For years, niobium was considered an underperformer when it got here to superconducting qubits. Now scientists supported by Q-NEXT have found a approach to engineer a high-performing niobium-based qubit and so benefit from niobium’s superior qualities.
In terms of quantum technology, niobium is making a comeback.
For the past 15 years, niobium has been sitting on the bench after experiencing a number of mediocre at-bats as a core qubit material.
Qubits are the elemental components of quantum devices. One qubit type relies on superconductivity to process information.
Touted for its superior qualities as a superconductor, niobium was all the time a promising candidate for quantum technologies. But scientists found niobium difficult to engineer as a core qubit component, and so it was relegated to the second string on Team Superconducting Qubit.
Now, a bunch led by Stanford University’s David Schuster has demonstrated a approach to create niobium-based qubits that rival the state-of-the-art for his or her class.
“This was a promising first foray, having resurrected niobium junctions. … With niobium-based qubits’ broad operational reach, we open up an entire recent set of capabilities for future quantum technologies.” — David Schuster, Stanford University
“We have shown that niobium is relevant again, expanding the chances of what we are able to do with qubits,” said Alexander Anferov of the University of Chicago’s Physical Science division, one in all the lead scientists of the result.
The team’s work is published in Physical Review Applied and was supported partly by Q-NEXT, a U.S. Department of Energy (DOE) National Quantum Information Science Research Center led by DOE’s Argonne National Laboratory.
By harnessing niobium’s standout features, scientists will have the opportunity to expand the capabilities of quantum computers, networks and sensors. These quantum technologies draw on quantum physics to process information in ways in which outclass their traditional counterparts and are expected to enhance areas as varied as medicine, finance and communication.
The niobium advantage
In terms of superconducting qubits, aluminum has ruled the roost. Aluminum-based superconducting qubits can store information for a comparatively very long time before the information inevitably disintegrates. These longer coherence times mean more time for processing information.
The longest coherence times for an aluminum-based superconducting qubit are a number of hundred millionths of a second. In contrast, lately, the most effective niobium-based qubits yielded coherence times which can be 100 times shorter — a number of hundred billionths of a second.
Despite that short qubit lifetime, niobium held attractions. A niobium-based qubit can operate at higher temperatures than its aluminum counterpart and so would require less cooling. It might also operate across an eight-times-greater frequency range and a large 18,000-times-wider magnetic field range in comparison with aluminum-based qubits, expanding the menu of uses for the superconducting-qubit family.
In a single respect, there was no contest between the 2 materials: Niobium’s operating range trounced aluminum’s. But for years, the short coherence time made the niobium-based qubit a nonstarter.
“Nobody really made that many qubits out of niobium junctions because they were limited by their coherence,” Anferov said. “But our group desired to make a qubit that would work at higher temperatures and a greater frequncy range — at 1 K and 100 gigahertz. And for each of those properties, aluminum is just not sufficient. We would have liked something else.”
So, the team had one other have a look at niobium.
Losing the lossiness
Specifically, that they had a have a look at the niobium Josephson junction. The Josephson junction is the information-processing heart of the superconducting qubit.
In classical information processing, data is available in bits which can be either 0s or 1s. In quantum information processing, a qubit is a combination of 0 and 1. The superconducting qubit’s information “lives” as a combination of 0 and 1 contained in the junction. The longer the junction can sustain the data in that mixed state, the higher the junction and the higher the qubit.
The Josephson junction is structured like a sandwich, consisting of a layer of nonconducting material squeezed between two layers of superconducting metal. A conductor is a fabric that gives easy passage for electrical current. A superconductor kicks it up a notch: It carries electrical current with zero resistance. Electromagnetic energy flows between the junction’s outer layers within the mixed quantum state.
The standard, trusty aluminum Josephson junction is product of two layers of aluminum and a middle layer of aluminum oxide. A typical niobium junction is product of two layers of niobium and a middle layer of niobium oxide.
Schuster’s group found that the junction’s niobium oxide layer sapped the energy required to sustain quantum states. Additionally they identified the niobium junctions’ supporting architecture as an enormous source of energy loss, causing the qubit’s quantum state to fizzle out.
The team’s breakthrough involved each a brand new junction arrangement and a brand new fabrication technique.
The brand new arrangement called on a well-recognized friend: aluminum. The design did away with the energy-sucking niobium oxide. And as an alternative of two distinct materials, it used three. The result was a low-loss, trilayer junction — niobium, aluminum, aluminum oxide, aluminum, niobium.
“We did this best-of-both-worlds approach,” Anferov said. “The skinny layer of aluminum can inherit the superconducting properties of the niobium nearby. This fashion, we are able to use the proven chemical properties of aluminum and still have the superconducting properties of niobium.”
The group’s fabrication technique involved removing scaffolding that supported the niobium junction in previous schemes. They found a approach to maintain the junction’s structure while eliminating the loss-inducing, extraneous material that hampered coherence in previous designs.
“It seems just eliminating the rubbish helped,” Anferov said.
A brand new qubit is born
After incorporating their recent junction into superconducting qubits, the Schuster group achieved a coherence time of 62 millionths of a second, 150 times longer than its best-performing niobium predecessors. The qubits also exhibited a high quality factor — an index of how well a qubit stores energy — of two.57 x 105, a 100-fold improvement over previous niobium-based qubits and competitive with aluminum-based qubit quality aspects.
“We have made this junction that also has the great properties of niobium, and we have improved the loss properties of the junction,” Anferov said. “We will directly outperform any aluminum qubit because aluminum is an inferior material in some ways. I now have a qubit that does not die at higher temperatures, which is the large kicker.”
The outcomes will likely elevate niobium’s place within the lineup of superconducting qubit materials.
