Phonons and solid-state qubits for quantum technology

Öney O. Soykal, Charles Tahan

Victor Chang Lee, James M. Rondinelli

Molecular qubits have emerged as versatile candidates for quantum information technologies.Their appeal lies in the chemical tunability, the possibility of large-scale synthetic control and their integration with solid-state and solution-phase architectures (Bayliss et al., 2022;Wojnar et al., 2024).In contrast to solid-state defects or superconducting qubits, molecular qubits can be engineered at the atomic level, allowing researchers to systematically tailor their electronic, magnetic, and vibrational properties (Laorenza & Freedman, 2022).However, the practical implementation of molecular qubits is limited by decoherence and relaxation processes, which shorten the time window over which quantum information can be stored and manipulated.

Pieter Kok, Brendon W. Lovett

Solid-state systems, by their very nature, have a vast number of different possible quantum degrees of freedom. In Chapter 11, we saw that some of these degrees of freedom make good qubits. However, there are plenty more which are less suitable, since they cannot easily be localized and externally controlled. Once the qubit has been chosen, it is important to think about how it interacts with the other, uncontrolled quantum excitations in its environment. Such an interaction leads to unpredictable behaviour and can cause decoherence – the irretrievable loss of quantum information from the qubit – and this will be the topic of this chapter. The most obvious decoherence mechanism for any optical manipulation scheme is the spontaneous emission of photons. The theory behind this follows analogously from the theory we discussed in Chapter 7, with a suitable definition of a transition dipole for the relevant transitions. However, solid-state systems bring with them lattice vibrations, or phonons, which have no direct atomic analogue. We will therefore focus on phonons in this chapter, first discussing how we model them, and second how they interact with the electron-based qubit that we discussed in the last chapter. Later, we will see how this leads to a loss of coherence, and how optical methods can be used to slow the rate of coherence loss. Phonon interactions are complex and not easy to model exactly, but we will show that with certain approximations very successful theories can be developed.

Chang‐Ling Zou, Luyan Sun

Mykyta Onizhuk, Giulia Galli

The usefulness of solid-state spins in quantum technologies depends on how long they can remain in a coherent superposition of quantum states. This Colloquium discusses how first-principles simulations can predict spin dynamics for different types of solid-state electron spins, helping design novel and improved platforms for quantum computing, networking, and sensing. We begin by outlining the necessary concepts of the noise affecting generic quantum systems. We then focus on recent advances in predicting spin-phonon relaxation of the spin-defect qubits. Next, we discuss cluster methods as a means of simulating quantum decoherence induced by spin-spin interactions, emphasizing the critical role of validation in ensuring the accuracy of these simulations. We highlight how validated cluster methods can be instrumental in interpreting recent experimental results and, more importantly, predicting the coherence properties of novel spin-based quantum platforms, guiding the development of next-generation quantum technologies.

David S. Simon