More efficient light gates

More efficiency for optical quantum gates

Vacuum room photo. Through the window to the discharge chamber in the center of the picture one can see the holder of resonant mirrors. Between the mirrors, the ultra-cold atoms generate the interaction of photons. Credit: Max Planck Society

Quantum computers of the future are expected not only to solve particularly challenging computing tasks, but also to be connected to a network for the secure exchange of data. In principle, quantum gates can be used for these purposes. But so far, it has not been possible to achieve them efficiently enough. Through a sophisticated combination of several techniques, researchers at the Max Planck Institute for Quantum Optics (MPQ) have now taken a major step toward overcoming this hurdle.

For decades, computers have been getting faster and more powerful with each of them New generation. This development makes it possible to constantly open up new applications, for example in systems with artificial intelligence. But further progress is becoming increasingly difficult to achieve with well-established computer technology. For this reason, researchers are now setting their sights on alternative and entirely new concepts that could be used in the future for some particularly challenging computing tasks. These concepts include quantum computers.

Its function is not based on combining digital zeros and ones – classical bits – as is the case with traditional microelectronic computers. Instead, a quantum computer uses Quantum bitqubits, or qubits for short, as basic units for encoding and processing information. They are analogues of qubits in the quantum world – but differ from them in one crucial feature: qubits cannot assume two constant values ​​or states such as zero or one, but also any values ​​in between. In principle, this provides the possibility to perform several computing operations simultaneously instead of processing one logical operation after another.

Click-proof optical qubit connection

“There are different approaches to the physical application of the concept of qubits,” says Thomas Stolz, who has done research on the basics of quantum computers at the Max Planck Institute for Quantum Optics (MPQ) in Garching. “One of them is the photons of light.” In their research, Stolz and colleagues on the team led by Dr. Stefan Dorr and MPQ Director Prof. Dr. Gerhard Remp also relied on such light particles from the visible spectral range. “One advantage of photons as carriers of information in a quantum computer is that they have a lower interaction with each other and with the environment,” explains Stolz. “This prevents coherence, which is essential to the existence of a qubit, from being rapidly destroyed by external disturbances.” In addition, photons can be transmitted over long distances, for example in optical fibers. This makes them an especially promising candidate for building quantum networks: connections of many quantum computers through which encrypted data can be safely transmitted unconditionally — and reliably protected against eavesdropping attempts, says Stolz.

The basic components of a quantum computer – and thus also of a quantum network – are quantum gates. Corresponds in the way it works with Logic gates, logic gates They are used in traditional computing machines, but are designed according to the special properties of qubits. “Quantum gates of qubits applied in trapped ions or superconducting materials are currently the most technologically advanced,” explains Stefan. “However, realizing such an element with photons presents a much greater challenge.” Because in this case, the advantage of weak interactions turns into a tangible disadvantage. In order to be able to process information, the particles of light must be able to influence each other. MPQ researchers show how this can be effectively achieved in a paper, which has now been published in the open access journal X . physical review.

Previous attempts to create quantum gates that link two photons together were only partially successful. They mainly suffered from their low efficiency, at best, 11%. This means that a large part of the light particles, and thus also the data, is lost during its processing in the quantum system – a disadvantage especially when multiple Quantum Gates They must be connected in series in a quantum network and losses increase as a result. “In contrast, for the first time we have achieved a two-qubit optical gate with an average efficiency of over 40%,” says Stefan Dorr — nearly four times the previous record.

More efficiency for optical quantum gates

Experimental setup. Depending on the initial state of the qubits, photons travel along different paths, some of which are reflected by the resonator. In this path, the photons experience an interaction mediated by Rydberg states. Credit: Max Planck Society

Very cold atoms in a resonator

“The basis for this success has been the use of non-linear components,” Stolz explains. It was included in a new experimental platform developed by the team at MPQ specifically for the experiment and installed in the lab. By doing so, the researchers were able to build on their experiences from previous work they published in 2016 and 2019. One conclusion from this was that it is useful to process information with photons to use a cold atomic gas with a small number of atoms. Actively very excited. “The atoms mediate the necessary interaction between the photons,” explains Stolz. “However, previous work has also shown that the density of atoms should not be too high, or else the encoded information is quickly erased by collisions between atoms.” So, the researchers have now used a low-density atomic gas, and cooled it to a temperature of 0.5 microKelvin – half a million degrees above absolute zero at minus 273.15 degrees Celsius. “As an additional amplifier for the interaction of photons, we set cold particles between mirrors of an optical resonator,” Stolz reports.

This led to the success of the experiment, as the quantum gate processed the optical qubits in two steps: the first Photon, called the control photon, was inserted into the resonator and stored there. Next, a second photon, called the target photon, entered the formation and was reflected from the resonator’s mirrors – “the moment the interaction occurred,” Stolz confirms. Finally, both photons left the quantum gate—along with the information printed on them. For this to work, the physicists used another trick. This is based on the electron excitation of gas atoms to very high energy levels, called Rydberg states. “This causes the excited atom, in the classical picture, to expand tremendously,” Stolz explains. Its radius reaches one micrometer – several thousand times the normal size of an atom. The resonant atoms inflated in this way make it possible for the photons to have a sufficiently strong effect on each other. This, however, only initially causes a phase shift to occur. In addition, the light is divided into different paths that are subsequently superimposed. Only quantum mechanical interference through this superposition converts the phase shift into a quantum gate.

The goal: scalable quantitative systems

The experiment was preceded by a detailed theoretical analysis. In particular, the MPQ team developed a comprehensive theoretical model to improve the design process of the new search platform. Additional theoretical investigations show the ways the researchers hope to improve the efficiency of their optical quantum gate in the future. They also want to know how the quantum gate can be expanded to larger systems — by manipulating many qubits simultaneously. “Our experiments so far have shown that this is possible in principle,” says Gerhard Rembe, group director. He is convinced: “Our new results will be of great use in the development of quantum computers and quantum networks.”

A new kind of quantum computer

more information:
Thomas Stolz et al., A quantum logic gate between two optical photons with average efficiency above 40%, X . physical review (2022). DOI: 10.1103/ PhysRevX.12.021035

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Max Planck Society

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