Research Bits: Aug. 16

Protein-based circuits; compact qubit controller; organic transistor for high-density circuits.


Protein-based circuits

Researchers from North Carolina State University and University of Cambridge created self-assembled, protein-based circuits that can perform simple logic functions and take advantage of an electron’s properties at quantum scales.

A challenge in creating molecular circuits is the unreliability as circuit size decreases. At the quantum scale, electrons behave like waves rather than particles, leading to a phenomenon called tunneling where an electron can be in multiple closely-spaced wires at once, making it difficult to control the direction of the current.

“Our goal was to try and create a molecular circuit that uses tunneling to our advantage, rather than fighting against it,” said Ryan Chiechi, associate professor of chemistry at North Carolina State University.

The researchers built the circuits by first placing two different types of fullerene cages on patterned gold substrates. They then submerged the structure into a solution of photosystem one (PSI), a commonly used chlorophyll protein complex.

The different fullerenes induced PSI proteins to self-assemble on the surface in specific orientations, creating diodes and resistors once top-contacts of the gallium-indium liquid metal eutectic, EGaIn, are printed on top. This process both addresses the drawbacks of single-molecule junctions and preserves molecular-electronic function.

“Where we wanted resistors we patterned one type of fullerene on the electrodes upon which PSI self-assembles, and where we wanted diodes we patterned another type,” Chiechi said. “Oriented PSI rectifies current – meaning it only allows electrons to flow in one direction. By controlling the net orientation in ensembles of PSI, we can dictate how charge flows through them.”

They then coupled the self-assembled protein ensembles with human-made electrodes and made simple logic circuits that used electron tunneling behavior to modulate the current.

“These proteins scatter the electron wave function, mediating tunneling in ways that are still not completely understood,” Chiechi said. “The result is that despite being 10 nanometers thick, this circuit functions at the quantum level, operating in a tunneling regime. And because we are using a group of molecules, rather than single molecules, the structure is stable. We can actually print electrodes on top of these circuits and build devices.”

The researchers created simple diode-based AND/OR logic gates from these circuits and incorporated them into pulse modulators, which can encode information by switching one input signal on or off depending on the voltage of another input. The PSI-based logic circuits were able to switch a 3.3 kHz input signal. The team noted that while this is not comparable in speed to modern logic circuits, is still one of the fastest molecular logic circuits yet reported.

“This is a proof-of-concept rudimentary logic circuit that relies on both diodes and resistors,” Chiechi said. “We’ve shown here that you can build robust, integrated circuits that work at high frequencies with proteins. In terms of immediate utility, these protein-based circuits could lead to the development of electronic devices that enhance, supplant and/or extend the functionality of classical semiconductors.”

Compact qubit controller

Engineers at the U.S. Department of Energy’s Fermi National Accelerator Laboratory (Fermilab) and University of Chicago developed a compact FPGA-based control and readout system for quantum computers, called the Quantum Instrumentation Control Kit, or QICK.

Control electronics use signals from the classical world as instructions for the qubits, while readout electronics measure the states of the qubits and convey that information back to the classical world.

“Currently, most control and readout systems for superconducting quantum computers use off-the-shelf commercial equipment not specialized to the task. As a result, researchers often must string together a dozen or more expensive components. The cost can quickly add up to tens of thousands of dollars per qubit, and the large size of these systems creates more problems,” the team noted.

“When you work with qubits, time is critical. Classical electronics take time to respond to the qubits, limiting the performance of the computer,” said Gustavo Cancelo, a senior principal engineer at Fermilab.

The new system is more compact, combining the capabilities of an entire rack of equipment in two boards about the size of a laptop. “We are designing a general instrument for a large variety of qubits, hoping to cover those that will be designed six months or a year from now,” Cancelo said. “With our control and readout electronics, you can achieve functionality and performance that is hard or impossible to do with commercial equipment.”

The team’s radio frequency (RF) board contains more than 200 elements: mixers to tweak the frequencies; filters to remove undesired frequencies; amplifiers and attenuators to adjust the amplitude of the signals; and switches to turn signals on and off. The board also contains a low-frequency control to tune certain qubit parameters. It is combined with a new commercial FPGA chip that integrates integrate digital-to-analog and analog-to-digital converters directly into the board, which the team said sped up the process of creating the interface between the FPGA and RF boards.

The researchers said that the two compact boards cost about 10 times less to produce than conventional systems. In their simplest configuration, they can control eight qubits. Integrating all the RF components into one board allows for faster, more precise operation as well as real-time feedback and error correction.

“You need to inject signals that are very, very fast and very, very short,” said Leandro Stefanazzi, a Fermilab engineer. “If you don’t control both the frequency and duration of these signals very precisely, then your qubit won’t behave the way you want.”

The team noted that the system is scalable. Frequency multiplexing qubit controls would allow a single RF board to control up to 80 qubits. Additionally, several dozen boards could be linked together and synchronized to the same clock as part of larger quantum computers. To improve future versions of its control and readout system, the team has started designing its own FPGA hardware.

Organic transistor for high-density circuits

Researchers from the National Institute for Materials Science (NIMS) and Tokyo University of Science developed an organic anti-ambipolar transistor capable of performing any one of the five logic gate operations (AND, OR, NAND, NOR, or XOR) by adjusting the input voltages to its dual gates.

By designing it to reduce its drain current when the gate voltage exceeds a certain threshold, the anti-ambipolar transistor can perform two-input logic gate operations.

When input voltages are applied to the top and bottom gates of the transistor, it produces an output signal (i.e., a drain current). This transistor demonstrated the ability to act as five different types of two-input logic gates at room temperature when the input voltages were adjusted; the team compared this to existing integrated circuit technology that requires four transistors to form a NAND circuit and 12 transistors to form a XOR circuit. By contrast, only one of these newly developed transistors is needed to form these circuits.

The team said that the transistor could substantially increase the integration density of organic circuits, which has been a major challenge in organic electronics, potentially making them suitable for use in mobile and IoT devices. In future research, the group plans to develop electrically reconfigurable integrated circuits using the new transistor.

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