**Description:**

**University of Oregon Researchers:** Hailin Wang, Mark Kuzyk

**Phononic Quantum Networks of Solid-State Spins with Alternating and Frequency-Selective Waveguides**

**Patent:** US11,113,622 issued on 9/7/2021 (UO-18-23)

__Technology Background:__ Quantum computing is the next frontier in complexity of operation of computing devices. The ability to tie together (“entangle”) the states of 2 or more bits of information (“qubits”) allows entirely new kinds of calculations to be performed and, importantly, physical systems to be modeled. Much theoretical work has been done to anticipate how these systems would function, but to be useful any effective quantum computing device must be embodied in a “real” physical system analogous to transistors and gates that form the basis of existing computers. While several existing quantum computing platforms exist, it’s not clear which will prove to be stable, scalable and cost-effective. Any new physical manifestation of a quantum computer could emerge as the basis of the next generation of research devices or even replacing the systems used today.

**Definition of Problem**: A promising approach for building a large-scale quantum computer is to network together smaller quantum systems, while preserving the relevant quantum properties. This modular approach is being pursued for the well-established platforms of superconducting qubits and atomic qubits and is also being explored for the emerging platform of solid-state spin qubits. Photons are excellent carriers of quantum information. As such, they have been the obvious choice of communication channels in quantum networks. Photons are ideally suited for long distance quantum communications; however, for on-chip communications, there are a few inherent limitations. For example, the speed of light is too fast for short distance communications. Electromagnetic waves easily propagate in vacuum, and so even with state-of-the-art nanofabrication technologies, scattering loss into the vacuum can be far too excessive in photonic nanostructures. In comparison, phonons, which are the quanta of mechanical waves, feature several inherent advantages for on-chip communications. The speed of sound is about five orders of magnitude slower than the speed of light. Mechanical waves cannot propagate in vacuum and are thus not subject to scattering losses into the vacuum. The relatively long acoustic wavelength also makes it easier to fabricate phononic nanostructures. There are, however, a number of technical challenges for developing a phononic quantum network. Mechanical modes couple easily to the surrounding environment, leading to decoherence. The relatively low phonon frequency also makes phononic communicating channels subject to thermal noise. A key function of a quantum network is to enable quantum state transfer between two neighboring quantum nodes. For a one-dimensional network, this requires either a cascaded network, for which the quantum state transfer between neighboring nodes is unidirectional, or a network with one-sided coupling, for which a given quantum node can couple selectively to only one of the two neighboring nodes. It has been shown theoretically that in both types of networks, quantum state transfer between neighboring nodes can take place via a photonic or phononic channel with high fidelity, even in the presence of relatively large thermal noise. Cascaded optical networks can be realized with chiral optical interactions or with the use of optical isolators. Thus far there have been no specific proposals or technical approaches to realize either cascaded phononic networks or phononic networks with one-sided coupling.

**Our Technology Solution**: University of Oregon researchers have developed a phononic quantum network of solid-state spin qubits with one-sided coupling. This quantum network takes advantage of spin qubits in diamond and exploits band gap enginnering of phononic crystal waveguides. The network is embedded in or attached to a phononic crystal lattice, which isolates and protects the network from coupling to the surrounding mechanical environment. The one-sided coupling also makes the phononic quantum communication robust against thermal mechanical noise.

**Applications**: The new ideas and concepts used in the diamond-based phononic quantum network we have developed can be extended to other material systems, to quantum networks of other qubit systems, and to photonic quantum networks.