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(QuantEOM) Quantum-coherent electro-optic microwave-to-optical conversion with GaP and BaTiO3

Applicant Seidler Paul F.
Number 186364
Funding scheme Sinergia
Research institution IBM Research GmBH
Institution of higher education Companies/ Private Industry - FP
Main discipline Interdisciplinary
Start/End 01.03.2020 - 28.02.2023
Approved amount 2'091'708.00
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All Disciplines (6)

Information Technology
Theoretical Physics
Material Sciences
Microelectronics. Optoelectronics
Other disciplines of Physics

Keywords (11)

quantum-coherent transduction; frequency conversion; electro-optic conversion; Pockels effect; gallium phosphide; barium titanate; microwave qubits; optical quits; quantum information theory; quantum error correction; rate-optimal codes

Lay Summary (German)

In Quantencomputern steckt das Potenzial, Rechenprobleme zu knacken, die unsere heutigen klassischen Rechner nie werden lösen können. Als elementare Einheit zukünftiger Quantencomputer sind supraleitende Schaltkreise die aktuell führende Technologie. Dabei sind die Informationen als Quantenbits-die sogenannten Qubits-in Mikrowellen-Photonen kodiert. Die Qubits benötigen jedoch die ultrakalte Umgebung eines Verdünnungskryostats, um zu verhindern, dass thermisches Rauschen die fragilen Quantenzustände zerstört. Die Übertragung der Quanteninformationen zu und von Rechenknoten erfordert deshalb die Umwandlung der "stationären" supraleitenden Qubits in "fliegende" Qubits, die sich zwischen verschiedenen Standorten bewegen. Robuste optische Photonen stellen eine besonders attraktive Option für die Realisierung fliegender Qubits dar.
Lay summary
Unser Hauptziel ist, die Verknüpfung von Quantensubsystemen durch die Entwicklung eines quantenkohärenten, bidirektionalen Wandlers zwischen Mikrowellenfrequenzen und optischen Frequenzen zu ermöglichen. Dafür nutzen wir die direkte elektrooptische Kopplung über den sogenannten Pockelseffekt in den Materialien Galliumphosphid (GaP) oder Bariumtitanat (BaTiO3), die beide aussergewöhnliche optische nichtlineare Eigenschaften aufweisen. Zudem erforschen wir die Rechnerarchitektur und informationstheoretischen Möglichkeiten solcher Geräte für praktische Kommunikationskanäle, insbesondere fehlerkorrigierende Codes und deren Leistungsgrenzen.

Unsere Arbeit könnte die Skalierung von Quantenrechnern auf ein sonst unerreichbares Leistungsniveau bringen. Die Vorteile der Quanteninformationsverarbeitung könnten sogar auf eine ganz neue Klasse von Aufgaben erweitert werden. Beispiele sind die sichere Datenübertragung und die Vernetzung von Quantenrechnern.


Direct link to Lay Summary Last update: 06.09.2019

Responsible applicant and co-applicants


Associated projects

Number Title Start Funding scheme
205378 Cryogenic RF Probe Station 01.08.2022 R'EQUIP
192293 Soliton Microcombs: Exploring driven dissipative Kerr cavities 01.06.2020 Project funding
198164 ICP-CVD of silicon nitride and silicon oxide for novel integrated photonics and MEMS devices 01.03.2021 R'EQUIP
204927 Cavity Quantum Electro-optomechanics 01.12.2021 Project funding


The ability to coherently convert microwave photons to optical photons is an outstanding scientific and technological challenge of particular relevance to quantum computing and future quantum networks. Superconducting/Josephson junction circuits, in which qubits are encoded in microwave photons, are leading contenders as the platform for future quantum computers. The qubits however are bound to the ultracold environment of a dilution refrigerator to prevent thermal noise from destroying the fragile quantum states. Connecting computing nodes, even within a “quantum data center”, will require conversion of the stationary superconducting qubits to flying qubits, for which robust optical photons represent a particularly attractive option. Only with such networking capability can the power of quantum information processing be brought to a whole new class of tasks, such as secret sharing and blind quantum computing, in addition to linking quantum subsystems. It is our overall objective to address the lack of networking capability by developing a quantum-coherent, bidirectional transducer between microwave and optical frequencies and to explore the architectural and information theoretic possibilities afforded by such devices.To date, the most successful approach to microwave-to-optical transduction utilizes a mechanical system as an intermediary. Here we propose to make use, instead, of direct electro-optic coupling via the Pockel’s effect in either of two exceptional optical materials, GaP or BaTiO3. The transduction will exploit the large vacuum electric field strength achievable with a microwave co-planar waveguide resonator to couple electric signals at GHz frequencies to an optical mode in an optical waveguide resonator. While BaTiO3 is expected to provide the ultimate performance because of its enormous Pockels coefficient, microfabrication is easier with GaP. Moreover, the lower Pockels coefficient of GaP is counterbalanced by its higher refractive index and lower dielectric constant at microwave frequencies. The resulting devices will constitute exceptionally efficient optical modulators and microwave receivers as well as low-noise microwave amplifiers that vastly outperform today’s devices. Our ultimate goal, however, is quantum coherent transduction of microwave qubits into the optical domain.The main challenge facing the use of GaP and BaTiO3 in photonics has been in fabrication, specifically the lack of methods for integrating them on low-refractive-index substrates and patterning them into structures with nanometer precision while maintaining good material quality. The proposed project exploits recent breakthroughs made by IBM in the materials science and fabrication capabilities for integrating high-quality, epitaxially-grown, monocrystalline films of both GaP and BaTiO3 onto a variety of substrates via direct wafer bonding. Moreover, techniques for microprocessing the resulting GaP-on-insulator or BaTiO3-on-insulator wafers have been developed to fabricate a catalogue of integrated photonic devices such as high-quality-factor microcavities.A second challenge for the realization of electro-optic microwave-optical transduction has been the development of a device geometry with sufficiently strong coupling. Large spatial overlap and small mode volume of the microwave and optical modes are prerequisites for a high coupling rate but inherently conflict with the necessity of preventing absorption of the optical field in conducting electrodes, which would both reduce the quality of the optical cavity and disrupt the superconductivity of the microwave circuit. The EPFL has recently described a method to achieve dramatically increased coupling rates using sub-wavelength co-planar microwave resonators and brings to the collaboration deep and long-established expertise and capabilities in cavity quantum optomechanics, microwave electromechanics, and non-linear photonics.Coherent conversion of microwave to optical photons enables direct transmission of quantum information from point to point as well as generation of entanglement between distant points. How to best deploy these capabilities within or between quantum computing systems remains an open question for quantum information theory. We will investigate requirements for practical quantum communication by building on previous techniques to both construct error-correcting codes and find information-theoretic limits on coding scheme performance. In particular, codes based on complementarity of quantum information, developed partly at ETHZ, has narrowed the gap between theory and experiment, as they enable rate-optimal codes with efficient encoding/decoding operations. ETHZ has also developed computationally-tractable bounds on code performance.Our effort brings to bear the materials science and process technology expertise of IBM, the deep knowhow of the EPFL in cavity quantum electrodynamics, and the understanding of quantum information theory at the ETHZ to address the challenge and promise of quantum networking.