Diamond approach to artificial atoms

A new spectroscopic approach to measuring the energy levels of an atomic system has been developed by US researchers. Amplitude spectroscopy can be used to measure the energies of certain natural and artificial atoms and molecules over extremely broad bandwidth by scanning the amplitude of the applied radiation rather than its frequency. The new technique allows the characterization of multiple energy levels in the system, and so overcomes a key challenge to realization of powerful quantum computers. It is applicable to systems with strong coupling to external fields, including artificial atoms, spin systems, cold atoms and molecules, and molecular magnets.

It was Nobel physics laureate, Los Alamos veteran and conga player, Richard Feynman who suggested that quantum mechanics might be exploited in a novel approach to computing more than two decades ago. Ever since, researchers have strived to reach that goal, which holds the promise of our being able to solve complex problems not suited to conventional computers, such as high-level code breaking and simulations of quantum systems. Because of the uncertain nature of quantum reality, experimental approaches to such solid-state implementations usually have to operate in the chilling grip of temperatures close to absolute zero, where thermal vibrations are reduced considerably and error-inducing jitter is cut to a minimum. At such low temperatures, superconducting devices can be persuaded to behave as artificial atoms: lithographically defined and fabricated micron-scale devices that behave like atoms by exhibiting specific, discrete energy levels. So, each artificial atom could function as a qubit in a quantum computer, occupying multiple energy states simultaneously.

Unfortunately, artificial atoms do not commonly succumb to the probings of conventional spectroscopic techniques. Characterizing the energy levels of quantum-coherent device is obviously fundamental to understanding and engineering such a system. But artificial atoms are broadband entities; their energy levels cover a wide swath of frequencies from tens to hundreds of gigahertz. It is this characteristic that precludes the use of standard spectroscopy without great effort and cost. "The application of frequency spectroscopy over a broad band is not universally straightforward," William Oliver of MIT Lincoln Laboratory's Analog Device Technology Group and MIT's Research Laboratory for Electronics (RLE) explains.

Now, Oliver and co-authors have developed a new spectroscopic technique overturns convention. Oliver and colleagues graduate students David Berns and Mark Rudner together with Sergio Valenzuela, of MIT's Francis Bitter Magnet Laboratory, Karl Berggren and Terry Orlando professors in the Department of Electrical Engineering and Computer Science (EECS), Leonid Levitov in physics, explain the details in a paper published in Nature this month.

The MIT team has developed a complementary approach in amplitude spectroscopy, which the researchers say, offers them a way in which to characterize quantum entities over an unprecedented range of frequencies. This procedure is "particularly relevant for studying the properties of artificial atoms," Oliver adds.

Amplitude spectroscopy gleans information about a superconducting artificial atom by probing its response to the driving-field amplitude at a single, fixed frequency that is strategically chosen to be benign. "Conventionally, spectra are measured using frequency spectroscopy, whereby the frequency of a harmonic electromagnetic driving field is tuned into resonance with a particular separation between energy levels," the researchers explain.

The amplitude, in contrast, pushes the atom throughout its energy levels; the larger the amplitude, the further through the energy levels it goes. At the amplitudes that reach a point where a pair of levels cross, a quantum transition can occur, a so-called "Landau-Zener transition," whereby the artificial atom can hop between energy levels provided it is being driven quickly enough to jump the energy gap. This effectively allows an very broad range of quantum leaps to be made simply by adjusting the amplitude of the fixed-frequency source. Repeated passages through the level crossing causes the artificial atom to self-interfere, leading to quantum interference between different paths through the energy levels.

The resulting radiation emitted by the artificial atom in response to this probing generates a characteristic interference pattern, which Oliver refers to as "spectroscopy diamonds" (see image). The diamonds represent regions in parameter space where transitions between specific pairs of energy levels can occur. The striking geometric regularity of these patterns serves as a fingerprint of the artificial atom's energy spectrum. In an accompanying News & Views item, Frank Wilhelm of the Institute for Quantum Computing, at University of Waterloo, in Ontario, Canada, explains that the central principle of amplitude spectroscopy is that the researchers exploit the metaphorical coupling of two pendulums. They carry out quantum interference at an "avoided" energy-level crossing, in which energy levels brought close together do not actually cross.

"Amplitude spectroscopy provides a means of manipulating and characterizing systems over an extremely broad bandwidth, using only a single driving frequency that may be orders of magnitude smaller than the energy scales being probed," the researchers conclude.