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 flying qubits – the term used for qubits transmitted between separate locations. Robust optical photons represent a particularly attractive option for flying qubits. Now, IBM-researchers report on the demonstration of an optomechanical device aimed at creating such a microwave-to-optical quantum link.
Artist’s rendition of a photonic crystal cavity made of gallium phosphide exhibiting strong optomechanical coupling with minimal two-photon absorption and reduced heating. (Source: S. Hönl, IBM Research)
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 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, a 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.
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. Now, the team has 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 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 the researchers are trying to manipulate as well as constrain the ability to control these states.
With the observation of efficient optomechanical coupling in GaP devices, the scientists have addressed one half of the challenge of interconverting microwave and optical quantum information. They still need to demonstrate the coupling between microwave qubits and mechanical motion in their devices, for which they 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. (Source: IBM Research)
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