Researchers develop technique to control quantum states of light in a 3D cavity

Quantum technology researchers at Chalmers University of Technology have succeeded in developing a technique for controlling quantum states of light in a three-dimensional cavity. In addition to generating already known states, the researchers are demonstrating the long-sought cubic phase state for the first time. The breakthrough is an important step towards efficient error correction in quantum computers.

“We have shown that our technology can compete with the best in the world”, says Simone Gasparinetti, head of a research group in experimental quantum physics at Chalmers and one of the lead authors of the study.

Just as a conventional computer is based on bits that can take the value 0 or 1, the most common way to build a quantum computer uses a similar approach. Quantum mechanical systems with two different quantum states, so-called quantum bits (qubits), serve as building blocks. One of the quantum states is assigned the value 0 and the other the value 1. However, due to the quantum mechanical state of superposition, qubits can occupy both states 0 and 1 at the same time, which allows a quantum computer to process huge amounts of data and the ability to solve problems far beyond the reach of today’s supercomputers.

For the first time ever for the cubic phase state

A major obstacle to realizing a practical quantum computer is that the quantum systems used to encode the information are prone to noise and interference, leading to errors. Correcting these errors is a key challenge in the development of quantum computers. A promising approach is to replace qubits with resonators – quantum systems that have many of them instead of just two defined states. These states can be compared to a guitar string that can vibrate in many different ways. The method is called Quantum computing with continuous variables and allows the values ​​1 and 0 to be encoded in several quantum mechanical states of a resonator.

However, controlling the states of a resonator is a challenge that quantum researchers around the world are grappling with. And the Chalmers results offer an opportunity to do so. The technique developed at Chalmers allows researchers to generate virtually all previously demonstrated quantum states of light, such as Schrödinger’s cat or Gottesman-Kitaev-Preskill (GKP) states, and the cubic phase state, a condition that has so far only been described theoretically.

“The cubic phase state is something that many quantum researchers have been trying to create in practice for twenty years. Being able to do this for the first time is a testament to how well our technique works, but most importantly.” The benefit is that there are so many states of varying complexity, and we found a technique that can create any of them “, says Marina Kudra, a PhD student at the Institute of Microtechnology and Nanosciences and lead author of the study.

Gate speed improvement

The resonator is a three-dimensional superconducting aluminum cavity. Complex superpositions of photons trapped in the resonator are generated by interaction with a secondary superconducting circuit.

The quantum mechanical properties of the photons are controlled by applying a series of electromagnetic pulses called gates. The researchers first succeeded in using an algorithm to optimize a specific sequence of simple displacement gates and complex SNAP gates in order to generate the state of the photons. When the complex gates proved to be too long, the researchers found a way to shorten them using optimal control methods to optimize the electromagnetic pulses.

“Dramatically improving the speed of our SNAP gates allowed us to mitigate the effects of decoherence in our quantum controller and take this technology a step forward. We have shown that we have full control over our quantum mechanical system.” says Simone Gasparinetti.

Or to put it more poetically:

“I’ve captured light in a place where it thrives and turned it into really beautiful forms.” says Marina Kudra.

Achieving this result also depended on the high quality of the physical system (the aluminum resonator itself and the superconducting circuit). Marina Kudra has previously shown how the aluminum cavity is created by first milling it and then making it extremely clean using methods including heating to 500 degrees Celsius and acid and solvent washing. The electronics that apply the electromagnetic gates to the resonator were developed in cooperation with the Swedish company Intermodulation Products.

Research Part of the WACQT research program

The research was carried out in Chalmers as part of the Wallenberg Center for Quantum Technology (WACQT), a comprehensive research program that aims to make Swedish research and industry leaders in quantum technology. The initiative is led by Professor Per Delsing and a key goal is to develop a quantum computer.

“At Chalmers, we have the complete stack for building a quantum computer, from theory to experiment, all under one roof. Solving the error correction challenge is a major bottleneck in the development of large quantum computers, and our results are proof of that our culture and way of working”, says Per Delsing.

• The item Robust preparation of Wigner negative states with optimized SNAP displacement sequences was published in the magazine PRX Quantum and was written by Marina Kudra, Mikael Kervinen, Ingrid Strandberg, Shahnawaz Ahmed, Marco Scigliuzzo, Amr Osman, Daniel Pérez Lozano, Mats O. Tholén, Riccardo Borgani, David B. Haviland, Giulia Ferrini, Jonas Bylander, Anton Frisk Kockum, Fernando Quijandria, Per Delsing and Simone Gasparinetti.
• The researchers work at Chalmers University of Technology.

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