# Why are crystals so useful for quantum isolation?

Some implementations of quantum gates (in the hopes of building a quantum computer one day) use crystals to isolate the qubits (to prevent decoherence). Why is a crystal so much better than an amorphous solid? My reasoning is that crystals tend to pack atoms more tightly than other substances, which means that lattice vibrations are less of an issue (vibrational excitations get "frozen out"). On the other hand, atoms near defects or atoms in a non-crystal are more loosely bound. Is this correct?

Crystals also contain an ordered arrangement of atoms. Does this perfect ordering help with isolation as well?

• If you believe this article hal.inria.fr/file/index/docid/222521/filename/…, the specific heat of glass has a $\sim T$ dependence, while crystalline silica shows the expected $\sim T^3$ dependence. If phonon coupling is the major problem in qubit isolation, then glass would be the worse choice. Having said that, I don't think that your reasoning about the density of crystals is correct, but since I have not looked at the phonon theory of amorphous solids, I couldn't tell you what the correct explanation for the differences is. Dec 28 '14 at 7:18

A crystal is one of the macroscopic manifestations of quantum mechanics, like super conductivity and super fluidity. It is a coherent whole.

In principle it could be described by a single wave function for the whole crystal. There are wavefunctions modeled for the electrons in an ordered lattice.

As quantum computing is based on the coherence of the wave functions it seems a good idea to start with a coherent environment before introducing changes to be used in computing.

In contrast, amorphous material can only be described statistically, because the phases are lost even though the basic units might be small crystals.

• So crystal periodicity is conducive wave function coherence, it seems. Dec 29 '14 at 1:53