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Quantum phase modulator - Nature Photonics
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The conversion of information from single photons in the microwave region to the optical region is an essential task for enabling the realization of a long-range network of superconducting quantum processors. However, this conversion is technically challenging due to the difference in photon energy of more than five orders of magnitude. Now, Poolad Imany and co-workers from National Institute of Standards and Technology (NIST) and University of Colorado, USA have reported an integrated, chip-scale solution to the problem that is compatible with information encoded at gigahertz modulation speeds. The team integrated superconducting electrical circuits, surface acoustic wave (SAW) resonators, and quantum emitters into a GaAs wafer (pictured) to generate efficient electro-optic interactions, and observed strong gigahertz-regime modulations of single photons ( Optica 9 , 501–504; 2022 ).
Credit: Optica Publishing Group
The key to the microwave-to-optical conversion is the usage of InAs quantum dots (QDs), which act as a single photon emitter and also a transducer because they are sensitive to local strain in a host crystal. A benefit of the approach is that the exciton energy shift due to strain is about two orders of magnitude more sensitive (~10 GHz pm –1 ) than that of small optical cavities (~100 MHz pm –1 ), which are also used for spectral conversion. Furthermore, GaAs is a convenient material to couple SAWs to superconducting circuits piezo-electrically.
In the integrated structure built by the authors, InAs QDs are sandwiched in a distributed-Bragg-reflector (DBR) optical cavity fabricated on a GaAs substrate. Then, the substrate is processed to add SAW cavities and interdigitated transducers (IDTs) made from superconducting niobium (pictured). Both the DBR and IDT structures are designed for microwave frequencies around 3.6 GHz. The IDT drives the SAW cavity to generate optical photons from microwave photons. The phonons interact with optical photons from the InAs QDs via the piezoelectric effect. As a result, the optically excited QDs exhibit sidebands in the scattering spectrum.
Phonons modulate the photons scattered from the QDs, shifting their frequency by increasing the SAW frequency. In order to confirm the ability to perform single-phonon transduction, an excitation laser is red-detuned by one SAW frequency. The obtained sideband spectrum becomes asymmetric with respect to the excitation frequency, indicating that the photon scattering process on average removes photons from the SAW cavity. In the limit of low microwave power, the resonant photon counts scale linearly with the SAW power, verifying that only one phonon is involved in the scattering process.
These results open the door for not only quantum transduction, but also sideband cooling of an acoustic cavity mediated by a quantum emitter and photon–photon entanglement generation.
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Nature Photonics https://www.nature.com/nphoton
Noriaki Horiuchi
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Horiuchi, N. Quantum phase modulator. Nat. Photon. 16, 478 (2022). https://doi.org/10.1038/s41566-022-01034-7
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Published : 01 July 2022
Issue Date : July 2022
DOI : https://doi.org/10.1038/s41566-022-01034-7
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