In a step toward building better quantum machines, researchers at Oak Ridge National Laboratory recently measured the electrical current between an atomically sharp metallic tip and a superconductor. This new method can find linked electrons with extreme precision in a move that could help detect new kinds of superconductors, which have no electrical resistance. “Superconducting circuits are the current front-runner for building quantum bits (qubits) and quantum gates in hardware,” Toby Cubitt, the director of Phasecraft, a company that builds algorithms for quantum applications, told Lifewire in an email interview. “Superconducting qubits are solid-state electrical circuits, which can be designed with high accuracy and flexibility.”
Spooky Action
Quantum computers take advantage of the fact that electrons can jump from one system to another through space using the mysterious properties of quantum physics. If an electron pairs with another electron right at the point where metal and superconductor meet, it could form what’s called a Cooper pair. The superconductor also releases another kind of particle into the metal, known as Andreev reflection. The researchers looked for these Andreev reflections to detect Cooper pairs. The Oak Ridge scientists measured the electrical current between an atomically sharp metallic tip and a superconductor. This approach lets them detect the amount of Andreev reflection returning to the superconductor. “This technique establishes a critical new methodology for understanding the internal quantum structure of exotic types of superconductors known as unconventional superconductors, potentially allowing us to tackle a variety of open problems in quantum materials, Jose Lado, an assistant professor at Aalto University, which provided theoretical support to the research, said in a news release. Igor Zacharov, a senior research scientist at the Quantum Information Processing Laboratory, Skoltech in Moscow, told Lifewire via email that a superconductor is a state of matter in which electrons do not lose energy by scattering on the nuclei when conducting the electric current and the electric current can flow unabated. “While electrons or nuclei have quantum states that can be exploited for computation, superconducting current behaves as a macro quantum unit with quantum properties,” he added. “Therefore, we recover the situation in which a macro state of matter may be used to organize information processing while it has manifestly quantum effects that may give it a computational advantage.”
The Superconducting Future
Superconducting quantum computers currently beat rival technologies in terms of processor size, Cubitt said. Google demonstrated so-called “quantum supremacy” on a 53-qubit superconducting device in 2019. IBM recently launched a quantum computer with 127 superconducting qubits, and Rigetti has announced an 80-qubit superconducting chip. “All quantum hardware companies have ambitious roadmaps to scale their computers in the near future,” Cubitt added. “This has been driven by a range of advances in engineering, which have enabled the development of more sophisticated qubit designs and optimization. The biggest challenge for this particular technology is improving the quality of the gates, i.e., improving the accuracy with which the processor can manipulate the information and run a computation.” Better superconductors may be key to making practical quantum computers. Michael Biercuk, the CEO of quantum computing company Q-CTRL, said in an email interview that most current quantum computing systems use niobium alloys and aluminum, in which superconductivity was discovered in the 1950s and 1960s. “One of the biggest challenges in quantum computing today relates to how we can make superconductors perform even better,” Biercuk added. “For instance, impurities in the chemical composition or the structure of the deposited metals can cause sources of noise and performance degradation in quantum computers—these lead to processes known as decoherence in which the ‘quantumness’ of the system is lost.” Quantum computing requires a delicate balance between the quality of a qubit and the number of qubits, Zacharov explained. Every time a qubit interacts with the environment, such as receiving signals for ‘programming,’ it could lose its entangled state. “While we see small advances in each of the indicated technological directions, combining them into a good working device is still elusive,” he added. The ‘Holy Grail’ of quantum computing is a device with hundreds of qubits and low error rates. Scientists can’t agree on how they will achieve this goal, but one possible answer is using superconductors. “The increasing number of qubits in a silicon superconducting device stresses a need for giant cooling machines that can drive large operational volumes close to absolute zero temperature,” Zacharov said.
