More efficient optical quantum gates

Photo of the vacuum chamber. Through the window of the vacuum chamber in the center of the image, one can see the support of the mirrors of the resonator. Between the mirrors, ultracold atoms generate the interaction between the photons. Credit: Max Planck Society

Future quantum computers should not only solve particularly delicate computing tasks, but also be connected to a network for the secure exchange of data. In principle, quantum gates could be used for these purposes. But so far it has not been possible to carry them out with sufficient efficiency. Using a sophisticated combination of several techniques, researchers at the Max Planck Institute for Quantum Optics (MPQ) have taken a major step towards overcoming this hurdle.

For decades, computers have gotten faster and more powerful with each new generation. This development makes it possible to constantly open up new applications, for example in artificial intelligence systems. But further progress is becoming increasingly difficult to achieve with established computer technology. This is why researchers are now turning to completely new alternative concepts that could be used in the future for certain particularly difficult computing tasks. These concepts include quantum computers.

Their function is not based on the combination of digital zeros and ones – the classic bits – as is the case with conventional microelectronic computers. Instead, a quantum computer uses quantum bits, or qubits for short, as the basic units for encoding and processing information. They are the counterparts of bits in the quantum world, but differ from them in one crucial feature: qubits can not only take on two fixed values ​​or states such as zero or one, but also all values ​​in between. In principle, this offers the possibility of carrying out several calculation processes simultaneously instead of processing one logical operation after another.

Eavesdropping communication with optical qubits

“There are different ways to physically implement the concept of qubits,” says Thomas Stolz, who undertook research into the fundamentals of quantum computers at the Max Planck Institute for Quantum Optics (MPQ) in Garching. “One of them is optical photons.” And in their research, Stolz and his colleagues from the team led by Dr. Stephan Dürr and MPQ director Prof. Dr. Gerhard Rempe also relied on these luminous particles from the visible spectral range. “One of the advantages of photons as information carriers in a quantum computer is their weak interaction with each other and with the environment,” Stolz explains. “This prevents the coherence, necessary for the existence of qubits, from being quickly destroyed by external disturbances.” In addition, photons can be transported over long distances, for example in an optical fiber. “This makes it a particularly promising candidate for building quantum networks,” says Stolz: connections of multiple quantum computers over which encrypted data can be transmitted securely and reliably against eavesdropping attempts.

The basic components of a quantum computer – and therefore also of a quantum network – are quantum gates. They correspond in their mode of operation to the logic gates used in classical computer machines, but are adapted to the particular properties of qubits. “Qubit quantum gates implemented in trapped ions or superconducting materials are currently the most technically advanced,” explains Stephan Dürr. “However, making such an element with photons is much more difficult.” Because in this case, the advantage of weak interactions turns into a tangible disadvantage. Because, in order to be able to process information, the light particles must be able to influence each other. MPQ researchers have shown how to achieve this effectively in an article, which has just been published in the open access journal Physical examination X.

Previous attempts to make quantum gates that link two photons together have been only partially successful. They suffered above all from their low yield of, at best, 11%. This means that a large part of the light particles, and thus also data, is lost during processing in the quantum system – a shortcoming, especially when many quantum gates have to be connected consecutively in a quantum network and the losses s ‘add up as a result. “On the other hand, we have succeeded for the first time in realizing a two-qubit optical gate with an average efficiency of more than 40%”, reports Stephan Dürr, almost four times the previous record.

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

Ultracold atoms in a resonator

“The very basis of this success was the use of non-linear components,” explains Stolz. They are contained in a new experimental platform that the MPQ team developed specifically for the experiment and installed in the lab. In doing so, the researchers were able to build on the experience of previous work they had published in 2016 and 2019. One of the conclusions was that it is useful for information processing with photons of use a cold atomic gas in which some atoms are very energetically excited. “Atoms mediate the necessary interaction between photons,” says Stolz. “However, previous work has also shown that the density of atoms should not be too high, otherwise the encoded information is quickly erased by collisions between atoms.” Therefore, the researchers now used a low-density atomic gas, which they cooled to a temperature of 0.5 microkelvin, or half a millionth of a degree above absolute zero at minus 273.15 degrees Celsius. “As an additional amplifier for the interaction between photons, we placed the ultracold atoms between the mirrors of an optical resonator,” Stolz reports.

This led to the success of the experiment, in which the quantum gate processed the optical qubits in two stages: a first photon, called the control photon, was introduced into the resonator and stored there. Then a second photon, called the target photon, entered the pattern and was reflected by the resonator mirrors – “the moment the interaction took place”, Stolz points out. Finally, the two photons left the quantum gate, with the information imprinted on them. To make it work, physicists used another trick. This is based on electronic excitations of gas atoms to very high energy levels, called Rydberg states. “This causes the excited atom to expand dramatically – in the classic image -,” says Stolz. It reaches a radius of up to one micrometer, which is several thousand times the normal size of the atom. The atoms of the resonator thus inflated then allow the photons to have a sufficiently strong effect on each other. This, however, initially only causes a phase shift. In addition, the light is split into different paths which then overlap. Only the quantum mechanical interference during this superposition transforms the phase shift into a quantum gate.

The goal: scalable quantum systems

The experiment was preceded by an elaborate theoretical analysis. The MPQ team had specially developed a complete theoretical model to optimize the design process of the new research platform. Further theoretical investigations show how the researchers hope to improve the efficiency of their optical quantum gate in the future. They also want to find out how the quantum gate can be extended to larger systems, by processing many qubits simultaneously. “Our experiences so far have already shown that this is possible in principle”, says Gerhard Rempe, Group Director. He is convinced: “Our new discoveries will be of great use in the development of quantum computers and light-based quantum networks.”


A new type of quantum computer


More information:

Thomas Stolz et al, Quantum logic gate between two optical photons with an average efficiency greater than 40%, Physical examination X (2022). DOI: 10.1103/PhysRevX.12.021035

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Max Planck Society

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