NIST toggle switch can help quantum computers eliminate noise

A blue-tinted drawing shows a schematic of the two qubits and resonator above a white rectangle, representing the SQUID device that controls the connections and relationships between qubits and resonator elements.

This photo shows the central working area of ​​the device. In the lower section, the three large (blue) rectangles represent the two quantum bits, or qubits, to the left and right, and the resonator in the center. At the top, magnified, driving microwaves through the antenna (large dark blue rectangle at bottom) induces a magnetic field in the SQUID loop (smaller white square in center, sides about 20 micrometers long). The magnetic field activates the toggle switch. The frequency and amplitude of the microwaves determine the position of the switches and the strength of the connection between the qubits and the resonator.


R. Simmonds/NIST

What’s the use of a powerful computer if you can’t read its output? Or readily reprogram it to do different jobs? People designing quantum computers face these challenges, and a new device could make them easier to solve.

The device, pioneered by a team of scientists at the National Institute of Standards and Technology (NIST), includes two superconducting quantum bits, or qubits, which are quantum computers analogous to the logic bits in a classic computer processing chip. The heart of this new strategy is based on a toggle switch device that connects the qubits to a circuit called a read resonator that can read the output of the qubit calculations.

This toggle switch can be moved to different states to adjust the strength of the connections between the qubits and the readout resonator. When deactivated, all three elements are isolated from each other. When the switch is flipped to connect the two qubits, they can interact and perform calculations. Once the calculations are complete, the toggle switch can connect one of the qubits and the readout resonator to retrieve the results.

Having a programmable toggle switch goes a long way in reducing noise, a common problem in quantum computer circuits that makes it difficult for qubits to make calculations and display their results clearly.

The goal is to keep the qubits happy so they can compute without distractions, while still being able to read them when we want, said Ray Simmonds, a physicist at NIST and one of the paper’s authors. This device architecture helps protect qubits and promises to improve our ability to make the high-fidelity measurements needed to build quantum information processors from qubits.

The team, which also includes scientists from the University of Massachusetts Lowell, the University of Colorado Boulder and Raytheon BBN Technologies, describes its findings in a paper published today in Physics of nature.

Quantum computers, which are still in a nascent stage of development, would harness the bizarre properties of quantum mechanics to perform tasks that even our most powerful classical computers find intractable, such as aiding in the development of new drugs by running sophisticated simulations of chemical interactions.

However, designers of quantum computers still face many problems. One of them is that quantum circuits are disturbed by external or even internal noise, which results from defects in the materials used to make computers. This noise is essentially random behavior that can create errors in qubit calculations.

Today’s qubits are inherently noisy in their own right, but that’s not the only problem. Many quantum computer designs have what’s called a static architecture, where each qubit in the processor is physically connected to its neighbors and its readout resonator. Fabricated wiring that connects qubits together and reading them can expose them to even more noise.

Such static architectures have another disadvantage: they cannot be easily reprogrammed. A static architectures qubit might do some related jobs, but for the computer to perform a wider range of tasks, it would have to replace a different processor design with a different qubit organization or layout. (Imagine switching the chip in your laptop every time you need to use different software, then consider that the chip needs to be kept a pinch above absolute zero, and you understand why this could prove inconvenient.)

The team’s programmable toggle switch sidesteps both of these problems. First, it prevents circuit noise from creeping into the system through the readout resonator and prevents qubits from talking to each other when they should be quiet.

This reduces a key source of noise in a quantum computer, Simmonds said.

Second, the opening and closing of switches between elements are controlled with a train of microwave pulses sent remotely, rather than through physical connections of static architectures. Integrating more of these toggle switches could be the basis of a more easily programmable quantum computer. The microwave pulses can also set the order and sequence of logic operations, meaning a chip built with several of the team’s toggle switches could be instructed to perform any number of tasks.

This makes the chip programmable, Simmonds said. Rather than having a completely fixed architecture on the chip, changes can be made via software.

A final benefit is that the toggle switch can also trigger the measurement of both qubits simultaneously. This ability to ask both qubits to reveal themselves as a pair is important for tracking down quantum computational errors.

The qubits in this demonstration, as well as the toggle switch and readout circuit, were all made from superconducting components that conduct electricity without resistance and must operate at very low temperatures. The toggle switch itself is made of a superconducting quantum interference device, or SQUID, which is very sensitive to magnetic fields passing through its ring. Driving a microwave current through a nearby antenna loop can induce interactions between the qubits and the readout resonator when needed.

At this point, the team has only worked with two qubits and a single readout resonator, but Simmonds said they are preparing a design with three qubits and a readout resonator, and plan to add more qubits and resonators as well. Further research could offer insights into how many of these devices could be brought together, potentially offering a way to build a powerful quantum computer with enough qubits to solve the kinds of problems that, for now, are insurmountable.

Paper: T. Noh, Z. Xiao, XY Jin, K. Cicak, E. Doucet, J. Aumentado, LCG Govia, L. Ranzani, A. Kamal and RW Simmonds. Strong parametric dispersive shifts in a statically decoupled two-qubit cavity QED system. Physics of nature. Published online June 26, 2023. DOI: 10.1038/s41567-023-02107-2

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