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In upcoming years, quantum machines may swiftly emulate novel materials or assist researchers in creating quicker machine-learning algorithms, paving the way for numerous fresh opportunities.
However, these uses will only be viable if quantum devices can execute tasks exceptionally rapidly, allowing scientists to gather data and implement corrections before accumulating error rates compromise their precision and dependability.
The effectiveness of this measurement technique, referred to as readout, is contingent on the strength of the interaction between photons—particles of light carrying quantum data—and artificial atoms, which are units of matter frequently utilized to hold information in a quantum device.
Currently, MIT scientists have showcased what they consider to be the most potent nonlinear light-matter interaction ever realized in a quantum environment. Their experiment marks progress toward achieving quantum maneuvers and readout that could be accomplished in mere nanoseconds.
The researchers employed an innovative superconducting circuit design to demonstrate nonlinear light-matter coupling that is approximately an order of magnitude stronger than earlier efforts, potentially allowing a quantum processor to operate about 10 times more swiftly.
There remains significant work ahead before the design could be applied in a functional quantum computer, but illustrating the fundamental physics behind the method is a significant milestone, states Yufeng “Bright” Ye PhD ’24, the primary author of a study on this investigation.
“This would truly remove one of the obstacles in quantum computing. Typically, you must measure the outcomes of your computations in between cycles of error correction. This could hasten our journey toward achieving fault-tolerant quantum computing and enable us to derive practical applications and benefits from our quantum devices,” Ye remarks.
Alongside him on the paper is senior author Kevin O’Brien, an associate professor and principal investigator at the Research Laboratory of Electronics at MIT, who oversees the Quantum Coherent Electronics Group within the Department of Electrical Engineering and Computer Science (EECS), along with collaborators from MIT, MIT Lincoln Laboratory, and Harvard University. The research is published today in Nature Communications.
A novel coupler
This physical demonstration advances years of theoretical inquiries within the O’Brien group.
After Ye joined the laboratory as a PhD candidate in 2019, he began crafting a specialized photon detector to improve quantum information processing.
Through this effort, he devised a new variant of quantum coupler, which is a device that facilitates interactions between qubits. Qubits serve as the foundational elements of a quantum computer. This so-called quarton coupler had numerous potential applications in quantum maneuvers and readout that it swiftly became a central focus of the lab.
This quarton coupler is a unique type of superconducting circuit with the ability to generate exceedingly strong nonlinear coupling, which is critical for executing most quantum algorithms. As the researchers increase the current flowing into the coupler, it produces an even more potent nonlinear interaction. In this context, nonlinearity indicates that a system operates in a manner that surpasses the sum of its individual components, revealing more intricate properties.
“Most valuable interactions in quantum computing stem from the nonlinear coupling of light and matter. If you can achieve a broader range of diverse couplings and enhance their strength, you can effectively boost the processing speed of the quantum machine,” Ye elaborates.
For quantum readout, scientists direct microwave light at a qubit, and based on whether that qubit is in state 0 or 1, there is a frequency shift on its respective readout resonator. They observe this shift to ascertain the qubit’s status.
Nonlinear light-matter coupling between the qubit and resonator facilitates this measurement process.
The MIT team designed a layout featuring a quarton coupler linked to two superconducting qubits on a chip. They convert one qubit into a resonator and utilize the other as an artificial atom to store quantum information, which is transferred through microwave light particles known as photons.
“The interaction between these superconducting artificial atoms and the microwave light that directs the signal is fundamentally how an entire superconducting quantum computer is constructed,” Ye explains.
Facilitating faster readout
The quarton coupler establishes nonlinear light-matter interaction between the qubit and resonator that is roughly an order of magnitude more robust than previous achievements by researchers. This could make a quantum system capable of extremely rapid readout.
“This work is merely a chapter in a larger narrative. This is a fundamental physics demonstration, but the group is currently engaged in efforts to achieve truly rapid readout,” O’Brien states.
This will involve incorporating additional electronic components, such as filters, to create a readout circuit that can be integrated into a more extensive quantum system.
The researchers also exhibited remarkably strong matter-matter coupling, a different form of qubit interaction crucial for quantum operations. This is another avenue they intend to investigate in future work.
Swift operations and readout are particularly vital for quantum computers because qubits have limited lifetimes, denoted as coherence time.
Enhanced nonlinear coupling permits a quantum processor to operate more rapidly and with fewer errors, enabling the qubits to execute a greater number of tasks within the same timeframe. This translates to the qubits being able to complete more rounds of error correction during their lifespans.
“The more iterations of error correction you can incorporate, the lower the error in the results will be,” Ye asserts.
Ultimately, this research could aid scientists in constructing a fault-tolerant quantum computer, which is indispensable for effective, large-scale quantum computation.
This study was partially funded by the Army Research Office, the AWS Center for Quantum Computing, and the MIT Center for Quantum Engineering.
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