Artist’s rendition of a photonic crystal cavity made of gallium phosphide exhibiting strong optomechanical coupling with minimal two-photon absorption and reduced heating. Credit: Simon Hönl, IBM Research
Quantum computers hold the promise of tackling computational problems that our current classical computers will never be able to solve. Leading contenders as the platform for future quantum computers are superconducting/Josephson junction circuits, in which qubits are encoded in microwave photons.
The qubits however are bound to the ultracold environment of a dilution refrigerator to prevent thermal noise from destroying the fragile quantum states. Transferring quantum information to and from computing nodes, even within a quantum data center, will require conversion of the “stationary” superconducting qubits to so-called “flying” qubits – the term used for qubits transmitted between separate locations. Robust optical photons represent a particularly attractive option for flying qubits. In a paper recently published in the peer-reviewed journal Optica, we report on the demonstration of an optomechanical device aimed at creating such a microwave-to-optical quantum link.
To date, one of the most successful approaches to microwave-to-optical transduction utilizes a mechanical system as an intermediary. In this case, the fact that photons possess momentum—light pushes things—is used to excite the motion of a device on a chip that is also connected to a microwave electrical circuit. Photonic devices exploiting such optomechanical coupling are often plagued by the deleterious effects of heating due to absorption of the high-intensity light. Instead of using silicon, the typical material for optomechanics, our group of scientists at IBM Research-Zurich has taken advantage of the exceptional optical properties of gallium phosphide (GaP) to create on-chip integrated devices with strong optomechanical coupling and minimal heating.
Why gallium phosphide (GaP)?
GaP possesses an attractive combination of a large refractive index (n > 3 for vacuum wavelengths up to 4 μm) and a large electronic bandgap (2.26 eV). The former allows light to be confined to a small volume; the latter implies a wide transparency window. There are few materials which exhibit these inherently conflicting properties, as there is typically a tradeoff between index of refraction and bandgap. GaP offers a unique possibility of creating devices with strong light confinement (small mode volumes), transparency into the visible (λvac > 550 nm) and enhanced light-matter interaction. Perhaps most importantly, two-photon absorption at the typical data communication wavelengths of 1310 nm and 1550 nm is dramatically diminished in comparison to silicon photonics, permitting the use of high intensities as may occur in nanophotonic devices.
Scanning electron microscope images of a one-dimensional GaP photonic crystal cavity. (a) Freestanding device with width and height of 542 nm and 300 nm, respectively. (b) Magnification of the central part of the device showing smooth and straight sidewalls.
In our latest work just published in Optica, we have achieved strong optomechanical coupling with a “photonic crystal cavity” made of GaP. The photonic crystal cavity is fabricated with a newly developed process for chip-level integration of GaP devices based on direct wafer bonding onto low-refractive-index substrates. The optomechanical coupling is large enough to permit amplification of the mechanical motion of the structure into the so-called mechanical lasing regime at relatively low optical power.
Although mechanical lasing per se is not our goal, its observation provides a clear indication that the coupling is sufficient to easily reach the threshold for realization of quantum-state-transfer protocols.
Moreover, the study confirms that heating due to the high-intensity optical fields in these devices is dramatically reduced. Heating can destroy the coherence of the very quantum states we are trying to manipulate as well as constrain the ability to control these states.
With the observation of efficient optomechanical coupling in GaP devices, we have addressed one half of the challenge of interconverting microwave and optical quantum information. We still need to demonstrate the coupling between microwave qubits and mechanical motion in our devices, for which we intend to take advantage of the piezoelectric properties of GaP.
Nevertheless, this work represents a significant step forward toward the overall objective of developing a quantum-coherent, bidirectional transducer between microwave and optical frequencies for quantum networking. With such networking capability, the power of quantum information processing could be brought to a whole new class of tasks, such as secure data sharing, in addition to linking quantum subsystems.