature is still more powerful than our strongest computers.
- nixpulvis 7y ago
  Well technically the photons only interact in "quanta" themselves anyway. And suddenly we're back at the level of quantum computing! I'm not sure what transformations you have in mind that "our current mathematics" cannot do.

grumpy8 7y ago

Overly simplified, but in classical computing, a bit is an abstraction representing a 0 or 1. In quantum computing, a bit IS the electron (or some other particle). And electrons are weird and can behave in very surprising ways. Quantum computers are controlling these electrons allowing to do things that classical computers can't. One example of such thing is Shor's algorithm which, taken from wikipedia, "is almost exponentially faster than the most efficient known classical factoring algorithm".

cdumler 7y ago

In classical computing, a bit is a binary state. The implementation isn't important. What is important is there are only two positions: true/false, on/off, electricity flowing/no electricity flowing, a wooden stick pointed to the left/right. If you treat combinations of state as binary numbers (0000=0, 0001=1, 0010=2, 0011=3, etc) and you create functions that manipulate these bits (AND, OR, NOT, etc gates) such that they generate new patterns of bits consistent with doing math. A classical computer relies on classical physics: something exactly is something. You either have a 1 or 0, because the implementation is either one thing or the other thing. Quantum computing relies on quantum effects, say the spin of an electron. The spin of the electron can be measured and when does it will either be up or down. At first it seems like it's just like a classical computer; however, it's possible to "entangle" multiple qbits together. The orientation becomes fuzzy. In this fuzzy state is possible to create quantum gates that again do math, but these results become "fuzzy." So I create the function: "Give x such that x has the reminder of 1 after dividing by some huge number?" In classical physics there is no one answer to modulus. There could be an infinite number of possible answers. Solving it with a classical computer does not mean much more than trying values and seeing what works. But, asking that question with a quantum computer does something different. Being "fuzzy," it exists across all possible states at the same time until you measure it. When you measure it, you will get a random answer but that answer will work for the equation. Say you ask the question, "Give me x where x divided by 5 has a remainder of 1." The quantum computer might first spit out 56 then next time 91 then next time 6.

Symmetry 7y ago
Your answer is a good one for helping to understand the problem and is true of almost all computers in active use today. But I want to point out that you can have a classical computer that isn't binary. Analog computers have a long history and are still used in some places today. Also ternary computers where bits can be 1, 0, or -1 had a role in Soviet computing and let you used Balanced Ternary[1] which is even nicer than Two's Complement. But these are classical in the sense of having one particular value in the way the rest of your answer lays out nicely. [1]https://en.wikipedia.org/wiki/Balanced_ternary https://en.wikipedia.org/wiki/Balanced_ternary

ridewinter 7y ago

As wild as it sounds, parallel universes are IMO the best way to explain a quantum computer. I posted an article about it: https://news.ycombinator.com/item?id=21337739 https://news.ycombinator.com/item?id=21337739

achenatx 7y ago

In the end a classical computer just "processes" data. It takes data in, processes it (math operations), then outputs it. The foundation is boolean math, which is then used to execute regular math. At the very most basic level a computer is made of transistors that can solve the problem of basic boolean math. Gates are AND (or NAND), OR, XOR etc. An XOR gate produces the following output from the following inputs: A B Output 0 0 0 0 1 1 1 0 1 1 1 0 an AND gate produces the following outputs: A B Output 0 0 0 0 1 0 1 0 0 1 1 1 with those gates you can make an adder. A single bit adder has the following inputs and outputs A B Output 1 Output 2 0 0 0 0 (decimal 0) 1 0 0 1 (decimal 1) 0 1 0 1 (decimal 1) 1 1 1 0 (decimal 2) Notice that Output 2 is an XOR of inputs A and B Output one is an AND of inputs A and B. So a single bit adder is simply the inputs feeding an XOR gate and an AND gate. This can be scaled up to add any number of bits. Other math operations can also be done. Transistors can be used to make memory bits that hold its state and can be set and unset. They are essentially binary gate loops that feed outputs back to inputs and rely on the delay of electron flows to reach stable states. A clock is used to sequentially count (and pull instructions from memory) and set and unset memory bits. Each cycle of the clock allows the computer to execute the next instruction (by incrementing a number that determines which instruction to fetch). From there you are right, the bits are used to encode meaning. Which could be alphabets, images, audio etc.

IIAOPSW 7y ago

A quantum computer is "a sensor" in the same sense that every laptop is "an electronics experiment". A quantum computer still fits your model because you arbitrarily represent anything with an alphabet of quantum states, automatically manipulate it with a quantum algorithm, and map back the result at the end so we learn something about whatever. I do not see the contradiction.

glandium 7y ago

Minute Physics has a really good explanation of Shor's algorithm, and by proxy, Quantum computing. https://www.youtube.com/watch?v=lvTqbM5Dq4Q https://www.youtube.com/watch?v=lvTqbM5Dq4Q And the followup: https://www.youtube.com/watch?v=FRZQ-efABeQ https://www.youtube.com/watch?v=FRZQ-efABeQ