System Bits: April 29

Beyond graphene
Researchers at The University of Manchester have shown how they can control the properties of stacks of 2D materials, opening up the potential for new, previously-unimagined electronic devices.

The isolation of graphene at the University in 2004 led to the discovery of many other 2D crystals and while graphene has an unrivaled set of superlatives, these crystals cover a large range of properties: from the most conductive to isolating, from transparent to optically active.

The next step is to combine several of these crystals in a 3D stack in order to create ‘heterostructures’ with novel functionalities – capable of delivering applications as yet beyond the imagination of scientists and commercial partners.

The first examples of such heterostructures already exist: tunneling transistors, resonant tunneling diodes, and solar cells. The scientists have demonstrated that layers in such stacks can interact strongly, which helps them learn how to control the properties of such heterostructures.

By controlling the relative orientation between graphene and underlying boron nitride – one of the 2D materials and an excellent insulator – the researchers can reconstruct the crystal structure of graphene. This leads to creation of local strains in graphene and even opening of a band-gap, which might be useful for the functionality of many electronic devices.

Superconducting qubit array points way to quantum computers
A fully functional quantum computer is one of the holy grails of physics. Unlike conventional computers, the quantum version uses qubits (quantum bits), which make direct use of the multiple states of quantum phenomena. When realized, a quantum computer will be millions of times more powerful at certain computations than today’s supercomputers. To this end, a group of UC Santa Barbara physicists has moved one step closer to making a quantum computer a reality by demonstrating a new level of reliability in a five-qubit array.

Quantum computing is anything but simple. It relies on aspects of quantum mechanics such as superposition. This notion holds that any physical object, such as an atom or electron — what quantum computers use to store information — can exist in all of its theoretical states simultaneously. It is expected that this could take parallel computing to new heights.

Quantum hardware is very, very unreliable compared to classical hardware, one of the researchers pointed out. Even the best state-of-the-art hardware is unreliable. This new work shows that for the first time reliability has been reached.

While the UCSB researchers have shown logic operations at the threshold, the array must operate below the threshold to provide an acceptable margin of error. Qubits are faulty, so error correction is necessary. They recognize they need to improve and would like to scale up to larger systems. The intrinsic physics of control and coupling won’t have to change but the engineering around it is going to be a big challenge.

The unique configuration of the group’s array results from the flexibility of geometry at the superconductive level, which allowed the scientists to create cross-shaped qubits they named ‘Xmons.’

Superconductivity results when certain materials are cooled to a critical level that removes electrical resistance and eliminates magnetic fields. The team chose to place five Xmons in a single row, with each qubit talking to its nearest neighbor, a simple but effective arrangement.

The five cross-shaped devices are the Xmon variant of the transmon qubit placed in a linear array. (Source: UCSB)

The control signals for all five qubits. (Source: UCSB)

To build a quantum computer, a 2D array of qubits is needed, and the error rate should be below 1 percent. If the team can get one order of magnitude lower — in the area of 10-3 or 1 in 1,000 for all gates — the qubits could become commercially viable but there are more issues that need to be solved. There are more frequencies to worry about, and it’s more complex but the physics is no different.