AI News, Ultimate precision for sensor technology using qubits and machine learning

Ultimate precision for sensor technology using qubits and machine learning

A well-established rule of thumb is the so-called standard quantum limit: the precision of the measurement scales inversely with the square root of available resources.

When the device is cooled to a very low temperature, magic happens: the electrical current flows in it without any resistance and starts to display quantum mechanical properties similar to those of real atoms.

'We use an adaptive technique: first, we perform a measurement, and then, depending on the result, we let our pattern recognition algorithm decide how to change a control parameter in the next step in order to achieve the fastest estimation of the magnetic field,' explains Andrey Lebedev, corresponding author from ETH Zurich, now at MIPT in Moscow.

'This is a nice example of quantum technology at work: by combining a quantum phenomenon with a measurement technique based on supervised machine learning, we can enhance the sensitivity of magnetic field detectors to a realm that clearly breaks the standard quantum limit,' Lebedev says.

A New Method for Reducing Quantum Uncertainty

May 2, 2017 Using a creative and insightful approach, scientists from the Barcelona Institute of Science and Technology (ICFO) recently measured the spin of atoms more precisely than many scientists thought was possible.

With this new method comes the promise of instruments that can measure magnetic fields, brain activity, and even gravitational waves more accurately than ever before.

These terms make sense to most people because they mean the same thing in the everyday, macroscopic world as they do when you zoom way in to quantum realm of individual atoms.

A magnetic field will cause something called spin precession in the atom, in which the spin of the nucleus precesses around the direction of the magnetic field like a wobbling top or gyroscope.

Many high precision instruments, such as MRI systems, gather information about the tissue or sample based on the spin precession rate of atomic nuclei.

Spin precession can’t be directly measured, so spin-based instruments determine the precession rate by repeatedly measuring two variables, spin angle and amplitude.

The common assumption is that the Heisenberg uncertainty principle applies to spin, meaning that the more precisely the spin angle is measured, the more uncertainty there is in the amplitude measurement—and vice versa.

As the team showed in this research, to determine the precession rate it’s only necessary to measure two of the three spin variables—the spin amplitude and the spin angle associated with longitude.

By designing a measurement technique that pushes most of the uncertainty into the unnecessary angle (the spin latitude), the researchers we able to measure spin amplitude and the relevant spin angle to a precision much better than the standard quantum limit.

Like non-destructive testing methods that look for weak spots in metal infrastructure without disturbing the metal, non-destructive measurement techniques gather information without disturbing the things they are measuring.

By measuring the properties of the light after it passed through the cloud of atoms, the researchers could determine the magnetization, and therefore the spin in the relevant directions, of the atoms in the cloud.

“Next we plan to study how this kind of spin tracking can be used in spin-based instruments to measure magnetic fields (for example from the heart and brain), and to measure time in atomic clocks,” says Mitchell.

‘Quantum Radio’ May Aid Communications and Mapping Indoors, Underground and Underwater

Researchers at the National Institute of Standards and Technology (NIST) have demonstrated that quantum physics might enable communications and mapping in locations where GPS and ordinary cellphones and radios don’t work reliably or even at all, such as indoors, in urban canyons, underwater and underground.

The NIST team is experimenting with low-frequency magnetic radio—very low frequency (VLF) digitally modulated magnetic signals—which can travel farther through building materials, water and soil than conventional electromagnetic communications signals at higher frequencies.

As a step toward that goal, the NIST researchers demonstrated detection of digitally modulated magnetic signals, that is, messages consisting of digital bits 0 and 1, by a magnetic-field sensor that relies on the quantum properties of rubidium atoms.

The sensor detected digitally modulated magnetic field signals with strengths of 1 picotesla (one millionth of the Earth’s magnetic field strength) and at very low frequencies, below 1 kilohertz (kHz).

(This is below the frequencies of VLF radio, which spans 3-30 kHz and is used for some government and military services.) The modulation techniques suppressed the ambient noise and its harmonics, or multiples, effectively increasing the channel capacity.

The researchers also performed calculations to estimate communication and location-ranging limits. The spatial range corresponding to a good signal-to-noise ratio was tens of meters in the indoor noise environment of the NIST tests, but could be extended to hundreds of meters if the noise were reduced to the sensitivity levels of the sensor.

The measured uncertainty in location capability was 16 meters, much higher than the target of 3 meters, but this metric can be improved through future noise suppression techniques, increased sensor bandwidth, and improved digital algorithms that can accurately extract distance measurements, Howe explained.

Detecting radio waves with entangled atoms

They applied a static magnetic field to the trapped atoms to make the atomic spins precess (rotate) synchronously (coherently) at a precise frequency of 42.2 kHz, which is within the low frequency band used for AM radio broadcasting.

They then applied a weak resonant radio frequency field in an orthogonal direction, which perturbed the atomic spin precession—this was the signal they wanted to detect.

In a standard RF magnetometer, the atomic spins are allowed to evolve freely for some time under the influence of this perturbation to allow the coherent buildup of signal before the change in the atomic state is detected.

Second, they used a new technique developed in the group to allow the coherent detection of an RF field with a changing frequency—as is used, for example, in an FM radio broadcast.

During the free evolution time, they used the applied static magnetic field to continuously shift the resonance frequency of the atoms to match the changing frequency of the RF field.

They then detected the perturbed atoms using a second stroboscopic quantum non-demolition measurement in order to measure the signal due to the RF field, and verify the entanglement generated among the atomic spins.

They were able to measure the weak RF magnetic field signal with a 25 percent reduction in experimental noise due to the quantum entanglement of the atoms, and a sensitivity comparable to the best RF magnetometers used to date.

Detecting radio waves with entangled atoms

They apply a static magnetic field to the trapped atoms, so that the atomic spins precess (rotate) synchronously (coherently) at a precise frequency of 42.2 kHz, within the low frequency band used for AM radio broadcasting.

In a standard rf magnetometer, the atomic spins are allowed to evolve freely for some time under the influence of this perturbation to allow the coherent buildup of signal, before the change in the atomic state is detected.

They then detect the perturbed atoms using a second stroboscopic quantum non-demolition measurement in order to measure the signal due to the rf field, and verify the entanglement generated among the atomic spins.

They were able to measure the weak rf magnetic-field signal with a 25% reduction in experimental noise due to the quantum entanglement of the atoms, and a sensitivity comparable to the best rf magnetometers used to date.

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