‘Giant atoms’ allow quantum processing and communication in one

MIT researchers have introduced a quantum computing architecture that can perform low-error quantum calculations while quickly sharing quantum information between processors. The work represents a key advance towards a complete quantum computing platform.

Prior to this discovery, small-scale quantum processors had successfully performed tasks at an exponentially faster rate than that of classic computers. However, it has been difficult to controllably communicate quantum information between distant parts of a processor. In classic computers, wired interconnects are used to route information from one processor to another during the course of a calculation. However, in a quantum computer, the information itself is quantum and fragile mechanics, and requires fundamentally new strategies to simultaneously process and communicate quantum information on a chip.

“One of the main challenges in scaling quantum computers is allowing quantum bits to interact with each other when they’re not located,” says William Oliver, associate professor of electrical engineering and computer science, a fellow at MIT Lincoln Laboratory and an associate. Director of the Electronics Research Laboratory. “For example, the closest neighboring qubits can easily interact, but how do I make ‘quantum interconnects’ that connect qubits at distant locations?”

The answer is to go beyond conventional light matter interactions.

Although natural atoms are small and pointed with respect to the wavelength of light with which they interact, in an article published in the magazine NatureThe researchers show that this does not have to be the case for superconducting “artificial atoms”. Instead, they have built “giant atoms” out of superconducting quantum bits, or qubits, connected in a tunable configuration to a microwave or waveguide transmission line.

This allows researchers to adjust the strength of qubit-waveguide interactions so that fragile qubits can protect themselves from decoherence, or from a kind of natural decay that would otherwise be accelerated by waveguide, while performing high Fidelity. Once those calculations are done, the strength of the qubit waveguide couplings is readjusted, and the qubits can release quantum data in the waveguide in the form of photons or light particles.

“Coupling a qubit to a waveguide is often quite bad for qubit operations, as doing so can significantly reduce the life of the qubit,” says Bharath Kannan, a MIT graduate fellow and first author of the paper. “However, the waveguide is necessary to release and route quantum information through the processor. Here, we have shown that it is possible to preserve the consistency of the qubit even though it is tightly coupled to a waveguide. So we have the ability determining when we want to release the information stored in the qubit. We have shown how giant atoms can be used to turn on and off interaction with the waveguide. “

The system made by the researchers represents a new regime of light matter interactions, the researchers say. Unlike models that treat atoms as point objects smaller than the wavelength of light with which they interact, superconducting qubits, or artificial atoms, are essentially large electrical circuits. When combined with the waveguide, they create a structure as large as the wavelength of the microwave light with which they interact.

The giant atom emits its information as microwave photons at multiple locations along the waveguide, so that the photons interfere with each other. This process can be adjusted to complete destructive interference, which means that the information in the qubit is protected. Furthermore, even when photons are not released from the giant atom, multiple qubits along the waveguide can still interact with each other to perform operations. At all times, qubits remain tightly coupled to the waveguide, but due to this type of quantum interference, they may be unaffected by it and protected from decoherence, while one and two qubit operations are performed with high fidelity.

“We use the quantum interference effects enabled by the giant atoms to prevent qubits from emitting their quantum information to the waveguide until we need it.” Oliver says

“This allows us to experimentally test a new physics regimen that is difficult to access with natural atoms,” says Kannan. “The effects of the giant atom are extremely clean and easy to observe and understand.”

The work appears to have a lot of potential for future research, adds Kannan.

“I think one of the surprises is actually the relative ease with which superconducting qubits can enter this regime of giant atoms.” he says. “The tricks we employ are relatively simple, and as such one can imagine using this for other applications without a huge additional burden.”

The coherence time of the qubits incorporated in the giant atoms, that is, the time they remained in a quantum state, was approximately 30 microseconds, almost the same for qubits not coupled to a waveguide, which have a range of between 10 and 100 microseconds, according to researchers

Additionally, research shows two-qubits entanglement operations with 94 percent fidelity. This is the first time that researchers have quoted a fidelity of two qubits for qubits that are tightly coupled to a waveguide, because the fidelity of such operations using conventional small atoms is often low in such an architecture. With more calibration, tuning procedures, and optimized hardware design, Kannan says fidelity can be further improved.