# Quantum Computing

1. Aug 1, 2006

### alias25

QC requires atoms to be isolated? because the slightest disturbance can cause its state to be changed and data loss. Are the spin/ angular momentum etc. of a particle affected by em waves?
if theres quantum fluctuations in space, so u have virtual particles/energy swapping, and its impossible to reduce this to 0, doesn't that mean you can never get an isolated atom?

2. Aug 1, 2006

### Gokul43201

Staff Emeritus
No, it does not.

3. Aug 1, 2006

### alias25

I thought that any environmental interaction can cause the particles to decohere so QC doesn't work unless theyre isolated.

4. Aug 1, 2006

### Gokul43201

Staff Emeritus
Did you mean that you need a system that's fairly isolated from surroundings or that you needed individual atoms? I thought you meant the latter.

The question of importance is "how long can we make the decoherence times?" I think typical values for some of the prospective systems are in the nanosecond range (or smaller, I'm not sure).

Last edited: Aug 1, 2006
5. Aug 1, 2006

### Rach3

There's actually a huge range of deoherence times, >15 orders of magnitude. I googled up this table:
http://beige.ucs.indiana.edu/B679/node117.html

Ions in electromagnetic traps have macroscopic decoherence times, they're very well isolated. Nuclear dipoles interact so weakly that NMR qubits are coherent for on the order of ~10^4 seconds (essentially forever).

Last edited by a moderator: Apr 22, 2017
6. Aug 1, 2006

### beautiful1

Just to add my two cents to this point..it is more about how many gate operations can be performed before the systems irrevocably decoheres.

The distinction being that (relatively) long decoherence times do not help if it also takes a (relatively) long time to perform an operation.

7. Aug 1, 2006

### steve_o

QC requires a well-isolated system, i.e. little coupling with the environment. But experimentally, that isolation is not perfect of course. However, decoherence can be "overcome" by error correction at the tradeoff of more qubits.

In NMR, the spin is controlled via emf tuned at the proper resonant frequencies, etc.. You should look up the Bloch sphere for information on this.

In QC processing, one has to control the qubit (ions, atoms, molecule ensemble, superconducting circuits, etc) and take measurements to get an answer. This inherently rules out a perfectly isolated qubit.

8. Aug 3, 2006

### Schrodinger's Dog

Are their any means of performing checks to assure that decoherence is accounted for: an analogy would be, parity checks in a simple computer system, that allow for manipulation of erata in data sets.

You'll have to excuse my ignorance about Qcomputing on this one; in a broader sense I'm asking if we have yet found a way to make quantum computing a good prospect, or is it still in the realms of the hypothetical?

9. Aug 4, 2006

### setAI

there is the repeat-until-success methodology of cluster states:

http://xxx.lanl.gov/abs/quant-ph/0508218

10. Aug 4, 2006

### Pythagorean

wait, don't atoms spontaneously decay? Wouldn't that be unproductive to data keeping? Or is there a method/technique? Or am I misunderstanding atoma?

11. Aug 4, 2006