Quantum sensors exploiting the strange properties of entangled particles can in principle outperform classical devices. To date, however, scientists have struggled to demonstrate how entanglement can provide a “quantum advantage” for useful applications.
Today, researchers in the United States have shown how a “crystal” of trapped ions can measure shifts and electric fields at sensitivities an order of magnitude beyond the classical limit. They say the technology is well suited for measuring putative dark matter that interacts with normal matter via weak electromagnetic fields (Science, doi: 10.1126 / science.abi5226).
Entanglement of a mechanical oscillator
Entanglement implies an interdependence between quantum particles, so that determining the state of one instantly fixes the state of the other (or others), regardless of their distance. The phenomenon could potentially allow all kinds of new sensors, from quantum radar to so-called phantom imagery. Indeed, scientists have already exploited entanglement to improve the sensitivity of gravitational wave detectors by reducing phase noise in interferometric detectors.
The latest work is based rather on entanglement to improve the sensitivity of a quantum mechanical oscillator. Such a device registers a small coherent variation in its amplitude over time when subjected to a low resonant force. The idea is to interweave the displacement of the oscillator with a measurable property of the system before force is applied, pushing accuracy beyond the limit usually imposed by quantum mechanics.
The oscillator in question was made by John Bollinger of the National Institute of Standards and Technology (NIST) in Boulder, Colorado, and colleagues, and builds on theoretical work by Ana Maria Rey and colleagues at NIST and JILA research. institute, also in Boulder. The device involves a 2D lattice of approximately 150 berylium-9 ions retained in a magneto-electric trap. The movement of the center of mass of this ionic crystal, which measures approximately 200 µm in diameter, constitutes the (high-Q) oscillator.
To measure the oscillations, Bollinger and his colleagues used microwaves to place all the ions in the same spin state. Next, they coupled the spins to the collective movement of the ions using a pair of laser beams, with the beams detuned so that their beat frequency was roughly equal to the resonant frequency of the center of mass. Once the entanglement is established, they then set the oscillator in motion by applying an alternating voltage at the resonant frequency to one of the electrodes of the trap.
The researchers found that they could measure the displacement of the oscillator very precisely by reversing the process of entanglement over time. Using the same microwave and laser frequencies that they used to do the entanglement, they were able to read the displacement by measuring the spin state (which they did using fluorescence).
Dark matter detector?
Regarding the displacement itself, Bollinger and his colleagues achieved a sensitivity of almost 9 dB below the standard quantum limit. The measurement of the electric field, on the other hand, gave an improvement of 4 dB compared to the conventionally limited measurements. The latter figure translates into a sensitivity of 240 nanovolts (nV) per meter per second.
The researchers say their new scheme could be used to detect certain forms of dark matter, the mysterious substance that is believed to make up about 80% of matter in the universe. The idea would be to measure the oscillations induced by the very weak electromagnetic interactions of what are called axions or hidden photons.
They point out that the sensitivity of the system to axions is limited, because the small spatial size of its magnetic field suppresses the electric field that axions can generate. Nevertheless, they estimate that with an improved electrical sensitivity of 10 nV / m / s, they could reach the current best limits of axion-photon coupling after taking data for just one day. This sensitivity, they suggest, could be achieved relatively easily by improving the stability of the oscillator’s resonant frequency as well as reducing thermal noise.
Beyond these improvements, the researchers are considering further improvements, such as using 3D crystals to house around a million ions, or increasing the size and strength of the trap’s magnetic field. These steps, they say, could further improve the detection sensitivity of dark matter, “up to three orders of magnitude.”