Quantum many body systems with qubits and phonons in the solid state

Öney O. Soykal, Rusko Ruskov, Charles Tahan

Öney O. Soykal, Charles Tahan

Ian S. Osborne

Quantum Physics Quantum information processing platforms typically require qubits to talk to each other. To date, photons (either optical or microwave) have been the carrier of choice to transfer quantum states between the qubits. For some solid-state systems, however, the vibrational properties of the materials themselves, phonons, could be advantageous. Bienfait et al. describe the deterministic emission and capture of itinerant phonons through an acoustic communication channel, enabling the phonon-based coherent transfer of quantum states from one superconducting qubit to another and the quantum entanglement of the two qubits over the acoustic channel. The results provide a route to couple hybrid quantum solid-state systems using surface acoustic waves. Science , this issue p. [368][1] [1]: /lookup/doi/10.1126/science.aaw8415

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.

Öney O. Soykal, Charles Tahan

Thanh Nguyen, Nina Andrejevic, Hoi Chun Po +17 more

Many-body localization (MBL) has attracted significant attention because of its immunity to thermalization, role in logarithmic entanglement entropy growth, and opportunities to reach exotic quantum orders. However, experimental realization of MBL in solid-state systems has remained challenging. Here, we report evidence of a possible phonon MBL phase in disordered GaAs/AlAs superlattices. Through grazing-incidence inelastic X-ray scattering, we observe a strong deviation of the phonon population from equilibrium in samples doped with ErAs nanodots at low temperature, signaling a departure from thermalization. This behavior occurs within finite phonon energy and wavevector windows, suggesting a localization-thermalization crossover. We support our observation by proposing a theoretical model for the effective phonon Hamiltonian in disordered superlattices, and showing that it can be mapped exactly to a disordered 1D Bose-Hubbard model with a known MBL phase. Our work provides momentum-resolved experimental evidence of phonon localization, extending the scope of MBL to disordered solid-state systems.