This fall, the Nobel Prize in Physics was awarded to John Clarke, Michel Devoret and John Martinis "for the discovery of macroscopic quantum mechanical tunnelling and energy quantization in an electric circuit."
At Lawrence Livermore National Laboratory (LLNL), these award-winning discoveries underpin two fronts of ongoing innovation: fundamental research in quantum computing hardware and designing ultrasensitive devices and methods to hunt for dark matter.
A prize with LLNL ties
For LLNL scientist Sean O'Kelley, this year's Nobel Prize is personal. Before joining the Laboratory, he earned his Ph.D. under the tutelage of John Clarke in his Berkeley lab.
"When I started with the Clarke lab, I didn't know too much about his previous work - I just knew he was working on some problems at the time that were particularly interesting to me. Only later did I come to appreciate the great body of foundational work already done in that lab," said O'Kelley. "The methods, technologies and conceptual frameworks that came out of the Clarke labs have become a kind of 'ABCs' for anyone in his and even related fields, and I'm very glad to see that recognized. The prize was richly deserved."
Because quantum effects were first observed in atoms, it's a common misconception that quantum phenomena only happen at the tiniest of scales. Experiments by the laureates in the 1980s proved that quantum has a bigger role to play.
"The key words in the prize are 'macroscopic' and 'quantum,'" said O'Kelley. "The idea of quantum is inexorably linked to the idea of small in the imagination of most people and maybe even most scientists, but that is a misunderstanding. If small things are quantum and big things are not, where is the boundary? There is none. Everything is quantum, all the time, at all sizes.
"This work demonstrated in a real, visceral way that quantum mechanics really is the way the world works, even for things 'big enough to get one's grubby fingers on,' to borrow a phrase from one of their 1988 papers."
Superconductivity reveals big quantum
The key to the Nobel work lay in superconductivity, a phenomenon in which materials at extremely cold temperatures will conduct electricity without any energy loss. An electrical current flowing around a ring of superconducting metal will persist indefinitely because it won't lose energy in the form of heat.
"No one knew how or why superconductivity worked when it was first discovered, but we now know that zero-resistance is just a nice side-effect of the really cool part: the conduction electrons in a superconductor are all doing the same thing right down to the quantum level," said O'Kelley.
The laureates proved the existence of this collective quantum state with measurements of palm-sized superconducting circuits.
"An important part of electrons all moving in the same quantum state is the formation of stable pairs of electrons called 'Cooper pairs,'" said O'Kelley.
While electrons normally repel each other, superconducting electrons pair up instead. When one electron travels through the superconductor's solid lattice structure at a cold enough temperature, it pulls positively charged atoms toward it, creating a sort of wake. The next electron in line sees the positive charge that has been pulled out of place and zips toward it (and the electron that created it). These electrons are called "Cooper pairs."
According to the laws of physics, electrons cannot all exist in the same exact state - but Cooper pairs can. Throughout the superconductor, every Cooper pair is in the exact same quantum state. Because of this, the entire circuit is in the same state and acts as a single, macroscopic quantum object.
This macroscopic quantum state causes big, observable quantum effects: the magnetic field becomes quantized, and the vibrational states of a circuit the size of your hand (or bigger) become discrete, just like the energy states of a single atom.
The experiments demonstrated quantum tunneling at palm-sized scales by including Josephson junctions in the circuit. These junctions create a barrier, akin to a brick wall, in the superconducting wire. Conventional electrical currents cannot cross the barrier, but this quantum supercurrent can tunnel through.
Below a certain current "speed limit", the electrons pass through the wall without any resistance. Their preferred state of finite current and no resistance or voltage is at the lowest possible energy. The electrons naturally want to stay in that state.
But when the laureates carefully increased the current, the electrons tunneled out of this low energy state, jumping to a higher energy state and generating a characteristic voltage pulse. That process is analogous to how an atom emits a photon when a single electron jumps between shells. Careful and innovative experimental design left no room to interpret this effect as anything other than quantum jumps in a macroscopic system.
Nobel foundations to LLNL creations
These findings are the building blocks of superconducting quantum computing at LLNL. Quantum bits, or qubits - the basic unit of information in quantum computers - can be built from superconducting circuits with Josephson junctions.
"With a superconducting platform, you aren't limited to just the quantum systems nature gives you, like single atoms - you can now basically make the metal any shape you want," said O'Kelley. "You can make your junctions any size you want. You can make your loops any size you want. You can design the exact quantum states you need."
With the Quantum Design and Integration Testbed (QuDIT), LLNL researchers are exploiting that flexibility to determine the optimal materials, fabrication methods and infrastructure for superconducting qubits that may power the next generation of computing.
Nobel research connects to ADMX
The Nobel-winning findings also led to advancements in the Axion Dark Matter eXperiment (ADMX), which began and operated at Livermore from 1996-2010. Now housed at the University of Washington, the experiment and LLNL continue to hunt for its target: the axion.
A hypothetical particle that could account for dark matter, the axion interacts primarily with gravity and only very weakly with anything else. That makes it extremely difficult to detect. By using an extremely strong magnetic field, ADMX aims to convert axions into measurable photons. But even as photons, their signature will be very, very small. For scientists to spot it, they'll need to amplify it, but every stage of amplification can add noise.
The experiment's original amplification technology was based on transistors, which have an added noise level equivalent to a 2 Kelvin blackbody. But the higher that temperature, the longer the detector must scan a certain frequency range for the axion.
"John Clarke came up with a clever new design that was able to basically make that noise near quantum limited, which is closer to 50 milli-Kelvin in the frequencies we operate," said LLNL scientist and ADMX co-spokesperson Gianpaolo Carosi. "It really relied on that seminal paper, and the work that he did with Michel Devoret and John Martinis, to be able to make these devices that were instrumental for ADMX. It would have taken 100 years to do the experiment if we kept using the transistor technology."
Clarke's design, based on a superconducting quantum interference device (SQUID) coupled to a microstrip resonator, allowed for amplifiers to be built in the frequency range ADMX was operating (gigahertz). SQUIDs themselves are built from superconducting rings with Josephson junctions. These devices can detect and amplify tiny magnetic changes that arise after an axion converts into a photon, making them indispensable to ADMX.
"The SQUID, because it's using the superconducting Josephson junction, provides this quantum state that is extraordinarily sensitive to changes. It doesn't dissipate almost any energy, and that allows it to amplify while adding a minimal amount of noise," said Carosi.
ADMX has since moved on to scan higher frequencies for the axion, which requires a different form of SQUID-like technology. The key features, though, remain the same.
Different on their face, same scientific base
O'Kelley emphasized that while these challenges - quantum computing and dark matter searches - may seem different, they share the same critical, newly lauded basis.
"I think this has been worthy of a Nobel Prize for a long time," he said. "It affirms the importance of the physics that underlies so much of what we're doing at LLNL today."
Carosi agreed and added that the application space extends even further - to fields like brain imaging.
"The work that they've done has built the underpinnings for all the quantum computing efforts based on superconductors," he said. "And the opportunities stemming from this are enormous."
