Sunday Science: Did Quantum Sensors Help Find a U.S. Pilot Shot Down in Iran? Experts Doubt It
If true, a claim reported by the New York Post and repeated by President Donald Trump would be perhaps the single biggest breakthrough to date in the budding field of quantum sensors. On 7 April, the Post reported that the United States rescued a jet pilot shot down in Iran last week with the help of magnetic quantum sensors that detected his heartbeat. Scientists say such devices can sense a beating heart, but at a distance of a few centimeters and not, as Trump suggested the day before at a press conference, at more than 60 kilometers.
“That really sounds implausible,” says Ronald Wakai, a medical physicist at the University of Wisconsin–Madison who has spent decades developing techniques to monitor the heart magnetically. Dmitry Budker, a physicist at Johannes Gutenberg University Mainz who works with the same type of sensor mentioned in the Post story, shares those doubts. “I would be extremely skeptical,” he says.
Called Ghost Murmur, the purported technology was developed by Lockheed Martin, the Post reported. “In the right conditions, if your heart is beating, we will find you,” an anonymous source told the paper. According to the story, signals are detected by tiny defects in diamonds in which a carbon atom is replaced with nitrogen and a neighboring atom is missing. That nitrogen-vacancy (NV) center can trap a single electron that in a magnetic field twirls like a compass needle, making it a quantum sensor.
The heart does emit magnetic signals, driven by ions flowing along cardiac neurons. However, those signals are extremely weak. On the surface of the chest of an adult, a few centimeters from the heart, they measure about 0.1 nanotesla, roughly a half-millionth as strong as Earth’s magnetic field. To monitor the heart, physicians instead rely on its stronger electric field, which can be easily sensed with electrodes on the chest, a technique called electrocardiography.
Still, since the 1960s, scientists have traced magnetic signals from the heart with ever-greater precision, thanks to improving sensors. For example, Wakai and colleagues use a device called an optically pumped magnetometer, which contains a small volume of magnetically polarized rubidium gas, to sense magnetic fields as weak as 1 picotesla, one-hundredth that of a beating heart’s signal. Similar sensitivity can be attained with electrical gizmos called superconducting quantum interference devices (SQuIDs).
But detecting such magnetic signals from a distance is another matter, Wakai says. The strength of the heart’s oscillating magnetic field fades with the square of the distance, he says, so a signal that is just barely detectable at a few centimeters will be only one-trillionth as strong at tens of kilometers. Wakai has tried to detect cardiac signals from adult patients at a distance. “Once you get past about a meter or so, you can’t see anything,” he says. However, Trump reiterated the claim that Ghost Murmur helped find the pilot.
Another problem is environmental noise, which can easily swamp the faint magnetic signal, notes Janette Strasburger, a pediatric and fetal cardiologist at the Medical College of Wisconsin with whom Wakai collaborates. Such interference would abound if a detector was placed in a vehicle such as a helicopter, she notes. “Just the rotor alone would be a killer, I’d think.” Even if the detector could sift the tiny signal from the noise, it couldn’t tell which direction it was coming from or where the person is, Wakai says.
Wakai also questions the use of diamond NV centers for remote sensing. Such detectors are not as sensitive as SQuIDs or optically pumped magnetometers, but they are very small and biologically inert. So they’re better for getting very close to tiny magnetic sources such as individual cells, Wakai says. In January, Budker and 29 colleagues reported using diamond NV centers to detect a heartbeat, although doing so required hovering over the patient’s chest for 5 minutes, according to a paper posted to the arXiv preprint server. “I thought we were the first,” Budker says.
Magnetic monitoring of the heart, or magnetocardiography, does have its uses. It’s ideal for monitoring the heart of a developing fetus, even though it produces signals only one-tenth as strong as those from an adult. Electrocardiography doesn’t work on a fetus, Wakai says, because it is covered with a fatty substance that blocks electrical signals.
Strasburger and Wakai have examined more than 1100 pregnant patients with magnetic sensors, which can reveal details ultrasound will miss. In 40% of the tests, they either confirmed the fetus had a suspected abnormality or found a different one, some controllable with measures as simple as diet and medication. The researchers’ goal is to help prevent stillbirths, Strasburger says. When speaking with Science, she was awaiting the arrival of a woman whose fetus had a heart rate only half as fast as normal.
Strasburger worries that if the claims for remote sensing turn out to be hype, they may trigger a backlash that will discredit magnetocardiography. She also notes that the National Institutes of Health had an initiative to encourage small businesses to develop quantum sensors for such applications, but the Trump administration canceled it last year. “Look at the excitement of saving one pilot,” she says. “We could be saving half a million fetuses every decade.”
Adrian Cho covers mainly physics, cosmology, and some scientific policy. He has been a staff writer at Science since 2005 and has been writing for a living since 1999, when he completed the Science Communication Program at the University of California, Santa Cruz. Cho earned a Ph.D in experimental particle physics in 1997, working on the CLEO experiment at Cornell University. He received his A.B. from the University of Chicago in 1987. He lives in the suburbs of Detroit with his wife and two children. In his spare time, he picks a little guitar and plays beer-league hockey. On the ice, he's small but slow.
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