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Quantum Sensor Approaches Theoretical Limits

New technique could improve sensors used in atomic clocks, GPS, and many other applications

2 min read
Glowing sphere with 4 smaller yellow spheres next to a glowing clock with circuitry.

Physicists from Innsbruck, Austria, have developed a programmable quantum sensor based on entangled atoms.

Harald Ritsch/University of Innsbruck

Quantum sensors promise new levels of accuracy. Now researchers in Austria have developed what they say is the first programmable quantum sensor, a device capable of an unprecedented level of sensitivity operating near the fundamental limits imposed by the laws of quantum mechanics.

Quantum technology relies on quantum effects that can emerge based on how the universe becomes a fuzzy place at its very smallest levels. For instance, the quantum effect known as superposition allows atoms and other building blocks of the cosmos to essentially spin in two opposite directions at the same time or exist in two or more places simultaneously. By placing many components known as qubits into superposition and linking them via a quantum-mechanical phenomenon known as entanglement, a quantum computer can in theory perform a mind-boggling number of computations at once.

Quantum computers and other quantum technology are notoriously very fragile to any outside interference. Yet, quantum sensors capitalize on this vulnerability to achieve exceptional sensitivity to the smallest disturbances to the environment, and have many potential applications in medicine, nanotechnology, telecommunications, and satellite navigation.

In the new study, researchers sought to bring recent advances made in quantum computing to improve quantum sensing. Their experiments involved quantum circuits made of up to 26 qubits, each consisting of an electrically trapped calcium ion.

The new technique adopted a hybrid approach in which each quantum circuit was partnered with a classical supercomputer. The scientists programmed this platform to run a variational quantum algorithm that sought the optimal solution to a problem—in this case, the best settings of the quantum circuit with which to measure the states of the qubits.

In experiments, the scientists found their programmable quantum sensor could optimize itself enough to approach the fundamental sensing limit up to a factor of about 1.45. (The ultimate sensing limit is a factor of 1; the more a sensor approaches this limit, the better its performance.)

Surprisingly, the sensor got “exceedingly close” to estimates of its best performance despite noise in the system, says study lead author Christian Marciniak, a physicist at the University of Innsbruck, in Austria. “This shows how robust the scheme is, which makes us quite hopeful about its practical uses in many scenarios,” he says. “It can actually work in realistic settings; provide tangible benefits under real-world conditions.”

The most immediate applications of this new work involve sensors dealing with trapped ions, including devices such as atomic clock and global positioning systems, as well as magnetic and inertial sensors. For example, this new technique could improve the measurement resolution of magnetometers, for “more accurate diagnostics,” Marciniak says.

Similarly, improved atomic clocks may lead “to a higher position-precision of GPS,” Marciniak adds. “Likewise, maybe a measurement has no constraints on space but maybe does on time. Getting to the same precision in a shorter time could mean that you can do the same work in shorter time or reduce someone’s time they are stuck in a diagnostic machine.”

Ultimately, this new technique may help improve many other kinds of sensor hardware platforms, Marciniak notes. Incorporating more and more entangled particles into this new class of programmable quantum sensors may then result in a “quantum advantage” over previous devices, the researchers say.

The scientists detailed their findings online 23 March in the journal Nature.

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Europe Expands Virtual Borders To Thwart Migrants

Our investigation reveals that Europe is turning to remote sensing to detect seafaring migrants so African countries can pull them back

14 min read
A photo of a number of people sitting in a inflatable boat on the water with a patrol ship in the background.

Migrants in a dinghy accompanied by a Frontex vessel at the village of Skala Sikaminias, on the Greek island of Lesbos, after crossing the Aegean sea from Turkey, on 28 February 2020.


It was after midnight in the Maltese search-and-rescue zone of the Mediterranean when a rubber boat originating from Libya carrying dozens of migrants encountered a hulking cargo ship from Madeira and a European military aircraft. The ship’s captain stopped the engines, and the aircraft flashed its lights at the rubber boat. But neither the ship nor the aircraft came to the rescue. Instead, Maltese authorities told the ship’s captain to wait for vessels from Malta to pick up the migrants. By the time those boats arrived, three migrants had drowned trying to swim to the idle ship.

The private, Malta-based vessels picked up the survivors, steamed about 237 kilometers south, and handed over the migrants to authorities in Libya, which was and is in the midst of a civil war, rather than return to Malta, 160 km away. Five more migrants died on the southward journey. By delivering the migrants there, the masters of the Maltese vessels, and perhaps the European rescue authorities involved, may have violated the international law of the sea, which requires ship masters to return people they rescue to a safe port. Instead, migrants returned to Libya over the last decade have reported enslavement, physical abuse, extortion, and murders while they try to cross the Mediterranean.

If it were legal to deliver rescued migrants to Libya, it would be as cheap as sending rescue boats a few extra kilometers south instead of east. But over the last few years, Europe’s maritime military patrols have conducted fewer and fewer sea rescue operations, while adding crewed and uncrewed aerial patrols and investing in remote-sensing technology to create expanded virtual borders to stop migrants before they get near a physical border.

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