Quantum Optics in Topological Waveguides

Artistic view of the atoms that work as qubits close to a topological waveguide. (Source: MPQ)

Intensive research is being carried out on quantum simulators: they promise to precisely calculate the properties of complex quantum systems, when conven­tional and even super­computers fail. In a cooperative project, theorists from the the Max Planck Institute of Quantum Optics in Garching anf the Consejo Superior de Inves­tigaciones Científicas (CSIC) have now developed a new toolbox for quantum simulators. It uses the principle of topology to allow quantum bits, for example individual atoms, to communicate with each other via topo­logical radio channels.

The radio channels are provided by a light field that travels in waveguide in a robust manner with the help of topology. The concept offers room for completely new ideas, ranging from basic research to quantum information. “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!” Together with Cirac and two Spanish colleagues from the Instituto de Ciencias de Materiales de Madrid, he has intro­duced a completely new toolbox to photonics.

One possible application is the quantum simu­lation. If one wants to calculate the behaviour of a quantum system as accurately as possible on a conven­tional computer, the necessary computing power doubles with each new quantum particle in the system. Because of this mathe­matical avalanche, even relatively small quantum systems consisting of just a few dozen particles overrun the performance of even conventional super­computers. For this reason, Richard Feynman had the idea decades ago to simulate the behaviour of a quantum system with the help of another quantum system. In principle, such a quantum simulator is a specialized quantum computer whose individual quantum bits can be easily controlled from the outside – in contrast to the rather inac­cessible quantum system whose behaviour it is supposed to simulate.

Such quantum simu­lators have been the subject of intensive research for many years. For example, they promise to provide a better understanding of material properties such as super­conductivity or complex magnetism. They also play an important role at the Institute in Garching. For example, a simulator can consist of a cloud of ultracold atoms trapped in a spatial lattice of laser light. If these quantum bits are to interact with each other, they do so by exchanging light quanta, photons. However, an atom normally emits such a photon in some random direction. It would be much more efficient for quantum simulations if the qubit could target its photon directly to its next or next but one neighbour.

González-Tudela and his team have now developed a theo­retical principle that enables such a targeted photon radio between atoms. “We have to pack the qubits and photons into a waveguide,” explains the theorist. However, how do you wire an ensemble of atoms floating in a light grid in space with such waveguides and make them talk in a robust way? The answer of the four theorists is: with extremely tricky light. The trick is essen­tially to transfer the mathe­matical concept of topology from solid state physics to photonics. In solid state physics, it has triggered a real hype in recent years because it can produce completely new, previously unknown material properties. In principle, the question is how many holes a geometric body has. A coffee cup, for example, has a hole in its handle just like a doughnut ring in its center, and both have thus the topo­logical number one. The consequence: from a purely geometric point of view, the cup and donut can easily be trans­formed into each other. On the other hand, violent topological resistance is encoun­tered when a one-hole donut is to be transformed into a three-hole pretzel.

In physics, this hole number rule has the conse­quence that the topology can enormously stabilize certain physical properties against disturbances. And this leads to the second major challenge in quantum infor­mation and thus quantum simulation: ubi­quitous distur­bances cause the highly sensitive quantum information to decay rapidly. “This decoherence is the biggest problem of quantum information,” says González-Tudela. The capti­vating properties of topology soon led clever minds to the conclusion that the sensitive quantum bits could be packaged in physical systems with such topological properties. This is being researched in solid state physics, for example, and large companies such as Microsoft are also investing heavily in this research.

González-Tudela and his colleagues have now devised a toolbox with which such topo­logical concepts can be transferred into photonics. Some systems, such as ultracold atoms in light grids, are already very advanced in their controlla­bility. They therefore offer many possi­bilities for quantum simu­lation. The toolbox of the four theorists opens a new space for many creative ideas. Simply put, it consists of a set of quantum bits, for example single atoms arranged in a line. They can interact with a cleverly constructed, linear light bath that behaves like the waveguide the theo­retical physicists were looking for. If one now mani­pulates the various adjusting screws of the system, the quantum bits can exchange photons as desired via this waveguide. But not only that: For example, a qubit can send its information in one direction, but remain completely dark in the opposite direction. Such inter­actions are extremely difficult to be produced in the micro world of atoms.

Thus the toolbox of the four theorists offers many new possi­bilities to let quantum bits communicate with each other. This is exactly what future quantum simulators need. The concept is also universal: it can also be realized in some quantum systems composed by many qubits that are currently being researched. The new work of the four theorists could become the nucleus for completely new ideas, ranging from pure basic research to quantum information. (Source: MPQ)

Reference: M. Bello et al.: Unconventional quantum optics in topological waveguide QED, Sci. Adv. 5, eaaw0297 (2019); DOI: 10.1126/sciadv.aaw0297

Link: Theory Division, Max Planck Institute of Quantum Optics, Garching, Germany

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