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Optical quantum gates that are more efficient

Photo of the vacuum chamber. Through the window into the vacuum chamber in the centre of the picture, one can see the holder for the mirrors of the resonator. Between the mirrors, ultracold atoms generate the interaction between the photons. Credit: Max Planck Society

Quantum computers of the future will not only be able to do exceptionally difficult computations, but they will also be able to connect to a network and share data securely. Quantum gates might theoretically be utilised in this manner. The problem is that until now, it hasn’t been able to implement them effectively. Now, researchers at the Max Planck Institute of Quantum Optics (MPQ) have achieved an important milestone by employing an advanced combination of approaches.

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There has been an ever-increasing leap in computer speed and power over the years. Using this development, new applications, such as those involving artificial intelligence, can be constantly discovered. However, with the current state of computer technology, making future advancements is becoming increasingly challenging. Due to the difficulty of these jobs, researchers are currently focusing their efforts on developing fresh and innovative concepts that may one day be applied to them. Quantum computers are one of these ideas.

Conventional microelectronic computers, on the other hand, are built on the combination of digital zeros and ones—the classical bits—as is the case here. Quantum computers use qubits, or “quantum bits,” as the building blocks of information storage. Bits’ quantum equivalents, qubits vary from one other in one essential way: qubits can take on any value between zero and one, while bits can only take on two fixed values or states, zero or one. In theory, this allows for the execution of multiple computing tasks at the same time, rather than one logical action at a time.

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The ability to communicate with optical qubits in a way that cannot be tapped

Researchers at the Max Planck Institute of Quantum Optics (MPQ) in Garching have found a variety of ways to physically apply the concept of qubits, explains Thomas Stolz. “Optical photons are one of them.” Dr. Stephan Dürr and MPQ director Prof. Dr. Gerhard Rempe’s study team also relied on light particles from the visible spectral region. Quantum computers use photons as information carriers because they have a low level of inter- and inter-environment interference, according to Stolz. A qubit’s survival is dependent on its ability to maintain its coherence in the face of external perturbations. Optical fibre, for example, can be used to carry photons large distances. “This makes them a very intriguing choice for establishing quantum networks,” adds Stolz: links of many quantum computers, via which encrypted data may be transported unconditionally securely—and dependably safeguarded against eavesdropping efforts.

Quant gates are the fundamental building blocks of a quantum computer and, by extension, a quantum network. They are similar to the logic gates used in conventional computers, but they are adapted to the qubits’ unique features. Trapped ions or superconducting materials are now the most advanced quantum gates for qubits,” says Stephan Dürr. “However, it is far more difficult to realise such an element with photons.” Because the benefit of weak contacts becomes a real negative in this situation. Because the light particles must be able to impact one another in order to process information, this is a prerequisite for information processing. In a publication published in the open-access journal Physical Review X, researchers from the MPQ have demonstrated how this can be done efficiently.

Attempts to create quantum gates that connect two photons have only been somewhat successful in previous attempts. They had a maximum efficiency of just 11%, which was a major problem for them. When many quantum gates are connected sequentially in a quantum network—which results in the loss of a substantial percentage of light particles, and hence the loss of data—then the problem becomes much more pronounced. A new record has been set by Stephan Dürr and his colleagues, who have achieved an optical two-qubit gate with an average efficiency of more than 40 percent—almost four times the previous record.

Experimentation in progress. The resonator reflects some of the photons based on the starting state of the qubits, which move in different directions. Rydberg states mediate an interaction between the photons along this path. In honour of the Max Planck Society

In a resonator, extremely cold atoms

By utilising non-linear components, Stolz claims that this success was built on. The MPQ team designed and built a custom experimental platform just for this experiment, and they have it set up in the lab. The researchers were able to build on their earlier work from 2016 and 2019 by publishing this new study. One of the outcomes of this research was the discovery that a cold, atomic gas with a few highly excited atoms can be used for photon information processing. When it comes to photons, “the atoms facilitate the required interaction,” Stolz says. When it comes to atom densities, though, earlier research has shown that the coded information is soon wiped out if they’re too dense. For this reason, scientists currently utilise an atomic gas of low density that has been cooled to 0.5 microkelvin, or 0.001 degree Celsius above absolute zero. As an additional amplifier for photon-photon interaction, we positioned the ultracold atoms between the optical resonator’s mirrors,” says Stolz.

For the experiment to be a resounding success, it had to process optical qubits in two stages: Initially, a control photon was inserted into the resonator and held there until further experimentation could take place. It was then that “the encounter took place,” says Stolz. Both photons finally exited the quantum gate, carrying the data that had been encoded in them. A second trick was employed by the physicists to make this work. This is based on Rydberg states, which are extremely high-energy electron excitations of gas atoms. Stolz argues that this leads the excited atom to grow enormously in the classical representation. Up to one micrometre in diameter, it is many thousand times larger than the average atom. When the atoms in the resonator are expanded in this manner, the photons are able to interact with each other strongly enough. This, however, just shifts the phase of the wave. It’s also broken into multiple pathways, which are later combined. The phase shift can only be used as a quantum gate due to the quantum mechanical interference that occurs during this superposition.

The end goal is to have quantum systems that are scalable

An extensive theoretical investigation preceded the experiment. The new research platform’s architecture has been specifically optimised by the MPQ team using a comprehensive theoretical model. Theoretical studies are underway to see if the researchers’ optical quantum gate might be made even more efficient in the future. A further objective of the research is to determine whether or not the quantum gate can be scaled up to bigger systems by simultaneously processing multiple qubits. Gerhard Rempe, the group’s director, says, “Our experiments so far have already proved that this is doable in theory.” For him, “our newly discovered information is extremely valuable in the creation of light-based quantum computers and networks.”

Further information: Thomas Stolz et al, Quantum-Logic Gate between Two Optical Photons with an Average Efficiency above 40%, Physical Review X (2022). DOI: 10.1103/PhysRevX.12.021035

Journal information: Physical Review X

Source: Max Planck Society

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