Quantum technology achieves unprecedented control over captured light

Quantum technology researchers at Chalmers University of Technology have successfully developed a technique to control quantum states of light in a three-dimensional cavity. In addition to creating previously known states, researchers are the first to demonstrate the long-sought cubic phase state. This breakthrough is an important step towards effective error correction in quantum computers.

“We have shown that our technology is on par with the best in the world,” says Simone Gasparinetti, who leads an experimental quantum physics research group at Chalmers and one of the lead authors of the study.

Just as a classical computer is based on bits that can take on the value 0 or 1, the most common method of building a quantum computer uses a similar approach. Quantum mechanical systems with two different quantum states, called quantum bits (qubits), are used as building blocks. One of the quantum states is assigned the value 0 and the other the value 1. However, due to the superposition quantum mechanical state, qubits can assume both 0 and 1 states simultaneously, allowing a quantum computer to process huge volumes of data with the ability to solve problems far beyond the reach of today’s supercomputers.

First time for cubic phase state

A major obstacle to achieving a practically useful quantum computer is that the quantum systems used to encode information are subject to noise and interference, which causes errors. Correcting these errors is a major challenge in the development of quantum computers. One promising approach is to replace qubits with resonators – quantum systems that, instead of having just two defined states, have a very large number of them. These states can be compared to a guitar string, which can vibrate in different ways. The method is called continuous variable quantum computation and makes it possible to code the values ​​1 and 0 in several quantum mechanical states of a resonator. However, controlling the states of a resonator is a challenge faced by quantum researchers around the world. And Chalmers’ results provide a way to do that. 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 cubic phase state, a state previously described only in theory.

“The cubic phase state is something that many quantum researchers have been trying to create in practice for twenty years. The fact that we managed to do this for the first time is a demonstration of how well our technique works, but the most important advance is that there are so many states of varying complexity and we have found a technique who can create any of them,” says Marina Kudra, a doctoral student in the Department of Microtechnology and Nanosciences and lead author of the study.

Improved door speed

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

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

“Drastically improving the speed of our SNAP gates allowed us to mitigate the effects of decoherence in our quantum controller, advancing this technology. We have shown that we have complete control of our quantum mechanical system,” says Simone Gasparinetti.

Or, to put it more poetically:

“I captured the light in a place where it flourishes and shaped it into very beautiful shapes”, explains 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 already shown how the aluminum cavity is created by first milling it and then making it extremely clean by methods such as heating it to 500 degrees centigrade and washing it with acid and solvent. The electronics that apply the electromagnetic gates to the cavity were developed in collaboration with the Swedish company Intermodulation Products.

Research under the WACQT research program

The research was conducted at Chalmers as part of the Wallenberg Center for Quantum Technology (WACQT), a comprehensive research program with the goal of making Swedish research and industry leaders in quantum technology. The initiative is led by Professor Per Delsing and a main objective is to develop a quantum computer.

“At Chalmers, we have the full stack to build 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-scale quantum computers, and our results are proof for our culture and ways of working,” says Per Delsing.

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