Optically switchable superradiance of surface acoustic wave phonons in diamond ✦
The interaction of surface acoustic waves (SAWs) with the spins of NV centers in diamond holds promise for quantum phononic manipulations on a chip. Ensembles of NV centers coupled to a common SAW mode can exhibit superradiance and collective control, yet a tunable superradiant phase transition has been difficult to achieve. It is shown that optical pumping of NV center transitions enhances the effective spin-phonon coupling, inducing a superradiant phase transition of SAW phonons in the weak coupling regime. It is demonstrated that above a critical threshold, the pump power rapidly switches on phonon superradiance—a dynamic effect that persists in ensembles with a finite number of NV centers. The results offer a controllable pathway to coherent control of NV center spins via phonons in solid-state quantum devices.
Context
Controlling the quantum properties of phonons is key to advancing quantum information technologies and quantum metrology. However, achieving strong coupling for superradiance in solid-state systems remains a challenge. The proposed method opens a new avenue for coherent control of phonons.
Methods
Using the superposition principle of collective spin states, the authors constructed an effective Hamiltonian describing an ensemble of NV centers in a diamond waveguide, coupled to a single mode of a surface acoustic wave (SAW) and driven by a classical laser field. Through unitary transformations accounting for dissipation, they reduced the problem to a Dicke model with tunable coupling. Then, using mean-field theory and numerical solution of the master equation, they investigated the steady states and dynamics.
Results
It is shown that when the laser intensity exceeds a critical value, the effective spin-phonon coupling G = λΩ/ωm (where λ is the bare coupling, Ω is the laser Rabi frequency, and ωm is the phonon frequency) triggers a superradiant phase transition. In the thermodynamic limit, the second-order transition is characterized by the order parameter—the amplitude of the phonon field. The critical bare coupling λc decreases with increasing Ω, enabling superradiance even for initially weak coupling. Numerically, it is demonstrated that upon switching on the laser, the system instantly transitions to the superradiant phase, acting as a quantum switch. For finite ensembles (N ~ 2–10), the phase transition smoothens into a crossover, but the Wigner function exhibits a characteristic three-peak structure—a signature of coexistence between normal and superradiant phases.
Implications
The work offers a universal mechanism for optical tuning of collective quantum effects in phononic systems. This expands the toolbox of quantum phononics and brings closer the realization of solid-state quantum networks, where phonons serve as mediators for transmitting quantum entanglement.
Future development
Future developments may include extension to multi-phonon modes, exploration of nonclassical states, and use of NV centers for quantum memory. Experimental implementation with state-of-the-art high-Q SAW resonators and small NV center ensembles appears achievable.
Impact
The results will impact the development of quantum acoustic devices, phonon-based quantum computers, and ultrasensitive sensors leveraging collective spin states.
Next steps
Experimental demonstration of the transition in diamond structures with tens of NV centers and verification of predictions for finite systems using quantum state tomography.
Key open problems
The study touches upon the fundamental challenge of achieving strong coupling in hybrid quantum systems and the issue of controlling quantum decoherence in solids. It also contributes to understanding nonequilibrium phase transitions in open many-body systems.
🎯 Surface acoustic waves travel 100,000 times slower than light, offering unique opportunities for manipulating quantum states 'on the fly.' And NV centers in diamond are true 'atoms in a solid,' maintaining quantum coherence even at room temperature!
Key numbers
- SAW speed relative to light speed: 5 orders of magnitude slower
- typical SAW frequency: 818 MHz
- single-emitter spin-phonon coupling (g0/2π): ~10 MHz
- SAW resonator quality factor: 10^3 – 10^5
- number of NV centers for collective coupling λ/2π~50 MHz: ~25