Photograph of Sven Albrecht (right) and Charles Marcus in the lab
Protected states: Sven Albrecht and Charles Marcus

An important property of Majorana quasiparticles has been measured for the first time by physicists at the Niels Bohr Institute in Denmark. They found evidence that electrons in tiny nanowires form entangled states that are highly isolated from noise and other external stimuli. Because they are protected from outside influences, these Majorana "zero modes" could be used as quantum bits (qubits) in quantum computers.

First predicted by the Italian physicist Ettore Majorana in 1937, the Majorana particle obeys "non-Abelian" statistics, which means that quantum information encoded in the particles would be highly resistant to decoherence. Decoherence is the bane of physicists who are trying to develop practical quantum computers, and so devices based on Majorana particles could be used in future quantum-information systems.

While physicists have yet to see isolated Majorana particles, some collective excitations of electrons in solids have the same properties as Majorana particles. These "Majorana quasiparticles" have already been glimpsed in several systems, including semiconductor nanowires coated in a superconducting layer. When these nanowires are cooled to near absolute zero, superconducting electrons can exist within the semiconductor. An electron in the wire becomes entangled with electrons on either side of it, creating an uninterrupted chain of entangled electrons along the entire length of the wire.

Electron halves

At either end of this chain are electrons that are entangled only with one electron, which can each be thought of as "half" an electron and are called Majorana modes. Together they form a Majorana quasiparticle. Quantum information stored in such a quasiparticle would be distributed between both ends of the nanowire, meaning it should be protected from being destroyed by external noise.

"The protection is related to the exotic property of the Majorana mode that it simultaneously exists on both ends of the nanowire, but not in the middle," explains Sven Albrecht, who was part of the Danish team. "To destroy its quantum state, you have to act on both ends at the same time, which is unlikely," he adds.

An important feature of the Majorana modes is that the energy required to add another electron to the nanowire decreases exponentially with the length of the nanowire. This exponential decay is a signature of the protected nature of the Majorana modes and is something that previous studies have not measured.

Aluminium coating

Now, Albrecht, Charles Marcus and colleagues in Denmark are the first to measure how much energy is required to add just one electron to such nanowires. They began by creating nanowires of the compound semiconductor indium arsenide that were around 1 μm long and 0.1 μm in diameter. These were then coated with aluminium, which is a superconductor at low temperatures. The wires were then deposited onto a silicon substrate, where each wire was surrounded by a set of gold electrodes used to apply voltages to the nanowires and measure the resulting currents.

Crucial to the success of the experiment, according to Marcus, is the fact that the interface between the superconductor and the semiconductor is perfectly crystalline – rather than having randomly positioned atoms. This allows superconducting electrons from the aluminium to flow into the semiconductor to create a state of matter called a "topological superconductor".

The team studied several different nanowires ranging in length from 330 nm to 1.5 μm. The researchers used a technique called Coulomb blockade spectroscopy to measure the energy needed to add an extra electron to the nanowires. As expected, they found that this energy decreased exponentially as the length of the nanowires increased.

Fast electronics

Marcus told physicsworld.com that the next step for the team is to use its nanowires to create a qubit and demonstrate that it is indeed protected from decoherence. This will require the development of fast electrical connections to the nanowires to read, write and manipulate quantum information in the Majorana modes.

Sankar Das Sarma of the University of Maryland in the US is one of the theoretical physicists who predicted the behaviour of Majorana modes in superconductor-coated nanowires. He describes this latest measurement as a "significant advance" that offers additional evidence that Majorana particles exist in nanowires. "These experiments provide further support for the semiconductor nanowires to be the best-available topological qubits among the many proposed such candidates," he adds.

The measurements are described in Nature.

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