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> No practical quantum computer exists, and nobody has a realistic concept for building one. Can you expand on that? I'm not involved in the field, but there c

by ascar 3y ago

> No practical quantum computer exists, and nobody has a realistic concept for building one. Can you expand on that? I'm not involved in the field, but there certainly exist many low qbit machines with varying technologies around the world that can already be used for calculations. Yes, they're nowhere near big enough to be useful yet, but qbit counts in the biggest machines are multiplying every year similar to Moore's law, so that seems to be just a matter of time.

Simon_O_Rourke 3y ago
> qbit counts in the biggest machines are multiplying every year similar to Moore's law, so that seems to be just a matter of time. That's pretty much my take on it too, just because it's pretty limited right now there appears to be healthy growth in qbit counts which would suggest a more usable quantum computer is mostly a matter of time and further research.
- upofadown 3y ago
  It doesn't matter how many qubits you have if those qubits are too noisy to do anything with. Noise is the ultimate limit when trying to get a useful signal out of something. Noise is how the universe stops us from knowing everything about everything.
fastneutron 3y ago
The big "if" in quantum computing is in our ability to engineer error correction and fault tolerance. Implementing fault tolerance requires a physical qubit overhead of 10-10,000 qubits per logical qubit, depending on the technology, base error rate and whose analysis you look at. It's not entirely accurate to say nobody knows how to do this. The theory of quantum error-correction has been rigorously developed since the 1990s, and experimental implementations have shown that improved fidelity is possible with known error-correction schemes. I think we'll know definitively within the next 3-5 years how hard it will be to engineer fault tolerance, and workable solutions will emerge from those findings.
fsh 3y ago
Increasing the number of qubits doesn't help unless the gate fidelities and coherence times also improve. Otherwise, the system quickly becomes a very expensive random number generator. In transmon qubit systems, I haven't seen much progress in these metrics over the last couple of years. IBM keep adding more qubits to their chips, but they don't seem to be able to actually use them. After almost thirty years of development, ion traps have now reached a few ten qubits with reasonable fidelities. However, this requires shuttling the ions between trapping zones which is enormously slow (many milliseconds per gate). And scaling this to hundreds of thousands of qubits is a completely open question (none of the current techniques will work).
vouaobrasil 3y ago
> Can you expand on that? I'm not involved in the field, but there certainly exist many low qbit machines with varying technologies around the world that can already be used for calculations. Yes, they're nowhere near big enough to be useful yet, but qbit counts in the biggest machines are multiplying every year similar to Moore's law, so that seems to be just a matter of time. I'll believe it when I see it. Yes, machines exist, but so far I have not seen one single convincing application that makes it better than a classical computer. Not saying it can't be done, but for all the hype around quantum computers, so far the results are dismal. (And I used to work in a relevant field.)
sampo 3y ago
> qbit counts in the biggest machines are multiplying every year similar to Moore's law 2001: Shor's algorithm was demonstrated by a group at IBM, who factored 15 into 3 × 5 2012: factorization of 21 was achieved 2019: an attempt was made to factor the number 35, but the algorithm failed because of accumulating errors https://en.wikipedia.org/wiki/Shor%27s_algorithm https://en.wikipedia.org/wiki/Shor%27s_algorithm
- andromeduck 3y ago
  You might fint twinkle/twirl interesting - it's prime factorization via optoelectronics https://en.m.wikipedia.org/wiki/TWIRL https://en.m.wikipedia.org/wiki/TWIRL

by ascar 3y ago

Simon_O_Rourke 3y ago

> qbit counts in the biggest machines are multiplying every year similar to Moore's law, so that seems to be just a matter of time. That's pretty much my take on it too, just because it's pretty limited right now there appears to be healthy growth in qbit counts which would suggest a more usable quantum computer is mostly a matter of time and further research.

upofadown 3y ago
It doesn't matter how many qubits you have if those qubits are too noisy to do anything with. Noise is the ultimate limit when trying to get a useful signal out of something. Noise is how the universe stops us from knowing everything about everything.

fastneutron 3y ago

The big "if" in quantum computing is in our ability to engineer error correction and fault tolerance. Implementing fault tolerance requires a physical qubit overhead of 10-10,000 qubits per logical qubit, depending on the technology, base error rate and whose analysis you look at. It's not entirely accurate to say nobody knows how to do this. The theory of quantum error-correction has been rigorously developed since the 1990s, and experimental implementations have shown that improved fidelity is possible with known error-correction schemes. I think we'll know definitively within the next 3-5 years how hard it will be to engineer fault tolerance, and workable solutions will emerge from those findings.

fsh 3y ago

Increasing the number of qubits doesn't help unless the gate fidelities and coherence times also improve. Otherwise, the system quickly becomes a very expensive random number generator. In transmon qubit systems, I haven't seen much progress in these metrics over the last couple of years. IBM keep adding more qubits to their chips, but they don't seem to be able to actually use them. After almost thirty years of development, ion traps have now reached a few ten qubits with reasonable fidelities. However, this requires shuttling the ions between trapping zones which is enormously slow (many milliseconds per gate). And scaling this to hundreds of thousands of qubits is a completely open question (none of the current techniques will work).

vouaobrasil 3y ago

> Can you expand on that? I'm not involved in the field, but there certainly exist many low qbit machines with varying technologies around the world that can already be used for calculations. Yes, they're nowhere near big enough to be useful yet, but qbit counts in the biggest machines are multiplying every year similar to Moore's law, so that seems to be just a matter of time. I'll believe it when I see it. Yes, machines exist, but so far I have not seen one single convincing application that makes it better than a classical computer. Not saying it can't be done, but for all the hype around quantum computers, so far the results are dismal. (And I used to work in a relevant field.)

sampo 3y ago

> qbit counts in the biggest machines are multiplying every year similar to Moore's law 2001: Shor's algorithm was demonstrated by a group at IBM, who factored 15 into 3 × 5 2012: factorization of 21 was achieved 2019: an attempt was made to factor the number 35, but the algorithm failed because of accumulating errors https://en.wikipedia.org/wiki/Shor%27s_algorithm https://en.wikipedia.org/wiki/Shor%27s_algorithm

andromeduck 3y ago
You might fint twinkle/twirl interesting - it's prime factorization via optoelectronics https://en.m.wikipedia.org/wiki/TWIRL https://en.m.wikipedia.org/wiki/TWIRL