Insider Brief
In a study published recently in Physical Review Letters, researchers unveiled a new type of quantum sensor that they report leverages quantum entanglement to perform detections that, note the quote marks, “travel back in time”. The researchers add the findings could — one day — lead to novel quantum sensors that are ideally suited for astronomical detection and magnetic field investigations.
The study, led by Kater Murch, Charles M. Hohenberg Professor of Physics and Director of the Center for Quantum Leaps at Washington University in St. Louis, introduces a sensor that can probe past events in complex systems. The team, which also included scientists from the National Institute of Standards and Technology (NIST) and the University of Cambridge, described the innovation in the press release as a bit like “sending a telescope back in time to capture a shooting star that you saw out of the corner of your eye.”
The sensor operates by entangling two quantum particles in a quantum singlet state, where their spins point in opposite directions. The process begins with one particle, the “probe,” being subjected to a magnetic field that causes it to rotate. The key breakthrough comes when the second particle, the “ancilla,” is measured. This measurement effectively sends its quantum state back in time to the probe, allowing researchers to optimally set the spin direction of the probe qubit in what Murch refers to as hindsight.
“The beauty of hindsight is that it allows experimenters to set the best direction for the spin—in hindsight—through time travel,” Murch explained in the press release.
This method circumvents the usual one-in-three failure rate associated with unknown magnetic field measurements, offering a new approach to quantum metrology. Quantum metrology aims to enhance measurement sensitivities by utilizing quantum resources. Traditional single-qubit sensors fall short when the rotation axis is unknown. However, this new approach entangles the probe qubit with an ancilla qubit, achieving the maximum quantum Fisher information about a rotation angle regardless of the unknown rotation axis.
“We demonstrate this metrological advantage using a two-qubit superconducting quantum processor,” the research team wrote in the paper. This approach outperforms classical type-II sensors by harnessing the entanglement of type-III sensors.
The 50% improvement in quantum Fisher information weighs against the cost of entanglement manipulation. In other words, what the researchers gain in measurement precision and sensitivity achieved through entanglement justify the additional complexity and resource investment required for manipulating entangled states. This trade-off offers a glimpse into the advantages of using type-III sensors in applications where enhanced accuracy is critical.
The team adds that the research avoids the need for postselection, as all measurement outcomes inform the inference of the rotation angle.
In their detailed experimental setup, which is available in the paper, the researchers utilized a two-qubit subsection of a three-qubit device, as described in the paper. The setup includes two transmon circuits, one fixed-frequency and one frequency-tunable via a fast flux line, allowing for parametric entangling gates. Single-qubit rotations were applied via independent drive lines, and the qubits were coupled to separate readout resonators, probed by a common feedline.
Future Directions and Limitations
While the study presents a somewhat mind-boggling — although my mind becomes boggled easily in this job — new approach to quantum sensing, there are both exciting avenues for future research, as well as acknowledged limitations. On the future directions side, the researchers suggest extending the protocol to optical and solid-state systems with concrete metrological applications, benefiting phase estimation in quantum algorithms, and enhancing metrology under time constraints. The ability to send quantum sensors “back in time” could improve various applications, from detecting astronomical phenomena to measuring time-varying fields with greater precision, according to the researchers.
Although not explicitly stated, any research into the mechanics of entanglement add to the understanding of this phenomenon and — one might think — could have ramifications across quantum science and technology fields.
The study also highlights the potential for their technique to be useful in metrology requiring time-varying field measurements. “Our protocol entails optimal state preparations and measurements without prior knowledge about the unknown unitary’s generator,” the paper states, emphasizing the robustness of their approach.
However, as mentioned, the research is not without limitations. The current experimental setup was performed within a two-qubit superconducting quantum processor, which, while effective for demonstrating the concept, may face scalability issues when applied to more complex systems. Additionally, the cost and complexity of manipulating entangled states and the need for highly controlled environments could pose challenges for practical implementations outside of laboratory conditions.
The study’s findings open the door to numerous potential applications. As quantum scientists continue to explore the properties of entangled particle pairs, more innovative uses for these time-traveling quantum sensors might emerge. For more information, the full study can be accessed in a recent issue of Physical Review Letters.
I also relied on an earlier version available on the pre-print server ArXiv. A synopsis is also available.
