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September 18, 2013

Toward Stable Quantum Computing

Tiffany Trader

What good is computing if it’s not reliable? As with standard computers, reliability is also the key to establishing practical quantum computing. The challenge for fielding such systems is immense, but the potential payoff is even more compelling. Quantum computers would be several magnitudes more powerful than today’s best technology, putting formerly intractable problems, for example strong decryption, within reach.

This schematic of a bismuth selenide/BSCCO cuprate (Bi2212) heterostructure shows a proximity-induced high-temperature superconducting gap on the surface states of the bismuth selenide topological insulator.

For this reality to be achieved, scientists must develop “fault-tolerant” quantum computers. An international team of researchers just got a little closer to this goal. Working at the DOE’s Advanced Light Source (ALS) facility, scientists from China’s Tsinghua University and the Lawrence Berkeley National Laboratory (Berkeley Lab) have reported the first demonstration of high-temperature superconductivity on the surface of a topological insulator, a first step toward stable quantum computing.

The experiment used premier beams of ultraviolet light at the ALS, a DOE facility for synchrotron radiation, to induce high-temperature superconductivity in a topological insulator, a material class that is electrically insulating on the inside but conducting on the surface. This process paves the way for a theoretical quasiparticle to appear. The mysterious particle is known as the “Majorana zero mode” and it’s being pursued for fault-tolerant quantum computing.

“We have shown that by interfacing a topological insulator, bismuth selenide, with a high temperature superconductor, BSCCO (bismuth strontium calcium copper oxide), it is possible to induce superconductivity in the topological surface state,” stated Alexei Fedorov, a staff scientist for ALS beamline 12.0.1, where the event was confirmed.

While quantum computing has enormous potential, the essential computing unit – the quantum bit or “qubit” – is notoriously unstable. According to a Berkeley Lab piece on the subject, “the qubit is easily perturbed by electrons and other elements in its surrounding environment.” These perturbations can cause a quantum particle to decohere, that is to lose information, comprising the accuracy of computations. Scientists are looking to topological insulators to solve this “decoherence” problem. The qubits in a topological quantum computer would be made from Majorana zero modes, which are immune to decoherence. Thus states stored in the form of topologically protected qubits would be preserved.

The experimenters believe they have identified a promising substrate in the form of bismuth selenide/BSCCO heterostructures.

“Our studies reveal a large superconducting pairing gap on the topological surface states of thin films of the bismuth selenide topological insulator when grown on BSCCO,” Fedorov says. “This suggests that Majorana zero modes are likely to exist, bound to magnetic vortices in this material, but we will have to do other types of measurements to find it.”

The research was primarily funded by the National Natural Science Foundation of China. Findings were published in the journal Nature Physics in a paper titled “Fully gapped topological surface states in Bi2Se3 induced by a d-wave high temperature superconductor.”