What good is a powerful computer if you can’t read its output? Or easily reprogram it to do different jobs? People designing quantum computers face these challenges, and a new device could make them easier to solve.
Introduced by a team of researchers at the National Institute of Standards and Technology (NIST), the device includes two superconducting quantum bits, or qubits, which are a quantum computer’s analog to the logic bits in a classical computer’s processing chip. The heart of this new strategy relies on a “switch” device that connects the qubits to a circuit called a “readout resonator” that can read the output of the qubits’ calculations.
This toggle switch can be flipped to different states to adjust the strength of the connections between the qubits and the readout resonator. When turned off, all three elements are isolated from each other. When the switch is turned on to connect the two qubits, they can interact and perform calculations. Once the calculations are complete, the flip-flop can connect one of the qubits and the readout resonator to retrieve the results.
Having a programmable flip-flop switch goes a long way toward reducing noise, a common problem in quantum computer circuits that makes it difficult for qubits to perform 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 out whenever we want,” said Ray Simmonds, a NIST physicist and one of the paper’s authors. “This device architecture helps protect qubits and promises to improve our ability to make the high-quality measurements required to build quantum information processors out of qubits.”
The team, which also includes researchers from the University of Massachusetts Lowell, the University of Colorado Boulder and Raytheon BBN Technologies, describes its findings in a paper published today in Natural physics.
Quantum computers, which are still at a nascent stage of development, would harness the bizarre properties of quantum mechanics to perform tasks that even our most powerful classical computers find difficult, such as aiding the development of new drugs by performing sophisticated simulations of chemical interactions.
However, quantum computer designers still face many problems. One of these is that quantum circuits are kicked around by external or even internal noise, which arises from defects in the materials used to make the computers. This noise is essentially random behavior that can introduce errors into qubit calculations.
Today’s qubits are inherently noisy, but that’s not the only problem. Many quantum computer designs have what is called a static architecture, where each qubit in the processor is physically connected to its neighbors and to its readout resonator. The fabricated wires that connect the qubits together and for their readout can expose them to even more noise.
Such static architectures have another disadvantage: they cannot be easily reprogrammed. A static architecture’s qubits could perform a few related tasks, but for the computer to perform a wider range of tasks, it would have to be switched into a different processor design with a different qubit organization or layout. (Imagine changing the chip in your laptop every time you need to use a different piece of software, then consider that the chip needs to be held a little bit above absolute zero, and you understand why this can prove inconvenient. )
The team’s programmable toggle switch circumvents both of these problems. First, it prevents circuit noise from creeping into the system through the readout resonator and prevents qubits from having a conversation with each other when they should be quiet.
“This cuts down on an important source of noise in a quantum computer,” Simmonds said.
Second, the opening and closing of the switches between elements is controlled by a train of microwave pulses sent remotely, rather than through the physical connections of a static architecture. Integrating more of these flip-flops could be the basis for a more easily programmable quantum computer. The microwave pulses can also set the order and sequence of logic operations, meaning that a chip built with many of the team’s flip-flops can be instructed to perform any number of tasks.
“This makes the chip programmable,” Simmonds said. “Instead of having a completely fixed architecture on the chip, you can make changes via software.”
A final advantage is that the toggle switch can also turn on the measurement of both qubits at the same time. This ability to ask both qubits to reveal themselves as a pair is important for tracking quantum computation errors.
The qubits in this demonstration, as well as the toggle switch and readout circuit, were all made of superconducting components that conduct electricity without resistance and must be operated at very cold temperatures. The rocker switch itself is made of a superconducting quantum interference device, or “SQUID,” which is highly sensitive to magnetic fields passing through its loop. 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 they plan to add more qubits and resonators as well. Further research could provide insight into how to put many of these devices together, potentially offering a way to construct a powerful quantum computer with enough qubits to solve the kinds of problems that are so far 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. Natural physics. Published online June 26, 2023. DOI: 10.1038/s41567-023-02107-2
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