**"And the stars are projectors, yeah, projecting our lives down to this planet Earth."** - Modest Mouse

### 4.13.2004

## Decoherence

The Economist.com has an interesting article about how advances in quantum computing - and the roadblocks encountered along the way - are pushing research into the deep depths of quantum physics. At question is the gray zone between the quantum world of the miscroscopic and the classical world of the macroscopic. In this zone lies the phenomenon of observer-mediated decoherence of quantum probability into absolute measurement - the collapse of the waveform and the formality of occurence.

Until a qubit interacts with the macroscopic world, which follows the classical laws of physics, it behaves according to the laws of quantum mechanics, which are well understood, at least by physicists. However, the interaction with the classical worldâ€”decoherenceâ€”and hence exactly where the divide between the quantum and classical worlds lies, are not well understood. When decoherence is deliberately provoked, the process is known as measurement. Before a qubit is measured, for example, it could have a 90% chance of being 1, and a 10% chance of being 0. After the measurement, it takes on one of these two values. But the details of how it chooses between the two are something that, until the advent of quantum computing, most physicists chose to remain agnostic about. Some even quipped that the answer to that question was beyond the realm of physics.

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