Scientists design first programmable quantum sensor to demonstrate quantum superiority
The University of Innsbruck in Austria has announced that two teams of physicists led by Peter Zoller and Thomas Monz have designed and tested the first programmable quantum sensor in the laboratory. Simply put, they apply quantum information processing techniques to measurement problems. This innovative approach promises to bring the precision of quantum sensors close to the limits set by the laws of nature.
The research results were published in the journals "Physical Review X" [1] and "Nature" [2].
Quantum variational optimization of atomic clocks
Atomic clocks are the best sensors ever built by mankind. Today, they are used in national standards bodies or in satellites for navigation systems. Scientists around the world are working to further optimize the accuracy of these clocks. Now, a research team led by Peter Zoller, a theoretical physicist at the University of Innsbruck in Austria and winner of the Micius Quantum Prize, has developed a new concept [1] that can be used to operate sensors with greater precision, regardless of sensor usage which technology platform.
Denis Vasilyev and Raphael Kaubrügger of the Institute of Quantum Optics and Quantum Information of the Austrian Academy of Sciences in Zoller's team said: "We answered the question of the accuracy of the sensor under existing control capabilities and showed how to implement it."
Using a method in quantum information processing called variational quantum algorithms, Zoller's team describes a quantum gate circuit that depends on free parameters. With an optimization routine, the sensor automatically finds the best settings for the best results.
"We're applying this technique to a problem in metrology—measurement science," Vasilyev and Kaubrügger explained. Zoller also said: "This is exciting because historically advances in atomic physics have been driven by metrology. , and that's where quantum information processing comes from. So, we've gone around in circles here."
The atomic clock uses Ramsey interferometry, a type of interferometry that first places atoms in a superposition of electron energy levels and then passes them through an optical cavity. Thus, the quantum superposition accumulates a measurable phase shift, which depends on the properties of the photons in the cavity.
Here, Zoller's team entangled 64 atoms and used them to create a better Ramsey interferometric sensor. They demonstrated the effectiveness of the method with an optical atomic clock, in which Ramsey interferometry of the phase of the atomic ensemble was used to correct the clock's laser frequency (Figure 1). Due to entanglement, the state of each atom cannot be described completely independently of the other atoms.
Figure 1 In atomic clocks, the input states act as the hands of the clock. The visualization shows the Wigner function of the variational optimization measure (white sphere) and the input state (green wedge).
Entangled atom sensors have been available before, and the standard approach is to use GHZ states. However, these states are optimal for sensing only under certain assumptions about a priori knowledge of the phase shift values. These states are only suitable for sensing under certain assumptions about a priori knowledge of the phase shift values. Therefore, Zoller's team proposes to use variational quantum circuits to improve and surpass the GHZ state. These circuits have a set of free parameters and replace the fixed quantum circuits used to implement quantum algorithms. Variational quantum circuits have internal parameters (such as the angle of rotation around a particular Bloch sphere axis) that can be optimized to perform a given task.
The researchers propose using two sets of variational quantum circuits to prepare entangled states for sensing and measure the parameter they want to sense (i.e. optical phase). They called these circuits the entanglement circuit and the decoding circuit, respectively (Figure 2).
Figure 2 In an optical atomic clock based on entangled atoms, when the atoms interact with photons in the clock's laser, the phase of the wave function describing the transitions of the atomic ensemble is shifted (physical process). This phase shift measurement is used to correct for fluctuations in the clock laser frequency. The researchers propose the use of variational quantum circuits to optimize the entanglement (parameter θ) and decoding (parameter ϕ) processes, resulting in greatly improved sensing performance.
The study shows that excellent performance can be achieved using "shallow" circuits composed of Ramsey interferometry and the quantum resources inherent in the atomic clock platform. With only a few layers of quantum circuits, they not only exceed the standard quantum limit (suitable for measurements made with uncorrelated atoms), but also come very close to the Heisenberg limit - the ultimate limit of sensitivity that a quantum system can achieve, and therefore a quantum sensor. ultimate limit.
Self-calibrating programmable quantum sensor
In another work [2], a research team led by Thomas Monz and Rainer Blatt went on to combine concepts from the field of quantum information processing with metrology and experimentally succeeded in realizing a programmable quantum sensor with performance close to Fundamental limits imposed by the laws of quantum mechanics.
They achieved this by using low-depth, parameterized quantum circuits to achieve optimal input states and measurement operators to perform sensing tasks in trapped ion experiments.
For 26 ions, the results are close to the fundamental sensing limit with a coefficient of 1.45 ± 0.01, which is superior to conventional spin compression with a coefficient of 1.87 ± 0.03.
Compared to traditional methods that do not use an entanglement protocol, their method reduces the mean number of times to reach a given Allan bias by a factor of 1.59 ± 0.06. The team further optimized quantum-classical feedback on the device to "self-calibrate" a programmable quantum sensor with comparable performance.
This capability suggests that this new generation of quantum sensors can be used without prior knowledge of the device or its noisy environment.
Fig.3 Measurement and feedback of variational quantum Ramsey interferometry circuit
Christian Marciniak, first author of the paper and from the Department of Experimental Physics at the University of Innsbruck, explains: "During the development of quantum computers, we have learned to create tailored entangled states. We are now using this knowledge to make better sensors ."
While both studies used variational quantum circuits, the simulations in the first study did not apply to all sensors, so a second approach was demonstrated in the second study: the researchers used A method to automatically optimize parameters in the case. "Similar to machine learning, programmable quantum computers act as high-precision sensors to automatically find their best patterns," said experimental physicist Thomas Feldker, one of the authors of the paper, describing the underlying mechanism.
The reason for making frequency measurements based on variational quantum computations on ion trap quantum computers is that the interactions used in linear ion traps are still relatively easy to simulate on classical computers, so the researchers were able to check on a supercomputer at the University of Innsbruck. necessary parameters. Although the experimental setup was not perfect, the results were surprisingly consistent with theoretical predictions.
Peter Zoller said: "Our concept makes it possible to demonstrate the advantages of quantum technology over classical computers on a practically relevant problem. We demonstrate a key component of quantum-enhanced atomic clocks using variational Ramsey interferometry. The next step is to run it in a dedicated atomic clock. To be able to demonstrate quantum superiority with programmable quantum sensors in the near future."
Ye Jun, a professor of the Department of Physics at the University of Colorado, an academician of the National Academy of Sciences, and a foreign academician of the Chinese Academy of Sciences, spoke highly of this. "I think it's very exciting work," said Jun Ye, whose algorithms may not be exactly the same, but he's excited to explore the basic ideas behind Zoller's paper. He also believes that this "quantum optimization" will drive new frontiers in quantum science.
In February of this year, Ye Jun's team developed the world's most accurate atomic clock, which measured a time difference of about one hundred billion billionths of a millimeter in height, which is consistent with the predictions of general relativity.
Link:
[1] https://journals.aps.org/prx/abstract/10.1103/PhysRevX.11.041045
[2] https://www.nature.com/articles/s41586-022-04435-4
[3] https://physics.aps.org/articles/v14/172#
[4] https://www.inverse.com/innovation/quantum-atomic-clock
[5]https://www.uibk.ac.at/newsroom/quantum-sensors-measuring-even-more-precisely.html.en
