To link and measure

Quantum technologies have great potential over the last few years we have seen that the future of many areas of science depends on their progress. Once in 1994, Peter Shor invented a quantum algorithm for the decomposition of large numbers into factors, which solves the problem exponentially faster than any classical algorithm, there are many quantum solutions for classically intractable problems. Some of these decisions were implemented experimentally with the use of modern mnogoserijnyj quantum computers. Collaboration Google recently showed the superiority of quantum 53 Cubana superconducting CPU cycles in solving the problem of emulation of random quantum circuits.

Another type of tasks for which it is possible to use quantum resources to improve precision measurement. New discoveries are often made due to new precision instruments: telescopes help astronomers to look deeper into space, biologists rather see the microcosm with the help of new microscopes, and archaeologists need a mass spectrometer to determine the age of fossils. The limits of measurement accuracy slow the progress in many fields of science, from microphysics to medicine. Here comes in quantum Metrology, the science of measurement using quantum resources — it is able to solve many modern problems.

Quantum mechanics imposes limitations on the accuracy of the measurements, which are expressed in the principle of uncertainty of Heisenberg. For measurements, this principle turns out that the accuracy increases linearly with measurement time. Classical methods of measuring these limits typically do not reach because of the noise: according to the Central limit theorem, the accuracy grows as a square root of the measurement time. However, Heisenberg accuracy — that is, quadratic acceleration in comparison with the classics — can be achieved by using quantum tricks such as compression and entanglement. But to support the involvement of a large system is experimentally difficult to date managed to confuse only a few tens of qubits, and the creation of squeezed light requires complex nonlinear elementssuch as nonlinear optical crystals or parametric oscillators.

There is, however, another quantum approach, which allows to reach linear growth measurement accuracy. It uses quantum coherence. It distinguishes the fact that he has only one coherent quantum system without creating confusion or complicated nonlinear interactions. This approach was proposed in 1995 a graduate of the MIPT Alexei Kitaev, there is a quantum algorithm the phase estimation, using coherence to measure physical quantities.

First quantum object that has coherence, is prepared in a special state that is sensitive to the parameters of the system under study. Then between the quantum object and the system interact, which sets the phase of the wave function of a quantum object. And then you can know you are interested in system settings, simply by reading the value of the phase. This non-invasive approach, it does not spoil the state of the system, which is important, for example, to biology. Using the algorithm of the phase estimation based on superconducting qubits, it is possible to measure a weak magnetic field. In this case, the magnetic field associated with superconducting ring as the qubit, modulating its energy and phase, and then use the microwave pulses can be considered state of the qubit, which depends on the magnetic field. In 2018, our group demonstrated the superiority of such a quantum sensor over the classical analogues. Due to the quantum coherence of the qubit, this method is superior in accuracy and SQUID-magnetometers, in which the field is also associated with the superconducting ring.

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