With all of the current quantum computing chatter and, arguably, quantum hype, you'd think we had quantum computing figured at least to the point where we know what one will actually look like.Not quite. Connecting qubits across meaningful distances—entangling them, that is—remains enormously tricky business. In a study released this week in Physical Review X, researchers from the Max Planck Institute for Quantum Optics describe a theoretical model for connecting distant qubits using sound in place of wires or fiber optics etc. The result, the group argues, is a new and improved "quantum bus" that can constructed at micrometer scales with existing technology.
"The realization of long-range interactions between remote qubits is arguably one of the greatest challenges in developing a scalable, solid-state quantum information architecture," the researchers note. "Here, we propose and analyze quantum sound in the form of surface-acoustic-wave phonons in piezoactive materials as a universal mediator for long-range spin-spin couplings instead of photons."To clarify, a bus within a computer processor is a bundle of connections meant to move lots of data between two or more architecture components that need to regularly exchange, well, lots of data. The classic illustration is of a system bus, which links a CPU with system memory and input-output devices. A generic quantum bus realizes this as a connection between separated qubits.The new model isn't the first imagining of a quantum bus. Of particular note is a scheme described in 2008 using photons at microwave energies—where they can best couple to qubits—as the transmission link. This is known as cavity quantum electrodynamics, in part because the photon information carriers are packed into a tiny hollowed-out space, which is essentially the data bus.It turns out that this method works pretty OK. It seems natural enough.The acoustic method also uses cavities as data channels, but is more so a variation on another quantum bus framework, this time based on phonons, which can be imagined as collective excitations of particles. It's sort of a fancy way of saying "vibrations"—a phonon is the quantized (particle) component of a vibration, just as a photon is the quantized component of light.In a quantum bus, phonons are produced and manipulated with help from resonators, which presents a challenge when it comes to transmitting across large distances. The new method combines the best of both worlds, the researchers argue. It's based on phonon-like surface acoustic waves (SAWs), which are two-dimensional waves the travel across a material as flat ripples (imagine waves of density or pressure rather than 3D height). They have the advantage of being easily channeled using very fine etching."Because of the plethora of physical properties associated with surface acoustic waves, our approach is accessible to a broad class of systems such as quantum dots, trapped ions, nitrogen-vacancy centers, or superconducting qubits," the researchers write in the current study. "We show that our proposed system also bears striking similarities to the established fields of cavity (circuit) quantum electrodynamics, opening up the possibility to implement the on-chip many-quantum communication protocols well known from the context of optical quantum networks."