Amplitude Spectroscopy Contributes to Advances in Quantum Computing

  • Published
  • By Maria Callier
  • Air Force Office of Scientific Research
ARLINGTON, Va. --  Air Force-sponsored research at the Massachusetts Institute of Technology is accelerating the development of quantum or high-speed computers, which helps the Air Force with cryptoanalysis or 'code-breaking,' microwave electronics and materials science.

Chief researcher, Dr. William Oliver of MIT's Lincoln Laboratory and Research Laboratory for Electronics and his team, in collaboration with Professors Terry Orlando, Leonid Levitov and Karl Berggren at the MIT campus, have developed what they call amplitude spectroscopy. This technology analyzes how an atom responds to different amplitudes of electromagnetic radiation at a fixed frequency in order to extract its energy-level structure (spectroscopy) over broad bandwidth.

"We have demonstrated spectroscopy using superconducting artificial atom structures that use two superconductors linked by a non-conducting barrier. When the atoms are cooled to ultra low temperatures using dilution refrigeration followed by microwave-cooling (similar to laser-cooling with atoms), they exhibit energy levels akin to a natural atom or molecule," Oliver said.

The fabrication of such structures is unique and requires special tools. Most researchers use ultra thin aluminum film to create artificial atoms.

"At Lincoln, we use niobium in a semiconductor-based, multi-layer fabrication process that uses optical lithography and chemical-mechanical planarization. One of the key accomplishments of our work is that we have been able to fabricate the deep-submicron Josephson junctions necessary to realize artificial atoms," noted Oliver.

As the researchers learn more about these superconducting structures, they will advance the technology needed to develop quantum computers and technology for present day and the future.

"In the long-term, artificial atoms can be used as the "quantum bits"-- qubits-- of quantum computing, a challenging technology, which drives near-term innovations. For example, the deep-submicron Josephson junctions can also be used with a classical computing technology, which can reach 100 GHz clock speeds or more.

Oliver's team, whose work runs the gamut from experimental to theoretical work, has had several recent successes that were all funded by the Air Force Office of Scientific Research.
· Mach-Zehnder interferometry in a strongly driven superconducting qubit
· Microwave-induced cooling of a superconducting qubit.
· Amplitude spectroscopy of a solid-state artificial atom

"The key challenge to our research is maintaining funding stability and continuity. We have been fortunate to have been funded by the Air Force for several years, but we now find ourselves looking for new programs to continue this work. Once we lose the momentum, it is very challenging to get it back," Oliver said.

The scientists have been able to apply concepts from atomic physics to their solid-state artificial atoms, in many cases exceeding what could previously be done with natural atoms alone. In their future work, they will continue find connections to atomic, optical, and nuclear physics.

"We are looking to the NMR (nuclear magnetic resonance) community to develop optimal pulse sequences with which to control our artificial atom, and we hope to use these methods to probe and extend its quantum coherence," he said.

The work of the MIT researchers, important technology that will be used to safeguard our national security, is an impressive continuation of the work of Nobel prize-winning physicist Richard Feynman who first advanced the theory of quantum computing over twenty years ago.