Periodic Reporting for period 4 - QUNNECT (A Fiber Optic Transceiver for Superconducting Qubits)
Berichtszeitraum: 2022-08-01 bis 2023-01-31
Research in optical quantum networks and superconducting circuits has progressed largely independently so far.No solution exists to connect remote qubits via a room temperature link, because the small energy scales in the electrical circuit make the fragile information carriers susceptible to interference, thermal noise and losses.In QUNNECT we are working towards integrating low-loss fiber optic communication with superconducting circuits for quantum processing.As intermediaries we are focusing on quantum ground state cooled nano-scale mechanical and high bandwidth electro-optic nonlinear circuit elements.Our work will enable preparing remote entanglement of superconducting qubits,a first step to realize modular multi-node networks over larger distances.Quantum networks will be important for large-scale fault tolerant quantum computing and they might enable quantum enhanced sensor networks.On the technology side,the transducers developed in this project could show a new way for special purpose, ultra-low power consumption classical communication devices.
The first direction (WP1) was to design and fabricate a chip-scale nano-mechanical transducer with strong electro- and optomechanical couplings.This approach brings together the world of silicon nano-photonics in the form of photonic crystal cavities,the world of nano- and micromechanical systems (MEMS),and the new field of superconducting circuits.With this integrated approach we realized coherent and bidirectional conversion between microwave and telecom wavelength signals using pump powers in the pW range with a few percent efficiency,i.e. orders of magnitude higher than any commercially available modulator.Because this electro-opto-mechanical transducer is not operating in the so-called sideband resolved limit on the optics side,developing a complete understanding of the noise processes both with careful measurements and detailed theory was a major part of this work.This completed all WP1 tasks as proposed.The second direction (WP2) was to ramp up the fabrication and measurement capabilities of single crystal lithium niobate whispering gallery mode resonators for high bandwidth single-stage electro-optic conversion. Here we succeeded to develop a recipe for realizing optical quality factors of up to >10^8 even for lithium niobate discs as thin as 0.15 mm,an important aspect to get sufficient electro-optic coupling and conversion bandwidth.With such a device we were able to demonstrate the first bidirectional microwave – optical converter that operates certifiable in its quantum ground state.In order to improve the conversion efficiency of this device we introduced nanosecond timescale pulses with a few hundred mW power to pump the system without heating it up.This was extremely successful because it allowed us to demonstrate unity cooperativity in electro-optics for the first time.This in turn means unity (internal) conversion efficiency with high bandwidth.Even more importantly we found that the equivalent input noise reaches levels below 0.5 quanta for low duty cycles.Achieving simultaneously low noise,high efficiency and bandwidth enables experiments connecting the microwave and optical domain in the quantum limit for the first time. It is the precondition for the goals described in WP4 and has been published in Sahu et al., Nat. Commun. 13,2022.A lot of progress was achieved also in the third direction (WP3), i.e. integrating superconducting qubits into the converter devices.This includes a systematic study of the limitations of compact vacuum gap transmons on SOI and the development of a fully tuneable (frequency and bandwidth) microwave single photon source that is off-chip.As part of developing interesting input states to our transducer we have also been working on waveguide QED devices which showed on-demand photon scattering directionality, an important capability for on and cross-chip photon routing and quantum communication, as well as new non-classical photonic states that involve an ensemble of qubits for their preparation (in preparation).The main results of the third period were obtained in WP4 where we could demonstrate entanglement between microwave and light for the first time characterized by two-mode-squeezing of a hybrid microwave optical state of approximately 0.7 dB.While this is a manifestation of quantum backaction,we also carefully studied the coherent dynamical backaction between microwave and optical photons and found excellent agreement between theory and experiment in various pumping configurations with only minuscule excess backaction.This latter effect allows for on-demand coherent control of the microwave and optical modes in analogy to electromagnetically induced transparency.In the fourth reporting period we connected the superconducting qubits and cavities developed in WP3 with the electro-optic transducer developed in WP2.First experiments showed promising results towards all-optical qubit readout where the transducer demodulates an optical readout tone in the microwave domain and uses the reflection from the qubit readout cavity to again modulate the same light field that is sent to a heterodyne setup at room temperature (work in progress).Here we use 2 fibres and a EO transducer to replace all microwave components inside the dilution refrigerator, including bulky and pricy components (low noise amplifiers,circulators,isolators,attenuators,filters and coaxial cables) that are difficult to fully thermalize and problematic to scale up. In addition, we also established the basis for heralding type photon counting measurements and quantum network protocols that are resilient to losses (in progress).In summary,this project led to 9 published peer reviewed publications and 4 manuscripts that are currently in revision at major journals.We disseminated these results at tens of conferences to experts,and via press releases and social media (> 100,000 views on twitter) to the public.Certain results such as the mechanical entanglement generation or the proof of principle quantum illumination got featured widely in the popular press including Science Magazine with an Altimetrics score of up to 299.
There were two surprising results that we did not foresee at the time of writing the proposal.The first one appeared as we tried to approach the quantum limit of a mechanical transducer by replacing the optical mode with a microwave mode. This led to the first demonstration of two-mode squeezed (entangled) radiation generated by mechanical motion.A natural extension of this work was to think about how we can use such an entangled state for remote sensing protocols at room temperature and we realized a proof of concept study of quantum illumination.The second surprise was the low excess backaction and the ultra-low bath occupancy of the proposed electro-optic devices despite the required hundreds of mW pump power pulses necessary for high cooperativity physics.It turns out that the thermal management is maybe the most crucial aspect to successfully implement quantum coherent microwave optical experiments.With this technology we have now achieved the converter with the lowest microwave input noise to date the first demonstration of microwave-optical entanglement in any system.This is the resource needed for a non-zero quantum channel capacity,which represents the basis for quantum networks between cryogenic microwave processors.