U.S.A. How does one identify and catalogue atoms? To accomplish this scientists must be able to “finger print” atoms. Nature is kind in that we are given a visually pleasing way to identify atoms; all atoms emit and absorb light. These colors uniquely identify what kind of atom is under observation. By cataloguing these colors–spectra–scientists are able to know what atoms exist in distant stars without having to go to the star directly.
As unique as atoms, superconducting circuits are the only man made device that can absorb or emit different colors of light in their spectra– all other man made devices absorb or emit the same frequency of light. The ability to treat a superconducting circuit as an artificial atom is relatively new and to gain an appreciation for this advance, it is necessary to have a solid understanding of why atoms are special.
Atoms are truly beautiful from a physics standpoint. By studying them we have made great advances for humanity—one such advance being lasers. From a knowledge standpoint atoms offer scientists the ability to perform amazingly controlled experiments probing the depths of our knowledge on the interface of quantum mechanics and our classical world. The only downside of atoms is they are uniquely shy. Atoms do not want to easily exchange light with their environment. In some applications this can be an amazing quality. For instance, the definition of a second is based on the color of light—frequency—that a cesium atom emits or absorbs. How reluctant an atom is to interact with its environment determines how purely one color, or one frequency, the atom emits or absorbs.
In the late 1970s, Nobel laureate Anthony Leggett sought to answer the question of whether or not one could apply quantum mechanics to electrical circuits. He developed a formalism to treat a many atom system, mesoscopic system, as though it were a single atom–quantum in nature. In the mid 1980s Michel Devoret, John Martinis, and John Clark at the University of California at Berkley, experimentally demonstrated that a superconducting circuit consisting of trillions of atoms could behave as though it were truly just one atom. In a sense, a superconducting circuit could be thought of as an artificial atom. This is the equivalent of having a thousand times the earth’s population singing in harmony.
Although intellectually interesting, this innovation predated a practical application for it. It was not until the explosion of interest in the field of quantum computing in the mid 1990s that such a device had a niche to fill. Quantum computing promises to exponentially increase the ability for computers to solve complex problems: the traveling salesman problem for example. The computation speed up of a quantum computer comes from the smallest part a quantum bit, qubit, being in a superposition of excited and not excited all at once. In a quantum context superposition is an object being simultaneously in two dichotic states. The famous example of superposition is Schrödinger’s cat that is both dead and alive simultaneously. For a computer to be aptly named a quantum computer it will have demonstrably quantum signatures such as superposition.
Alternatively, the increase in computing power one gains in a classical computer by Moore’s law comes from being able to shrink the size of the fundamental unit of a classical computer, transistor, down further, but there is a limit to how small a transistor can be made. It is likely that within our lifetime we will hit a limit for computing power increases simply by reducing the size of the basic building block of a classical computer. As computers are made more powerful uses for them have been applied to virtually every field of science. We as a society are addicted to the success and progress increased computational power offers.
The most promising experimental attempts towards a quantum computer are trapped ions, quantum dots, impurities in diamond, and–the focus of this article–superconducting circuits. An advantage superconducting circuits have is that they are produced in the same manner as current computing manufacturing and utilize microwave techniques, which have existed since world war two. With artificial atoms, scientists can a priori design the observed spectra of their artificial atom. In the past fifteen years superconducting circuits have gone from proof of principle to approaching threshold tolerances required for a quantum computer. Now we stand on the threshold of observing glimpses of the computing power promised by the advent of a quantum computer.