Physicists demonstrate magnetometer that uses quantum effects and machine learning to measure magnetic fields more accurately than its classical analogues. Such magnetometer could be used to seek mineral deposits, discover distant astronomical objects, diagnose brain disorders and create better radars.
Researchers from the Moscow Institute of Physics and Technology (MIPT), Aalto University in Finland and ETH Zurich combinedly worked on this and made it the reality. Andrey Lebedev said, When you study nature, whether you investigate the human brain or a supernova explosion, you always deal with some sort of electromagnetic signal. So measuring magnetic fields is necessary across diverse areas of science and technology and one would want to do this as accurately as possible.
A magnetometer or magnetic sensor is an instrument that measures magnetism either the magnetization of a magnetic material like a ferromagnet or the direction, strength or relative change of a magnetic field at a particular location. It simply means an instrument that measures magnetic fields. A compass is an example of a primitive magnetometer. In an electronics store, one can find more advanced devices of this kind used by archaeologists. Military mine detectors and metal detectors at airports are also magnetometers.
There is a fundamental limitation on the accuracy of such instruments, known as the standard quantum limit. Basically, it says that to double the precision, a measurement has to last four times as long. This rule applies to any classical device, which is to say one that does not utilize the bizarre effects of quantum physics.
Achieving a higher accuracy, and therefore shorter measurement times is crucial when fragile samples or living tissue is examined. For example, when a patient undergoes positron emission tomography, also known as a PET scan, radioactive tracers are introduced into the bloodstream, and the more sensitive the detector is, the smaller the necessary dose.
A qubit is a particle that obeys the laws of quantum physics and can occupy two discrete basis states simultaneously in what is known as a superposition. This notion refers to a multitude of "intermediate" states, each of which collapses into one of the two basis states as soon as it is measured. An example of a qubit is a hydrogen atom whose two basis states are the ground and the excited state.
Superconducting qubits are distinguished by their sensitivity to magnetic fields, which can be used for making measurements. Once a suitable microwave radiation pulse is used to drive the device into a balanced superposition of the ground and excited states, this new state begins to change predictably with time. To track this state change, which is a function of the external magnetic field, the researchers sent a second microwave pulse to the device after a brief delay and measured the probability of finding the qubit in the excited state. This probability, which was calculated over many identical experiments performed in quick succession, indicates the strength of the magnetic field. The precision of this quantum technology surpasses the standard quantum limit.
Qubit training consists of making many preliminary measurements under controlled conditions with predetermined delays between pulses and in a range of known magnetic fields. The authors thereby determined the probability of detecting the excited state following the sequence of two pulses for an arbitrary field and pulse delay. The researchers plotted their findings on a diagram, which serves as a fingerprint for the individual device used in the study, accounting for all its imperfections. The point of the sample fingerprint is that the delay times between pulses can be optimized during repeated measurements.
The prototype has been tested on a static magnetic field, but time-varying or transient fields can be measured in the same way. The research team is already conducting experiments with variable fields, expanding the potential range of applications of their device.
Once the first microwave pulse is absorbed by the magnetometer, it enters a superposition of the ground and excited states. This can be visualized by picturing the two basis states of the qubit as the two poles of a sphere, where each other point on the sphere represents some state of superposition. In this analogy, the first pulse drives the state of the qubit from the north pole the ground state to some point on the equator. A direct measurement of this state of balanced superposition would result in the ground or excited state being detected with even odds.
The main challenge is to distinguish between the different states on the equator: Unless some trick is used, the measurement would return the excited state exactly 50 percent of the time. This is why the physicists sent a second microwave pulse to the qubit and only then checked its state. The idea behind the second pulse is that it predictably shifts the state of the device off the equator, into one of the hemispheres. Now, the odds of measuring an excited state depend on how much the state has rotated since the first pulse, that is, angle X. By repeating the sequence of two pulses and a measurement many times, the authors calculated the probability of an excited state, and thus the angle X and the strength of the magnetic field. This principle underlies the operation of their magnetometer.
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