2.5D cQED: Multilayer circuit quantum electrodynamics

Publications & Patents

Planar Multilayer Circuit Quantum Electrodynamics  [ArXiv]
Z.K. MinevK. SerniakI.M. PopZ. LeghtasK. SliwaM. HatridgeL. FrunzioR. J. Schoelkopf, and M. H. Devoret
Phys. Rev. Applied 5, 044021
* Main manuscript on 2.5D circuit quantum electrodynamics (cQED)

[Patent] Techniques for coupling planar qubits to non-planar resonators and related systems and methods
Z.K. Minev
, K. Serniak, I.M. Pop, Y. Chu, T. Brecht, L. Frunzio, R.J. Schoelkopf,
and M.H. Devoret
Priority date: Feb. 27, 2015
International Publication No. WO/2016/138395

Planar superconducting whispering gallery mode resonators (WGMR)   [ArXiv]
Z.K. MinevI.M. Pop, and M.H. Devoret
Appl. Phys. Lett. 103, 142604 (2013)
* Manuscript on multilayer planar superconducting whispering gallery mode resonators (WGMR)


Popular summary

In the field of quantum technologies based on superconducting elements, there are two experimental platforms: the fully planar (2D) approach, which can benefit from the parallel fabrication of integrated circuits, and the machined cavity (3D) approach, which provides record quantum coherence, the crucial ingredient for advanced quantum operations. These two seemingly conflicting approaches raise the question: Is it be possible to reconcile the demand for robust quantum coherence with the demand of modularity and scaling arising from increased complexity?

We demonstrate an intermediate approach that can unite the benefits of both architectures. The experimental platform consists of a stack of 2D circuit layers separated by vacuum gaps. The circuit layers can be fabricated using conventional single-layer fabrication procedures. By properly designing the metallic structures that face each other in different layers, we confine the electromagnetic fields in the vacuum gap, therefore minimizing losses from the substrate and connecting elements.

In order to demonstrate the potential of these design principles, we implemented an integrated, two-cavity-modes, one-transmon-qubit system for cQED experiments. This has been possible by introducing a key concept, namely that of aperture coupling, which provides the manner in which an element patterned in one layer, such as a qubit, links to the electromagnetic fields in-between the layers.

The measured coherence times and coupling energies suggest that the 2.5D platform would be promising for integrated quantum-information-processing and for interfacing with a variety of mesoscopic or atomic systems.

Optical image  of a wafer patterned with several 2.5D cQED chip-stack layers, before dicing.  The wafer is diced into chips, which are stacked on top of each other, see next figure. Sample pattern, both large and fine structures, were defined with a 100 kV electron-beam pattern generator ( Raith EBPG 5000+ ) in a single step on a PMAA/MAA resist bilayer.

Optical image of a wafer patterned with several 2.5D cQED chip-stack layers, before dicing.  The wafer is diced into chips, which are stacked on top of each other, see next figure. Sample pattern, both large and fine structures, were defined with a 100 kV electron-beam pattern generator (Raith EBPG 5000+) in a single step on a PMAA/MAA resist bilayer.

Qubit-resonator coupling in 2D and 2.5D cQED. (a) In-plane coupling in 2D. The electric field lines of the resonator (blue) are aligned with the dipole moment of the qubit (red), both of which are in the plane of qubit fabrication. (b) Out-of-plane coupling in a multilayer planar device. The resonator is now represented as a section of a multilayer whispering-gallery-mode resonator, consisting of two superconducting thin-film rings deposited on different sapphire substrates that are separated by an electrically thin vacuum gap. The qubit is defined by an aperture carved directly from the conducting boundary of the resonator. The orange and blue arrows represent the resonator surface-current density and electric field lines, respectively.

Electromagnetic profile of aperture transmon. Surface-charge-density amplitude σ shown in color scale with overlayed white signs to indicate the relative charge polarity. The charges in the island and corresponding image charges in the opposite layer below determine the electric contribution to the qubit-resonator coupling. (c) Qubit-mode surface-current amplitude js shown in color scale with overlayed white arrows to represent the direction of the flow. The narrow rails on each side of the aperture are equivalent to a shared inductance between the qubit and resonator and determine the magnetic contribution to the qubit-resonator coupling.

Coherent parity revivals of storage cavity. Calibration of the photon-number parity measurement in the storage is achieved with a qubit-state revival experiment. For small storage-mode displacements n ~ 0 (blue), the decay is dominated by intrinsic qubit decoherence. For increasing displacements, up to n ~ 3 (red), the apparent increase in decoherence is due to the large qubit-cavity interaction rate, and we observe qubit-state revivals at integer multiples of 2t_p = 2π/ χ.