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If you added one word of qualification – practical quantum computer or quantum processor – this would actually be a reasonable position to argue.

gigel82 5y ago
No, I mean in the real sense, there is no actual quantum processor anywhere in the world (unless some secret organization actually built something amazing and is hiding it). Not a single quantum logical gate exists that actually behaves as described in the theory, let alone circuits of quantum gates that do even the most basic of computation. Actually, I'll take that one step further and say that no one has produced a single qubit (actual logical qubit as described in the theoretical literature), these 127 (if that) are what they call "physical qubits" or what you and me would call chaotic sources of uncontrolled entropy (i.e. random number generators). I'm not saying quantum computing in the physical world is impossible, I'm just saying no one has accomplished it (yet).
- krastanov 5y ago
  Like the parent comment, your absolutism seems unreasonable. A wave-plate is a darn-near-perfect single qubit gate for a dual-rail encoded photonic qubit. It is just that we can not really generate photons on demand in a scalable way. Hence agreeing both with "there are some small noisy unreliable quantum computers" and with "there are no scalable useful practical quantum computers".
- bnjemian 5y ago
  Uncontrolled entropy? If you can repeatedly throw a ball and hit a target but not the bullseye, that doesn't mean you have no control, it just means you need to practice more. In the spirit of that, there are recent experimental demonstrations of logical qubits: - NV in diamond qubits (https://arxiv.org/abs/2108.01646 https://arxiv.org/abs/2108.01646) - Superconducting qubits (https://journals.aps.org/prxquantum/pdf/10.1103/PRXQuantum.2.010339 https://journals.aps.org/prxquantum/pdf/10.1103/PRXQuantum.2...) True, a universal logical gate set has yet to be practically realized (at least to my knowledge). But, it seems you're looking for perfection and QC will never be that – even with error correction. Fundamentally, QCs are analog and probabilistic; all discrete states we define theoretically will be approximations in practice if and when they can be demonstrated. But if the approximation is exponentially close to the target state and that is sufficient for the practical purpose at hand...does it matter?

gigel82 5y ago

No, I mean in the real sense, there is no actual quantum processor anywhere in the world (unless some secret organization actually built something amazing and is hiding it). Not a single quantum logical gate exists that actually behaves as described in the theory, let alone circuits of quantum gates that do even the most basic of computation. Actually, I'll take that one step further and say that no one has produced a single qubit (actual logical qubit as described in the theoretical literature), these 127 (if that) are what they call "physical qubits" or what you and me would call chaotic sources of uncontrolled entropy (i.e. random number generators). I'm not saying quantum computing in the physical world is impossible, I'm just saying no one has accomplished it (yet).

krastanov 5y ago
Like the parent comment, your absolutism seems unreasonable. A wave-plate is a darn-near-perfect single qubit gate for a dual-rail encoded photonic qubit. It is just that we can not really generate photons on demand in a scalable way. Hence agreeing both with "there are some small noisy unreliable quantum computers" and with "there are no scalable useful practical quantum computers".
bnjemian 5y ago
Uncontrolled entropy? If you can repeatedly throw a ball and hit a target but not the bullseye, that doesn't mean you have no control, it just means you need to practice more. In the spirit of that, there are recent experimental demonstrations of logical qubits: - NV in diamond qubits (https://arxiv.org/abs/2108.01646 https://arxiv.org/abs/2108.01646) - Superconducting qubits (https://journals.aps.org/prxquantum/pdf/10.1103/PRXQuantum.2.010339 https://journals.aps.org/prxquantum/pdf/10.1103/PRXQuantum.2...) True, a universal logical gate set has yet to be practically realized (at least to my knowledge). But, it seems you're looking for perfection and QC will never be that – even with error correction. Fundamentally, QCs are analog and probabilistic; all discrete states we define theoretically will be approximations in practice if and when they can be demonstrated. But if the approximation is exponentially close to the target state and that is sufficient for the practical purpose at hand...does it matter?