Wi-Fi for a quantum computer

The basic picture of an optomechanical system, that even many scientists keep in mind, is that of a cavity with one movable mirror. But that is not the only way to achieve coupling between light and mechanical vibrations. Every time light is strong enough (and the mechanical oscillator light enough), the light can be used to control the vibrational state of the mechanical system.

Optomechanical systems can take on various forms, such as a vibrating mirror inside a cavity or a vibrating microdisk.

People have studied all sorts of different systems this way. One option is to use a cavity (with both mirrors fixed) and put a vibrating membrane inside. Other scientists work with microdisks where light travels around thanks to total internal reflection; if the disk can vibrate, strong light will excite mechanical vibrations of the disk. And there are optomechanical platforms that are more exotic than these examples.

The beauty of the theoretical description of such systems lies in the fact that they are all described by the same mathematics. This stays true even if we do not use visible light but a microwave field which cannot be trapped in a cavity using two simple mirrors. Instead, microwave cavities have the form of LC circuits — basic electrical circuits with an inductor (basically a coil) and a capacitor (two conducting plates separated by a thin layer of a dielectric material) that have been used in electronics for decades.

Optomechanics can be studied even in microwave systems, where the role of the optical cavity is taken by an LC circuit and vibrating mirror is replaced by an oscillating capacitor plate.

If such a circuit is to be used in the quantum regime, though, it is not that simple. The circuit has to be built from a superconducting material (and cooled down for the experiments) so that the electrical signals can travel through the circuit many times without being absorbed. If we now make one of the capacitor plates vibrating, usually by making it from a membrane, the following happens:

The microwave field acts as a varying electric field across the capacitor. Since the membrane can freely vibrate, it will move in accordance with the electric field. But that results in varying distance between the capacitor plates which affects the resonance of the LC circuit in a way similar to a moving mirror in an optical cavity. The whole system is then described in the same way as other optomechanical systems — even though we now use a microwave field, instead of visible light!

Imagine that we now take such an LC circuit with a vibrating membrane and put the membrane in an optical cavity (either by making it an end mirror or putting it inside a closed cavity). The microwaves as well as the visible light can now swap state with the vibrating membrane. Using such a system, we can, for example, swap the state of the microwave field and the membrane and then swap the state of the membrane and the visible light. Any signal that was initially encoded in the microwave field has now been converted to light.

Combining microwave and optical cavity with a vibrating membrane, we get a system that is capable of converting microwaves to visible light and vice versa.

Such a conversion is commonly done in the classical world — Wi-Fi uses microwaves to send signals between your computer and router and light is used in optical fibres to transmit these signals over long distances to a server. This is done by the router measuring the microwave signal, transmitting it to a modem via a cable where it is measured again, sent in the form of light to the other end where the process is repeated in reverse. That is something you cannot do in the quantum world where every measurement destroys the quantum nature of the signal. This is why more sophisticated methods — such as swapping the state with a mechanical oscillator — have to be used.

There is one immediate application for these opto-electromechanical systems (i.e., systems comprising an optical cavity, an LC circuit, and a mechanical oscillator). The conversion of microwave signals to visible light can be used to improve detection efficiency of weak microwave fields. That is a task that is very difficult to do. But if you could efficiently convert these signals to light, you would need to measure weak light pulses instead, which is easier. Radio astronomers, for instance, can then use these systems to detect weaker sources of radio waves in the universe. Magnetic resonance imaging can profit by reaching better accuracy than with current detection strategies, which could lead to earlier diagnoses of serious illnesses. But we still have to wait for these applications — there is a long way between a successful experimental demonstration and a practical use of an effect.