More efficient optical quantum gates – Verve times

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

For decades, computers have been getting faster and more powerful with each new generation. This development makes it possible to continuously open up new applications, for example in systems with artificial intelligence. But with the established computer technology, it is becoming increasingly difficult to make further progress. For this reason, researchers are now turning to alternative, completely new concepts that could be used in the future for 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 basic units for encoding and processing information. They are the counterparts of bits in the quantum world, but differ from them in one crucial characteristic: qubits can take not only two fixed values ​​or states such as zero or one, but also all values ​​in between. In principle, this offers the possibility of executing many calculation processes simultaneously instead of processing one logical operation after another.

Tap-proof communication with optical qubits

“There are several ways to physically implement the concept of qubits,” said Thomas Stolz, who has researched the fundamentals of quantum computers at the Max Planck Institute of Quantum Optics (MPQ) in Garching. “One of them is optical photons.” And in their research, Stolz and his colleagues relied on the team led by Dr. Stephan Dürr and MPQ Director Prof. dr. Gerhard Rempe also noted such light particles from the visible spectral range. “An advantage of photons as information carriers in a quantum computer is their low interaction with each other and with the environment,” explains Stolz. “This prevents the cohesion, which is necessary for the existence of qubits, from being quickly destroyed by external perturbations.” In addition, photons can be transported over long distances, for example in an optical fiber. “This makes them a particularly promising candidate for building quantum networks,” says Stolz: connections of multiple quantum computers over which encrypted data can be sent unconditionally securely – and reliably protected against eavesdropping attempts.

The basic components of a quantum computer – and therefore also of a quantum network – are quantum gates. They function similarly to the logic gates used in conventional calculators, but are tuned to the special properties of qubits. “Quantum gates for qubits implemented in trapped ions or superconducting materials are currently the most technically advanced,” explains Stephan Dürr. “However, realizing such an element with photons is much more challenging.” Because in this case, the advantage of weak interactions turns into a tangible disadvantage. Because in order to process information, the light particles must be able to influence each other. The researchers of the MPQ have shown how this can be done effectively in a paper, which has now been published in the open access journal Physical Assessment X†

Previous attempts to realize quantum gates connecting two photons have only partially succeeded. They were mainly affected by their low efficiency of at most 11%. This means that much of the light particles, and therefore of the data, are lost as they are processed in the quantum system – a shortcoming, especially when multiple quantum gates have to be connected in sequence in a quantum network and losses add up as a result. “In contrast, for the first time, we have succeeded in realizing a two-qubit optical gate with an average efficiency of more than 40%,” reports Stephan Dürr – almost four times the previous record.

Ultracold atoms in a resonator

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

This led to the success of the experiment, where the quantum gate processed the optical qubits in two steps: a first photon, called a control photon, was introduced into the resonator and stored there. Then a second photon, called the target photon, entered the setup and was reflected off the resonator mirrors — “the moment the interaction occurred,” Stolz points out. Eventually, both photons exited the quantum gate — along with the information imprinted on them. To make this work, the physicists used a different trick. This is based on electron excitations of the gas atoms to very high energy levels, the so-called Rydberg states. “This causes the excited atom – in the classical image – to expand enormously,” explains Stolz. It reaches a radius of up to one micrometer – several thousand times the normal size of the atom. The atoms in the resonator that are blown up in this way then ensure that the photons can influence each other sufficiently. However, this initially only causes a phase shift. In addition, the light is split into several paths that are later superimposed. Only the quantum mechanical interference during this superposition turns the phase shift into a quantum gate.

The goal: scalable quantum systems

The experiment was preceded by an extensive theoretical analysis. The MPQ team had specially developed a comprehensive theoretical model to optimize the design process of the new research platform. Closer theoretical research shows how the researchers hope to improve the efficiency of their optical quantum gate in the future. They also want to discover how the quantum gate can be scaled up to larger systems, by processing multiple qubits simultaneously. “Our experiments to date have already shown that this is in principle possible,” says Gerhard Rempe, director of the group. He is confident: “Our new findings will be of great use in the development of light-based quantum computers and quantum networks.”

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