Hybrid quantum devices

What is a hybrid quantum device?


Quantum information technology, though promising, provides a number of hurdles to its implementation that are not found in classical information theory. Among the most significant of these hurdles is that quantum state information is extremely fragile and will eventually decohere due to interaction with the environment.


Superconducting qubits (SCQs) have shown promise in their scalability [1],  but are hampered by their relatively short coherence time and inability to send quantum states outside of the local qubit environment (i.e. the cryostat). This has led to the development of hybrid quantum devices (HQDs), which attempt to combine the best aspects of more than one quantum system [2]. One particularly popular avenue of research is enhancement of the coherence of SCQs by pairing them with a more long-lived system, such as a spin ensembles [3]. Such coupling schemes have the potential to be used for long-lived quantum memories.


Spin-qubit HQDs could also be used to detect a single spin [4], and coherently convert between microwave and optical photons [5]. Such a conversion scheme could enable long-range communication between quantum processor nodes, in a sort of "quantum internet" [6].


What's so special about diamond? 


Impurities in diamond have long been of interest to researchers. Diamond itself is unique among materials for its large bandgap, good thermal conductivity, high refractive index, and physical robustness. [7]. The simplicity of the diamond lattice also allows for easy modeling and calculations. Diamond has low intrinsic magnetism, as well as a lack of magnetically active impurities, resulting in narrow ESR resonance lines [7]. In recent years, impurity centers in diamond, particularly nitrogen-vacancies (NV), have become popular in a wide variety of QIS applications [8].

In our research we primarily use three different impurity centers in diamond: NV centers, nitrogen (P1) centers, and silicon-vacancy (SiV) centers. 


The quantum internet? 


As mentioned above, the information produced by superconducting qubits cannot be sent out of the local low-temperature environment, as it will be quickly overwhelmed by room-temperature blackbody radiation. Optical photons, on the other hand, have outstanding coherence at room temperature. A way to convert the information between microwave and optical frequencies is thus highly desirable. 

SiV centers have been found to have outstanding optical properties [9]. By coupling an ensemble of SiV centers to both a microwave and optical resonator, we will attempt to achieve coherent, bidirectional conversion between microwave and optical photons, taking advantage of the narrow optical linewidths and large electric dipole moment of SiV centers.



[1] Neeley, M. et al. Generation of three-qubit entangled states using superconducting phase qubits. Nature 467, 570-573 (2010).
[2] Kurizki, G. et al. Quantum technologies with hybrid systems. PNAS 112, 3866-3873 (2015).
[3] Kubo, Y. et al. Hybrid quantum circuit with a superconducting qubit coupled to a spin ensemble. Phys. Rev. Lett. 107 (2011).
[4] Haikka, P., Kubo, Y., Bienfait, A., Bertet, P. & Mølmer, K. Proposal for detecting a single electron spin in a microwave resonator. Phys. Rev. A 95 (2017).

[5] Williamson, L. A., Chen, Y.-H. & Longdell, J. J. Magneto-Optic Modulator with Unit Quantum Eciency. Phys. Rev. Lett. 113, 203601 (2014).
[6] Kimble, H. J. The quantum internet. Nature 453, 1023-1030 (2008).
[7] Loubser, J. & van Wyk, J. Electron spin resonance in the study of diamond. Rep. Prog. Phys. 41, 1201-1248 (1978).
[8] Doherty, M. W. et al. The nitrogen-vacancy colour centre in diamond. Phys. Rep. 528, 1-45 (2013).
[9] Rogers, L. J. et al. All-Optical Initialization, Readout, and Coherent Preparation of Single Silicon-Vacancy Spins in Diamond. Phys. Rev. Lett. 113, 263602 (2014).