Researchers create protocols to enable quantum sensors

Yuan Liu, Assistant Professor of Electrical and Computer Engineering and Computer Science at NC State and corresponding author of the research, explained: “Quantum sensing shows promise for more powerful sensing capability that can approach the fundamental limit set by the law of quantum mechanics, but the challenge lies in being able to direct these sensors to find the signals we want.”

Liu, who was formerly a postdoctoral researcher at MIT, stated that their idea was inspired by classical signal processing filter design principles commonly used by electrical engineers. He added: “We generalised these filter designs to quantum sensing systems, which allows us to ‘fine-tune’ what is essentially an infinite dimensional quantum system by coupling it to a simple two-level quantum system.”

The researchers designed an algorithmic framework that couples a qubit to a bosonic oscillator. Qubits, or quantum bits, are the quantum computing equivalent of classical computing bits and can exist in a superposition of two states: 0 and 1. Bosonic oscillators are the quantum counterparts of classical oscillators, like a pendulum’s motion, but they are infinite-dimensional systems, meaning their states are not limited to a linear combination of only two basis states.

Liu elaborated on the complexity of manipulating the quantum state of an infinite-dimensional sensor: “Instead of trying to figure out amounts of our targets, we just ask a decision question: whether the target has property X. Then we can design the manipulation of the oscillator to reflect that question.”

By coupling the infinite-dimensional sensor to the two-dimensional qubit and manipulating that coupling, the sensor can be tuned to detect a specific signal. Interferometry is then used to encode the results into the qubit state, which is measured for readout.

Liu stated: “Once the signal happens, we undo the shaping, which creates interference in the infinite dimensional system that comes back as a readable result – a polynomial function determined by the original polynomial transformation of the oscillator and the underlying signal – in the qubit’s two-level system. In other words, we end up with a ‘yes’ or ‘no’ answer to the question of whether the thing we’re looking for is there. And the best part is that we only need to measure the qubit once to extract an answer – it’s a ‘single-shot’ measurement.”

The researchers see the work as providing a general framework for designing quantum sensing protocols for a variety of quantum sensors. Liu noted: “Our work is useful because it utilises readily available quantum resources in leading quantum hardware (including trapped ions, superconducting platform, and neutral atoms) in a fairly simple way. This approach serves as an alarm or indicator that a signal is there, without requiring costly repeated measurements. It’s a powerful way to extract useful information efficiently from an infinite dimensional system.”

The work appeared in Quantum and was supported by the Army Research Office under project number W911NF-17-1-0481, and by the U.S. Department of Energy under contract number DE-SC0012704. Jasmine Sinanan-Singh and Gabriel Mintzer, both graduate students at MIT, were co-first authors of the research. Isaac L. Chuang, professor of physics and electrical engineering and computer science at MIT, also contributed to the work.