Please don't answer: a QC already exist.
Some circuits are existing - using quantum particles, but these are no QC (Quantum Computers) which can be used to "compute" a greater amount of data in shorter time than classical computer can do it.
To answer your question, first let me answer another: What makes a quantum computer special?
Why, it operates on continuous quantities (e.g., the phase of the wavefunction) as opposed to the discrete representation found in a digital computer.
But... so does any good old 1930s style analog computer, used extensively on ships, airplanes and by artillery crews in WW2. Not exactly 21st century tech.
However, analog computers are notoriously unreliable. Only the best of the best of them have an accuracy of 4 significant digits or (a little) more.
The secret of quantum computers lies in the threshold theorem. The theorem basically says that if you can implement a quantum computer with a noise level that is below a certain threshold, then there exists error correction that allows this quantum computer to simulate an ideal (error-free) quantum computer: i.e., you have fault-tolerant quantum computing, which can be scaled to any number of qubits.
So this is what's missing: qubits with a sufficiently low error rate and no correlation of errors between qubits.
Some see the threshold theorem as a source of encouragement and believe it's just a matter of improving technology. Others are more skeptical: they believe that the threshold theorem represents something fundamentally unachievable, like the perpetual motion machine.
Dear Viktor,
thanks for your comment. I am surprised to hear of analog computers in the 30th used in WW2. My teacher has made the first full transistored computer in Europe. His name was Prof ZEMANEK. He called it "Mailüfterl" in contrast to these big machines. They where called "Thunderstorm".
Konrad ZUSE is said to having made the first programmable computer in about 1942-1945.
In Quantum Computing we are in state of analog quantum computers. If we don't find such elements like relays or bi-stable semiconductors for classical computers we never will find a functioning QC (Quantum Computer).
I think I know what's missing but I don't know how to find it.....
Dear Franz: The analog computers in question weren't necessarily electronic. From Wikipedia: "The first modern analog computer was a tide-predicting machine, invented by Sir William Thomson, later Lord Kelvin, in 1872. It used a system of pulleys and wires [...] The differential analyser, a mechanical analog computer designed to solve differential equations by integration using wheel-and-disc mechanisms, was conceptualized in 1876 by James Thomson [...] An important advance in analog computing was the development of the first fire-control systems for long range ship gunlaying. When gunnery ranges increased dramatically in the late 19th century [...] In 1912, British engineer Arthur Pollen developed the first electrically powered mechanical analogue computer [...] "
You might also enjoy this amusing vintage US Navy training film on the use of fire control computers: https://www.youtube.com/watch?v=_8aH-M3PzM0
Had it been built, the first programmable computer would have been Babbage's Analytical Engine. But it wasn't built so the honor indeed goes to Zuse and his electromechanical Z3.
Actually, the parts (qubits) in quantum computers are not the problem. The problem is reducing noise and eliminating noise correlation such that the threshold theorem could be exploited. My own feeling is that this is going to be theoretically impossible, but of course I could very well be wrong.
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