Exploring uncharted territories with quantum devices

May 24, 2024

(News from Nanowerk) Many modern quantum devices rely on collections of qubits, also called spins. These quantum bits only have two energy levels, “0” and “1”. However, unlike classical bits, qubits can exist in superpositions, which means they can be in a combination of states “0” and “1” at the same time. Spins in real devices also interact with light and vibrations called bosons, which greatly complicates calculations.

In a new publication in Physical inspection letters (“Fast Quantum State Preparation and Bath Dynamics Using Non-Gaussian Variational Ansatz and Optimal Quantum Control”) Amsterdam researchers demonstrate a way to describe spin-boson systems and use it to efficiently configure quantum devices in a desired state.

Quantum devices exploit the bizarre behavior of quantum particles to perform tasks beyond the capabilities of “classical” machines, including quantum computing, simulation, quantum sensing, quantum communications, and quantum metrology. These devices can take many forms, such as a collection of superconducting circuits or a network of atoms or ions held in place by lasers or electric fields.

Regardless of their physical implementation, quantum devices are usually described simplistically as a collection of interacting two-level quantum bits or spins. However, these spins also interact with other elements in their environment, such as light in superconducting circuits or oscillations in a lattice of atoms or ions. Examples of bosons are light particles (photons) and lattice vibrational modes (phonons).

Unlike spins, which have only two possible energy levels (“0” or “1”), the number of levels of each boson is infinite. As a result, there are very few computational tools for describing boson-coupled spins.

In their new work, physicists Liam Bond, Arghavan Safavi-Naini and Jiří Minář from the University of Amsterdam, QuSoft and Centrum Wiskunde & Informatica get around this limitation by describing systems composed of spins and bosons using so-called non-Gaussian states. Every non-Gaussian state is a combination (superposition) of much simpler Gaussian states. Spin (blue ball with arrow) interacts with surrounding bosons described by non-Gaussian states – a new computational method that allows us to precisely describe what is happening inside quantum devices. (Photo: Jiří Minář)

Each blue-red pattern in the image above represents a possible quantum state of the spin-boson system. “The Gaussian state would look like a plain red circle without any interesting blue-red patterns,” explains PhD student Liam Bond. An example of the Gaussian state is laser light, in which all light waves are perfectly synchronized.

“If we take many of these Gaussian states and start superimposing them (so that they form a superposition), these beautifully complex patterns emerge. We were particularly excited because these non-Gaussian states allow us to retain much of the powerful mathematical machinery that exists for Gaussian states, while allowing us to describe a much more diverse set of quantum states.”

Bond continues: “There are so many possible patterns that classical computers often have difficulty calculating and processing them. Instead, in this paper we use a method that identifies the most important of these patterns and ignores the rest. This allows us to study these quantum systems and design new ways to prepare interesting quantum states.”

The new approach can be used to efficiently prepare quantum states in a way that outperforms other traditionally used protocols.

“Fast preparation of a quantum state could be useful for a wide range of applications, such as quantum simulation and even quantum error correction,” notes Bond.

Scientists have also shown that they can use non-Gaussian states to prepare “critical” quantum states, which correspond to a system undergoing a phase transition. Beyond basic interest, such states can significantly increase the sensitivity of quantum sensors.

While these results are very encouraging, they are only a first step towards more ambitious goals. So far, the method has been demonstrated for a single spin. A natural but challenging extension is to consider multiple spins and multiple bosonic modes simultaneously. A parallel direction is to take into account the influence of the environment disturbing spin-boson systems. Both approaches are under active development.