Scientists reduce noise in quantum signals as way to boost accuracy of measurements

The nature of quantum computing is such that the noise that is part of the process can throw off measurements, thus increasing the error range.

Now, researchers at the Massachusetts Institute of Technology (MIT) have found a way to mitigate the problem, using parametric amplification to “squeeze” the noise over larger bandwidths. 

“Squeeze,” in this sense, refers to a situation in which the noise affecting one variable is decreased while the noise affecting its conjugate partners is increased. Although the noise level stays the same overall, the redistribution of it gives scientists a more accurate measurement when they consider the lower-noise variable.

This breakthrough could lead to more efficient readout of quantum information and enhance the performance of quantum systems. The findings were published in the journal Nature Physics, and MIT News published an article about it. 

Quantum systems inherently possess a certain amount of noise that limits the accuracy of measurements. However, the team of MIT scientists and outsiders has shown that by focusing on the lower-noise variable, researchers can make more accurate measurements. 

The researchers developed a superconducting parametric amplifier that achieves quantum squeezing over much larger frequency bandwidths. Previous microwave parametric amplifiers typically achieved bandwidths of only 100 megahertz or less, whereas the new amplifier demonstrated squeezing over a broad frequency range of up to 1.75 gigahertz while maintaining a high degree of squeezing. 

This new broadband device has the potential to significantly improve the efficiency of reading quantum information, leading to faster and more accurate quantum systems. This architecture could be particularly valuable for multiqubit systems and other applications that require precise measurements.

Jack Qiu, a graduate student in electrical engineering and computer science at MIT and lead author of the paper, highlighted the importance of broadband amplification as quantum computing advances. He explained that with their architecture, a single amplifier could theoretically read out thousands of qubits simultaneously, offering substantial benefits as the field of quantum computing grows. 

The senior authors of the study are William D. Oliver, the Henry Ellis Warren professor of electrical engineering and computer science and of physics, and Kevin P. O’Brien, the Emanuel E. Landsman Career Development professor of electrical engineering and computer science. 

The researchers utilized a Josephson traveling-wave parametric amplifier instead of the conventional resonator-based approach, the MIT write-up said. By chaining more than 3,000 Josephson junctions together, they created a system that allows photons to interact as they travel between junctions, enabling noise squeezing without stressing any individual junction. This approach allows for higher-power signals and broader bandwidths compared to previous amplifiers.

The new device reduces the noise power by a factor of 10 below the fundamental quantum limit, achieving a 3.5 gigahertz amplification bandwidth. The researchers also demonstrated the broadband generation of entangled photon pairs, which could improve the readout of quantum information with a higher signal-to-noise ratio. 

While the researchers acknowledge that there is room for improvement, they believe this work has tremendous potential for application in various quantum systems, MIT said in its assessment of the paper. The team plans to explore different fabrication methods to enhance the performance of the amplifier and further extend its operating frequency range. 

The research was supported by funding from the NTT Physics and Informatics Laboratories and the Office of the Director of National Intelligence IARPA program. The collaboration involved scientists from the University of Sydney, Atlantic Quantum, Princeton University, MIT Lincoln Laboratory, the University of California at Berkeley, and other institutions.