On-chip molecules made of matter and light
July 16, 2008 - ETH researchers report the creation of quantum states formed by two light particles and a single artificial atom on a microchip. The implementation has potential applications as an interface between matter and light in a future quantum information processor.
While the formation of molecules made of particles of matter constitutes the basis for all of chemistry and therefore for our own existence, the formation of molecules made of photons seems not to occur in nature since light quanta hardly ever interact with each other. In a clever experimental setup however the quantum mechanical laws allow strong interactions between light and matter which in turn can be used to mediate interactions between two otherwise unaffected light particles.
The rather simple configuration of just one photon bound to one atom has been observed in numerous experiments both in traditional optical setups with real atoms and more recently also in solid state implementations. While some of these results could, at least in principle, still be explained without quantum physics, the new results with more than one photon can only be explained with the laws of quantum mechanics; a basic research experiment which has been called for by the community by nearly 20 years as a clear proof of field quantization in cavity quantum electrodynamics in general, and an unambiguous proof for the quantum mechanical nature of the employed electrical circuits containing discrete packages of energy in particular.
Figure 1 (high res version): Part of the team in the Quantum Device Lab at ETH Zurich. The experiments were carried out in a dilution refrigerator below 0.02 Kelvin, seen in the background in blue and involved a considerable amount of high frequency electronics equipment (foreground right). From left: Andreas Wallraff, Martin Goeppl, Johannes Fink and Mathias Baur.
In their experiment the team at the Quantum Device Lab cooled down a small, about 300 by 30 micrometer sized, electronic circuit which consists of two Aluminum strips connected only by two weakly conducting tiny contacts (100 by 150 by 1 nanometer sized Aluminum oxide tunnel barriers). This circuit has an energy spectrum comparable to an ordinary atom in real nature, where the different quantum states are represented by different locations of the electrons inside the two metal strips. At temperatures of just some tens of thousandths of degrees above absolute zero the circuit comes to rest in its quantum mechanical ground state. This state can then be excited with single photons at gigahertz frequencies and spontaneously decay back to the ground state while sending out a photon with the same frequency. This effect is much like in an ordinary atom which can be excited with laser light and shows resonance fluorescence.
In order to establish a strong interaction strength to just one or two such photons the circuit is embedded inside a high quality microwave frequency resonator. By applying an extremely weak microwave tone, the resonator gets populated with a defined average number of light particles, say just one. Due to the relatively large size of the artificial atom and the quasi one dimensional layout of the resonator, the dipole coupling between the two can be achieved to be extremely strong. Under these conditions an injected photon is immediately absorbed by the atom and reemitted into the resonator over 300 000 000 times per second. When the photon enters the resonator the circuit and the resonator share this energy quantum and exchange it over 30 times before it leaves via the resonator again.
Figure 2 (high res version): The image shows microchips fabricated in the FIRST clean-room facilities at ETH Zurich. The background photograph depicts a set of microwave resonators that are used to store and manipulate single or multiple microwave photons. In the foreground a false-color microscope image of a superconducting artificial atom (zick-zack structure) which is integrated into such a resonator is shown together with an artist's impression of two photons traveling along the resonator and approaching the artificial atom.
In such an experiment both the atom and the photons lose their individual properties entirely and form a new entangled system - half matter half light. Quantum theory predicts the interaction rate to be enhanced by a factor of the square root of 2 in the presence of a second photon in the resonator, which is the key observation of the reported work. The main ingredient for the experiment to work cleanly was to inject two photons of different colors where the first one pumps the joint system and the second one probes the pumped state to energetically even higher matter-light superposition states. In order to obtain the full transmission spectrum these probe photons were measured which required the ability to detect signal powers down to 10 to the -17 Watts.
Cavity quantum electrodynamics (QED) represents a research area which has gained tremendous momentum by the recent advances with novel semiconducting and superconducting solid-state implementations. This is the case in particular due to its close relation to the fast evolving field of solid state quantum information processing, where it provides a natural interface between photons - ideal to transmit quantum information - and solid state quantum systems which are more suitable to process quantum information. Moreover the new results could potentially be used for the realization of a single photon transistor, parametric down-conversion or the generation and detection of individual microwave photons.
Johannes M. Fink