Exploring It

In this last lecture, I want to talk about quantum hybrid devices.

They mean coupling different quantum systems to each other and to some extent, we have already learned about this.

For example, when we discussed microwave resonators, I told you that you could also in principle

couple them to red-black atoms or to polar molecules and so on.

And also we talked briefly about having microwave resonators coupled to nano resonators.

And so I want to follow up on this with a few examples that have been realized in recent

experiments that are being proposed and are quite realistic.

One of the questions is of course why would you want to realize quantum hybrid devices?

And the answer is that the different setups, the different devices may be more suitable

for one or the other task.

For example, one may have a very long dephasing time so it's good as a storage.

But maybe it's hard to induce interactions so it's better to carry over the information

to another quantum system where you have strong interactions.

And yet another quantum system may be good, for example, as a flying qubit such as a photon

that can transport information from here to there.

I want to start with the first example that again deals with nanomechanical systems.

But with a nanomechanical system that instead of being coupled to a light field as an optomechanics

is coupled to a single superconducting qubit.

And the benefit of coupling to a qubit or a two-level system is that the two-level system

automatically is very unharmonic.

And so we will see that this can be used to read out or to produce box states in the nanomechanical

resonator.

So, the experiment I'm referring to here was published just in the beginning of this year

and was done in the groups of Kledant and Martinez at Santa Barbara.

So, with respect to the nanoresonator, instead of having one of these large continevers,

for example, that have relatively low frequencies, they chose to look at the bulk vibration of

a small piece of, in this case, aluminum nitride.

So, this really looks like a drum head.

And for reasons that will become clear immediately, this was placed between two electrodes, so

to speak, made of aluminum.

And these then can be connected to an electric circuit.

So the mode of vibration we're talking about is a compression and dilation mode of the

whole structure of the aluminum nitride piece of material.

And it turns out that for the dimension state shows, it can have a frequency of, say, five

or six gigahertz.

So, why is this good?

Well, if the frequency is that large, it's very easy to achieve temperatures, or relatively

easy to achieve temperatures, that are below h by omega, and so you are automatically in

the ground state.

And that's of course good because you don't then need to introduce any of these extra

cooling schemes that we discussed in the context of optomechanics.

So simply by placing this in a dilution refrigerator, you can have it cold.

That is one of the advantages, but then you still need to come up with a way of coupling

this to an interesting other quantum system, such as in this case a superconducting qubit.

Of course, the electrodes are already part of the solution, but the point is, how would

you then convert the mechanical vibration into an electric field?

There are straightforward solutions to this.

For example, if you have a cantilever that vibrates and it is charged, then automatically

you will have an oscillating electric field, but that typically gives rise to rather weak