Keeping the Noise Level Down

“The signals we measure are extremely weak: it’s like trying to detect on Earth the signal from a cell phone on the Moon.” Frederick William Beinecke Professor of Applied Physics & Physics Michel Devoret, armed with an extensive knowledge of quantum mechanics and a strong French accent, proceeds to explain why an amplifier is necessary to magnify the amplitude of a signal despite the fact that it necessarily adds to the signal an unavoidable amount of noise.

In collaboration with nine other Yale researchers, Devoret recently published a paper in Nature that outlines the design and operation of a Josephson Parametric Amplifier, an element necessary for the development of the futuristic quantum computer. This new amplifier can read the basic unit of a quantum computer—the quantum bit—and can preserve the phase of the signal, an essential property when the signal’s arrival time contains important information.

In fact, Devoret estimates that his amplifier will be twenty times less noisy than its commercial counterpart, which largely ignores the issue of signal phase in favor of signal amplitude magnification. Instead of conducting an experiment one thousand times and averaging the results for statistical significance, this drastic decrease in noise from amplification will allow Devoret and his colleagues to reach the same results by conducting an experiment a single time. This will not only cut down on the time and needless repetition but also make more types of experiments possible given that certain experiments were previously off limits due to changing physical conditions over the course of hundreds of trials.

Created using superconducting circuits cooled to frigid temperatures near absolute zero on the Kelvin scale, the Josephson Parametric Amplifier utilizes a rhombus-shaped network of four Josephson Junctions, each composed of two thin aluminum plates separated by a self-assembled layer of oxide. Josephson Junctions are powered using alternating current (AC) and are analogous to transistors in traditional amplifiers. Devoret adds, “Even though our circuits are cooled to temperatures as low as 10 milliKelvin (-273.14oC), the fact that they operate in the microwave range of 5-10 GHz allows us to use the relevant technology of cell phone electronics to simplify experiments.

The Josephson Parametric Amplifier will hopefully be used as a feedback controller for quantum bits; it will provide rapid response to the quantum computer in order to reduce the accumulation of physical errors by a process called quantum error correction—a process Devoret describes as “extremely beautiful.”

Devoret plans to continue his work on quantum circuits and conduct extensive testing of the Josephson Parametric Amplifier in terms of its impact and application to a quantum computer. He currently has a joint appointment at the Collège de France in Paris and is a distinguished member of the both the American Academy of Arts and Sciences and the French Academy of Science. He has most notably received the Ampere Prize in 1991 for the invention of the electron pump, and the Europhysics-Agilent Prize in 2004 for his work on superconducting quantum bits.