Superconducting Metamaterial Traps Quantum Light
Light is a wave whose electric and magnetic field undulates up and down, with a spatial separation between neighboring peaks (or troughs) corresponding to the wavelength. The way in which light propagates within materials can be strongly influenced not only by bulk material properties, but also by the arrangement and geometric structuring of materials at the sub-wavelength scale. This fact has been known and exploited for quite some time. In the natural world, these effects can be seen in the beautiful reflected color patterns of a peacock’s feathers. In our technology driven information age, this concept shows up in the design of lasers that power the backbone of the fiber optic network of the internet. We call these sub-wavelength structured materials by various names, but perhaps the most descriptive and general is the term “metamaterial”. The metamaterial concept applies not just to optical phenomena, but rather to electromagnetic waves at all frequencies, and is most commonly associated with microwaves like the ones used by your cell phone or wireless network router.
Metamaterials can be used to synthesize an almost magical set of electromagnetic responses, including extreme slowing of the velocity of light, waves which bend (refract) in the `wrong’ direction when passing through an interface of two materials, or beams of light which focus rather than spread (diffract) as they propagate forward. One of the more interesting metamaterial examples is one that creates frequency bands for which light is entirely forbidden from propagation, a so-called photonic bandgap. Photonic bandgap metamaterials give rise to curious behavior – first considered by Sajeev John some 30 years ago – for atoms embedded inside of them which naturally emit light at frequencies within the bandgap. What happens, for instance, to an excited sodium atom which emits yellow light when it is placed inside a metamaterial with a photonic bandgap covering the visible spectrum? John showed that in this case since the atom cannot radiate light, a type of atom-photon bound state forms in which the atom attaches to a surrounding localized “cloud” (wavepacket) corresponding to a single yellow-wavelength photon.
In recent work from Professor Oskar Painter’s lab in Caltech’s Applied Physics department, Mohammad Mirhosseini, along with colleagues Eun Jong Kim, Vinicius Ferreira, Mahmoud Kalaee, Andrew Keller, and Alp Sipahigil, was able to realize a similar atom-photon bound state in a superconducting quantum circuit. Superconducting quantum circuits represent one of the most exciting prospective technologies for building quantum computers. As their name indicates, they are fashioned from superconducting thin films that are patterned into traces on a microchip substrate like those on a circuit board. These superconducting trace patterns can be used to form resonant elements which oscillate at well-defined frequencies, or to create planar waveguides for transporting electrical signals from one part of the microchip to another. What makes them truly quantum, however, is the introduction of a highly non-linear inductive element called a Josephson junction, which consists of top and bottom superconducting electrodes that sandwich together an atomically thin insulating layer. Shunting a Josephson junction with a capacitor yields a device with two isolated states, like the ground and excited electronic states of an atom that are involved in the emission of light. In the language of quantum computing we call this device a ‘qubit’, since it can be used to store a quantum bit of information.