Breakthrough technique unveils new possibilities in quantum exploration

USC investigators have showcased a new quantum sensing method that greatly outshines traditional approaches, potentially hastening advancements in areas from medical imaging to foundational physics research, as detailed in a recent study released Tuesday in Nature Communications.

For many years, quantum sensor performance has been hampered by decoherence, which refers to erratic behavior brought about by environmental noise. “Decoherence leads to the state of a quantum system becoming randomly disordered, obliterating any quantum sensing signal,” stated Eli Levenson-Falk, the lead author of the study, associate professor of physics and astronomy at the USC Dornsife College of Letters, Arts and Sciences, and associate professor of electrical and computer engineering at the USC Viterbi School of Engineering.

Quantum sensing involves utilizing quantum systems such as atoms, photons, or qubits as sensors for measuring physical phenomena (like brain activity, ultra-precise clocks, or gravitational anomalies) with exceptional accuracy, frequently surpassing the limits of classical sensors. Sensing devices leverage quantum characteristics — such as superposition, entanglement, and coherence — to detect minuscule signals that would otherwise be drowned out by noise.

“Consider it as trying to hear a soft whisper in a loud environment,” noted Malida Hecht, a USC Dornsife doctoral student in physics and the primary author of the research. “Quantum sensing devices are capable of recognizing stimuli that are too minute or subtle for conventional measuring instruments to perceive.”

Mitigating decoherence with a novel coherence-stabilized approach

In the recent investigation, the research group temporarily addressed the persistent issue of decoherence by implementing a new pre-defined coherence-stabilized protocol on their experimental qubit, stabilizing one crucial aspect of the quantum state. The study’s protocol was based on theoretical foundations established by co-authors Daniel Lidar, who holds the Viterbi Professorship in Engineering and is a USC professor of electrical and computer engineering, chemistry, and physics and astronomy, alongside Kumar Saurav, a USC Viterbi doctoral candidate in electrical engineering. This experiment considerably enhanced the detection of minor frequency shifts in quantum systems. Levenson-Falk indicated that the study’s coherence-stabilized sensing protocol enables the sensing signal, represented as a change in the quantum state, to become more pronounced than it would under the conventional protocol measurement.

This stabilization could be crucial for scenarios where identifying subtle signals is vital. “The more substantial signal is simpler to identify, enhancing sensitivity,” Levenson-Falk remarked. “Our study showcases the highest sensitivity for measuring a qubit’s frequency observed to date. Most importantly, our protocol needs no feedback and no additional control or measurement resources, making it readily applicable across diverse quantum computing and quantum sensing technologies.”

Notable enhancement in sensing capability

The researchers applied their protocol on a superconducting qubit, attaining up to 1.65 times greater effectiveness per measurement than the traditional Ramsey interferometry protocol. Theoretical evaluations suggest possible improvements of up to 1.96 times in some systems.

Levenson-Falk mentioned that his experimental demonstration of sensing with a stabilized condition illustrates that it is feasible to enhance quantum sensors without resorting to complex methods like real-time feedback or entangling numerous sensors. “It also indicates that we have not yet utilized all the potential information from these kinds of measurements. Even superior sensing protocols exist, and we could harness them to drive immediate real-world benefits.”

About the investigation: The research was carried out by Matilda O. Hecht, Kumar Saurav, Evangelos Vlachos, Daniel A. Lidar, and Eli M. Levenson-Falk, all from USC.

The research was partially supported by the U.S. Army Research Laboratory and the U.S. Army Research Office under contract/grant number W911NF2310255, the National Science Foundation, the Quantum Leap Big Idea under Grant No. OMA-1936388, the Office of Naval Research under Grant No. N00014-21-1-2688, and Research Corporation for Science Advancement under Cottrell Award 27550. Devices were designed and provided by the Superconducting Qubits at Lincoln Laboratory (SQUILL) Foundry at MIT Lincoln Laboratory, funded by the Laboratory for Physical Sciences (LPS) Qubit Collaboratory.