Researchers from MIT and elsewhere have noted, for the first time, "temporal coherence" of qubit graphene – meaning how long it can maintain a special state that allows it to represent two logical states simultaneously. The demonstration, which uses a new type of graphene-based qubit, is a critical step forward for practical quantum computing, the researchers said.
Superconductor quantum bits (simple, qubits) are artificial atoms that use various methods to produce quantum information bits, the basic components of quantum computers. Similar to traditional binary circuits on computers, qubits can maintain one of two states that correspond to classic binary bits, 0 or 1. This qubit can also be a superposition of both conditions simultaneously, which can allow quantum computers to solve complex practical problems impossible for traditional computers.
The amount of time these qubits use in the state of superposition is called their "time of coherence." The longer the coherence, the greater the qubit's ability to calculate complex problems.
Recently, researchers have incorporated graphene-based materials into superconducting quantum computing devices, which promise faster, more efficient computing among other facilities. Until now, however, there was no coherence recorded for this sophisticated qubit, so no one knew whether they were feasible for practical quantum computing.
In a paper published today at Natural Nanotechnology, the researchers showed, for the first time, coherent qubits made from graphene and exotic ingredients. These materials allow qubits to change through voltage, like transistors in traditional computer chips today – and unlike most other superconducting qubits. In addition, the researchers placed the number on that coherence, with a clock of 55 nanoseconds, before the qubit returned to its base state.
The work combines the expertise of co-author William D. Oliver, a practice physics professor and Lincoln Laboratory Fellow whose work focuses on quantum computing systems, and Pablo Jarillo-Herrero, Cecil and Ida Green Professor of Physics at MIT who research innovation in graphene.
"Our motivation is to use unique graphene properties to improve the performance of superconducting qubits," said first author Joel I-Jan Wang, a postdoc in Oliver's group at the Research Laboratory of Electronics (RLE) at MIT. "In this work, we show for the first time that superconducting qubits made of graphene while coherent are quantum, the main requirement for building more sophisticated quantum circuits. Our device is the first device to show measurable coherence time – the main metric of a qubit – that's long enough for humans to be controlled. "
There were 14 other co-authors, including Daniel Rodan-Legrain, a graduate student in the Jarillo-Herrero group who contributed equally to work with Wang; MIT researchers from RLE, Department of Physics, Department of Electrical Engineering and Computer Science, and Lincoln Laboratory; and researchers from the Irradiation Solids Laboratory at the École Polytechnique and the Advanced Materials Laboratory from the National Institute for Materials Science.
Original graphene sandwich
Superconducting qubits depend on a structure known as the "Josephson junction," where insulators (usually oxides) are sandwiched between two superconducting materials (usually aluminum). In traditional tunable qubit designs, the current loop creates a small magnetic field that causes electrons to jump between superconducting materials, causing the qubit to switch status.
But this flowing current consumes a lot of energy and causes other problems. Recently, several research groups have replaced insulators with graphene, carbon layers as thick as atoms that are cheap for mass production and have unique properties that enable faster and more efficient computing.
To make their qubits, the researchers turned to a class of materials, called van der Waals materials – thin material atoms that can be stacked like Lego on top of each other, with little or no resistance or damage. These materials can be stacked in a special way to create various electronic systems. Although surface quality is almost perfect, only a few research groups have applied van der Waals materials to quantum circuits, and none of them have previously shown temporal coherence.
For their Josephson junction, the researchers sandwiched a sheet of graphene between two layers of van der Waals isolators called hexagonal boron nitride (hBN). Importantly, graphene takes the superconductivity of the superconducting material it touches. Selected van der Waals materials can be made to drive electrons around using voltage, instead of traditional currents based on magnetic fields. Because of that, graphene – and all the qubits.
When a voltage is applied to the qubit, electrons bounce back and forth between two superconducting cables connected by graphene, converting qubits from the ground (0) to an excited or superposition state (1). The lower hBN layer serves as the substrate for hosting graphene. The top hBN layer encapsulates graphene, protecting it from any contamination. Because the material is very pure, traveling electrons never interact with defects. This represents "ballistic transportation" which is ideal for qubits, where the majority of electrons move from one superconductor to another without scattering with dirt, making rapid and precise changes in state.
How voltage helps
The work could help overcome the "scaling problem" of the qubit, Wang said. At present, only about 1,000 qubits can be accommodated on one chip. Having a qubit controlled by voltage will be very important because millions of qubits start crammed into one chip. "Without voltage control, you also need thousands or millions of current loops too, and that requires a lot of space and leads to energy dissipation," he said.
In addition, voltage control means greater efficiency and targeting of individual qubits on a more local chip, more precisely, without "cross talk." That happens when a little magnetic field created by currents disrupts qubits that are not targeted, causing calculation problems.
For now, the researchers' qubits have a short life span. As a reference, conventional superconducting qubits that promise practical applications have documented coherence time of several tens of microseconds, several hundred times greater than the researchers' qubits.
But researchers have discussed a number of problems that have caused this short life span, most of which require structural modification. They also used a method of investigating new coherence to further investigate how electrons move ballistically around the qubits, with the aim of broadening the qubit coherence in general.
The ballistic junction of Josephson's graphene enters the microwave circuit
Coherent control of hybrid superconducting circuits made with graphene-based van der Waals heterostructures, Natural Nanotechnology (2018). DOI: 10.1038 / s41565-018-0329-2, https://www.nature.com/articles/s41565-018-0329-2