Aug 5 2019

**Rigorous studies are being performed on quantum simulators, which have the potential to accurately measure the characteristics of intricate quantum systems, when traditional and even supercomputers prove ineffective.**

In the cooperative project, physicists from the Max Planck Institute of Quantum Optics and Consejo Superior de Investigaciones Científicas (CSIC) have come up with a new toolbox for quantum simulators, where quantum bits can exchange light quanta through a waveguide. The study has been reported in Science Advances.

The toolbox utilizes the Nobel prize-winning principle of topology to enable quantum bits, for instance, individual atoms, to interact with one another through “topological radio channels.” These “radio channels” are offered by a light field that travels in the waveguide in an intense way with the aid of topology. The theory provides room for completely novel concepts, spanning from fundamental research to quantum data.

**“***How can we make two distant quantum bits 'talk' to each other*?" asks Alejandro González-Tudela and states: “*This is an essential challenge in the field of quantum information and simulation*!”

The theoretical physicist is currently a permanent researcher at the Instituto de Física Fundamental IFF-CSIC in Madrid. Earlier, he served as a postdoctoral fellow in the department of Ignacio Cirac and as a director at the Max Planck Institute of Quantum Optics in Garching.

Along with Cirac and two Spanish coworkers from the Instituto de Ciencias de Materiales de Madrid, González-Tudela has currently reported a scientific paper that introduces an entirely new toolbox to photonics. A branch of physics, photonics deals with the communication between light and matter and their technical applications.

The so-called quantum simulation is one potential application, which goes back to a concept of Richard Feynman, a well-known US Nobel Prize winner. If the behavior of a quantum system needs to be calculated as precisely as possible on a traditional computer, the required computing power will become two-fold with every new quantum particle present in the system. Due to this mathematical avalanche, even comparatively tiny quantum systems containing only a few dozen particles will outpace the performance of traditional supercomputers.

Owing to this reason, Feynman developed the concept many years ago to imitate the behavior of a quantum system using another quantum system. Theoretically, a quantum simulator like that is a unique quantum computer whose separate quantum bits can be effortlessly regulated from the outside, unlike the relatively inaccessible quantum system whose unique behavior it is expected to simulate.

Such kinds of quantum simulators have been the topic of considerable research for a number of years. For instance, quantum simulators have the potential to offer an improved understanding of material characteristics like complex magnetism or superconductivity. In addition, they have a significant role to play at the Institute in Garching. For instance, a quantum simulator can comprise a cloud of ultra-cold atoms confined in a spatial lattice of laser light.

When these quantum bits, also known as qubits for short, communicate with one another, they do so by switching photons, which are light quanta. Conversely, an atom typically discharges such a photon in a certain arbitrary direction. For quantum simulations, it would be relatively more efficient if the qubit can directly target its photon to its next or next but one neighbor.

Along with his team, González-Tudela has currently come up with a hypothetical principle that allows that targeted “photon radio” between atoms.

*We have to pack the qubits and photons into a waveguide.*

Alejandro González-Tudela, Theoretical Physicist and Researcher, Instituto de Física Fundamental, CSIC

Conversely, how does one “wire’ a group of atoms that are floating in a light grid in space with these waveguides and make them interact in a powerful manner? According to the four theorists, the answer is with extremely tricky light.

The trick is basically to transmit the mathematical idea of topology from solid-state physics to photonics. In the case of solid-state physics, it has generated a real hype in the recent past. This is because it can create completely new material characteristics that were not known before.

Eerier in 2016, the Nobel Prize in Physics was awarded to three British physicists David Thouless, Michael Kosterlitz, and Duncan Haldane for effectively introducing topological ideas theories to solid-state physics. In theory, the query is how many holes are present in a geometric body? For instance, a coffee cup has a hole in its handle similar to a doughnut ring in its middle, and both can thus be said to have the topological number one.

The outcome from a purely geometric standpoint is that both the donut and the cup can be effortlessly converted into each other. However, intense topological resistance is faced when a single-hole donut has to be changed into a three-hole pretzel.

In the physics field, this hole number rule means that the topology can considerably stabilize specific physical characteristics against disturbances. This leads to another significant challenge in quantum data and thus quantum simulation—the extremely sensitive quantum data decays quickly due to ubiquitous disturbances.

*This so-called decoherence is the biggest problem of quantum information.*

Alejandro González-Tudela, Theoretical Physicist and Researcher, Instituto de Física Fundamental, CSIC

The fascinating characteristics of topology soon led ingenious minds to conclude that the highly sensitive quantum bits can possibly be integrated into physical systems with such kinds of topological characteristics. For instance, this is being studied in solid-state physics, and large companies like Microsoft are also investing considerably in this study.

González-Tudela, along with his three co-authors, has currently created a toolbox using which such topological theories can be easily transferred into photonics. Certain systems, like ultra-cold atoms in light grids, are already quite advanced in their controllability. Hence, they provide several opportunities for quantum simulation.

The new toolbox developed by the four theorists paves the way for several creative concepts. In other words, it includes a series of quantum bits, for instance, single atoms organized in a line. They can communicate with an ingeniously designed, linear “light bath” that acts similar to the waveguide being searched by the theoretical physicists.

If the numerous adjusting screws of the system are exploited, the quantum bits can exchange photons as required through this waveguide. Besides that, for instance, a qubit is capable of sending its data in one direction, but stays completely dark in the reverse direction. Interactions like that cannot be produced easily in the micro realm of atoms.

Therefore, the toolbox developed by the four theorists offers several new possibilities to enable quantum bits to interact with one another. This is precisely what is needed for upcoming quantum simulators. The idea is also universal—it can also be achieved in certain quantum systems made up of several qubits that are presently being studied.

The latest study performed by the four theorists is expected to become the center for complete novel concepts, spanning from pure fundamental research to quantum data.

Source: https://www.mpq.mpg.de/