A new method developed by Amsterdam researchers promises advances in quantum computing and sensing by using non-Gaussian states to efficiently identify and structure quantum spin-boson systems.

Many modern quantum devices operate using groups of qubits or spins that have only two energy states: “0” and “1”. But in real devices, these spins also interact with photons and phonons, collectively known as bosons, making calculations much more complicated. In a recently published study Physical Examination LettersResearchers from Amsterdam have developed a method to efficiently identify these spin-boson systems. This invention could help efficiently tune quantum devices to achieve specific desired states.

Quantum devices use the interesting behavior of quantum particles to perform tasks beyond those that “classical” machines can perform, including quantum computing, modelling, sensing, communications and metrology. These devices can take many forms, such as a collection of superconducting chains or a lattice of atoms or ions held in place by lasers or electric fields.

Regardless of their physical implementation, quantum devices are often described in simplified terms as a collection of interacting two-level quantum bits or spins. However, these spins also interact with other things around them, such as light in superconducting circuits or vibrations in the lattice of atoms or ions. Examples of bosons are particles of light (photons) and vibrational lattice modes (phonons).

Innovative descriptions of quantum states

Unlike spins, which have only two possible energy levels (“0” or “1”), the number of levels for each boson is infinite. As a result, there are very few computational tools that describe the spins associated with bosons. In their new work, physicists Liam Bond, Argavan Safavi-Naini and Jiri Minarj from the University of Amsterdam, QuSoft and Centrum Wiskunde & Informatica overcome this limitation by describing systems composed of spins and bosons using non-Gaussian states. Every non-Gaussian state is a combination (superposition) of much simpler Gaussian states.

Each blue-red pattern in the image above represents one possible quantum state of the spin-boson system. Ph.D. “A Gaussian case is like a regular red circle without any interesting blue-red patterns,” explains Liam Bond. An example of the Gaussian case is laser light, in which all light waves are perfectly synchronized. “If we take a lot of these Gaussian states and start superimposing them (so they overlap), these wonderful entangled patterns emerge. “We are particularly excited because these non-Gaussian states allow us to describe a much more diverse set of quantum states while allowing us to preserve many of the powerful mathematical mechanisms that exist for Gaussian states.”

Bond continues: “There are so many possible patterns that it is often difficult for classical computers to calculate and process them. Instead, in this paper we use a method that identifies the most important of these patterns and ignores the others. This allows us to study these quantum systems.” “And it allows us to develop new ways to prepare interesting quantum states.”

The new approach can be used to efficiently prepare quantum states to outperform other traditionally used protocols. “Rapid preparation of the quantum state could be useful for a wide range of applications, such as quantum modeling and even quantum error correction,” notes Bond. The researchers also showed that they could use non-Gaussian states to prepare “critical” quantum states corresponding to a system undergoing a phase transition. Apart from fundamental interest, such states can significantly increase the sensitivity of quantum sensors.

While these results are very encouraging, they are only the first step towards more ambitious goals. So far the method has been demonstrated for a single rotation. A natural but difficult extension is to include many spins and many bosonic modes simultaneously. A parallel direction is the consideration of environmental effects that perturb spin boson systems. Both of these approaches are actively developing.