Quantum computing is one of the great scientific challenges of the 21st century. Small-scale
systems today promise to surpass classical computers in the coming years and to enable the
solution of classically intractable computational tasks in the fields of quantum chemistry,
optimization, cryptography and more.
In contrast to classical computers, quantum computers based on superconducting quantum
bits (qubits) can to date not be linked over long distance in a network to improve their computing
capacity, since devices, which preserve the quantumstate when it is transferred from one
machine to another, are not available. Several approaches are being pursued to realize such a
component, one of themost promising to date makes use of an intermediary, micromechanical
element that enables quantum coherent conversion between the information present
in the quantum computer and an optical fiber, without compromising the quantum nature
of the information, via optomechanical interaction. This approach could allow fiber-optic
quantum networks between separate quantum computers based on superconducting qubits
in the future.
In this work a platformfor such a microwave-to-optic link was developed based on the piezoelectric
material gallium phosphide. This III-V semiconductor offers not only a piezoelectric
coupling between the electric field of a microwave circuit and a mechanicalmode, but also a
wide optical bandgap E_g = 2.26eV which reduces nonlinear optical absorption in the device
and a large refractive index n(1550nm) = 3.01 which allows strong optical confinement at
near-infrared wavelengths.
Importantly and in contrast to other approaches with gallium phosphide, an epitaxially
grown, single crystal thin film of the material is integrated directly on a silicon wafer with
pre-structured niobium electrodes by direct wafer-bonding. This opens up the possibility of
integrating the device design presented here directly with superconducting qubits fabricated
with this material system.
A microwave-to-optical transducer design was simulated and fabricated in the galliumphosphideon-
silicon platform. The device was found to exhibit large vacuum optomechanical coupling
rates g0/2 pi ~ 290kHz and a high intrinsic optical quality factor Q >10^5 while at the same time
permitting electromechanical coupling to a microwave electrode. Coherent microwave-tooptical
transductionwas shown at room temperature for this device and the electromechanical
coupling rate could be extracted from a model derived by input-output theory.
The electromechanical coupling between the electro-optomechanical device and a superconducting
qubit was estimated to be g/2 pi = O(200kHz) which indicates that strong coupling
between the here presented device and a superconducting transmon qubit is achievable.
In addition, superconducting microwave cavities with high quality factor at single photon
energy Q ~ 5x10^5 were fabricated and measured to verify that fabrication process of the
microwave-to-optical transducer is compatible with high-quality superconducting microwave
circuits.