Session E31: Building Quantum Networks with Hybrid Systems

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Observing quantum phenomena at the macroscopic scale has captured both the attention of scientists and the imagination of the public for more than a century. Although quantum mechanics presumably applies to objects of all sizes, directly observing entanglement becomes harder as masses increase, requiring measurement and control with a vanishingly small error. Here we strongly and deterministically entangle two massive mechanical oscillators (~ 70 pg) and directly observe their state. Our technology allows for on-demand reproducible entanglement generation. For direct state observation, we implement a near quantum-limited measurement of the positions and momenta of both mechanical oscillators in every realization of the experiment. By repeating these measurements, we completely characterize their joint covariance matrix. This tomography demonstrates clear evidence of continuous variables (CV) entanglement in the measurement signals, without noise subtraction.

The amount of entanglement measured and the ability to directly ovserve it without noise subtraction encourages future research directions that include: entanglement distribution between separate dilution refrigerators, quantum illumination of objects, and quantum teleportation of mechanical states.   

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Future long-distance quantum networks based on local computational nodes of superconducting qubits connected by optical fibers will require a bidirectional, quantum-coherent transducer between microwave and optical frequencies. A converter can be realized by simultaneously coupling a MHz frequency mode of a SiN membrane to a superconducting LC circuit and an optical Fabry-Perot Cavity. We demonstrated such a converter in 2018 with 47% efficiency and 38 photons of added noise [1]. Significant contributions to this added noise come from the membrane thermal motion and fluctuation of the LC circuit parameters. Here, we present a new electromechanical circuit design that reduces the added noise by improving sideband cooling of the membrane.

[1] Higginbotham, A. P., et. al. “Harnessing electro-optic correlations in an efficient mechanical converter,” Nature Physics 14, 1038-1042 (2018)   

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Coherently converting quantum states between distinct elements via quantum transducers remains a crucial yet challenging task in quantum science. Especially in demand is quantum transduction between optical frequencies, which are ideal for low-loss transmission across long distances, and microwave frequencies, which admits high-fidelity quantum operations. We present a generic formalism for N-stage quantum transduction that covers all the leading microwave-to-optical linear conversion approaches such as electro-optics, electro-optomechanics, optomagnonics, and atomic ensembles. We then identify a generalized matching condition for achieving maximum conversion efficiency. The generalized matching condition requires resistance matching as well as frequency matching beyond the usual resonant assumption, with simple impedance matched transmission interpretation. Our formalism provides a universal toolbox for determining experimental parameters to realize efficient quantum transduction that can fulfill various practical requirements, and suggests new regimes of non-resonant conversions that can outperform all-resonant ones.   

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Quantum teleportation, one of the fascinating predictions of quantum mechanics, has been recently demonstrated with at least 90% fidelity in the telecommunication C-band at 1536 nm at the Caltech quantum network. Here we report upgrades of our system towards remote and replicable installations-- key requirements for deployment in real-world settings. Expensive Arbitrary Waveform Generators (AWGs) and high-bandwidth electrical amplifiers have been replaced by a homemade pulse shaper and amplifier, respectively, with almost negligible impact on the quality of the qubit states. Light in the telecommunication O-band at 1310 nm is distributed in the same fiber carrying entangled photons, thereby demonstrating not only the much needed co-existence of quantum light with classical communication for wide employability of quantum systems, but also the ability to synchronize remote nodes in a network. Finally, a novel data acquisition system with continuous monitoring of critical experimental parameters has also been developed, allowing uninterrupted data acquisition over long timescales.   

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Recent work in cavity optomechanics has demonstrated an unprecedented level of sensitivity in the detection of acoustic phonons through Brillouin scattering in optical cavities. [1] This work extends the experimental scheme by adding a resonant RF cavity, enabling the investigation of electro-acousto-optical couplings.
We demonstrate two important use cases: First, we measure the cavity-enhanced transduction efficiency of microwave to optical photons in x-cut Quartz, a piezoelectric material; and second, we further refine the bounds on anomalous piezoelectricity in Silicon, a centrosymmetric material. Future applications of our work are directly relevant in microwave to optical transduction for the construction of quantum networks and the understanding of phonon properties in substrates to utilize in superconducting qubits system.[2]
[1] Kharel, et al. Science Advances (2019)
[2] Vijay Jain, Acoustic Spontaneous Emission by a Superconducting Qubit, APS March Meeting 2021   

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With the potential exhibited by microwave photons in the field of quantum information coupled with the ease of transporting optical photons at room temperature, coherent quantum transduction – the process by which one can convert between the two energy profiles – has become a coveted goal. We are developing a centimeter scale polymeric dielectric microwave photonic crystal for electro-optic quantum transduction operating in the 5-20 GHz range. Using finite element simulations, we developed various iterations of high-Q microwave cavities, evolving from a basic circular unit-cell structure to a "bowtie" unit-cell structure with a highly confined mode, on a scale much smaller than the microwave wavelength. Based on these simulations, we manufacture our photonic crystals using machining and 3D printing processes. We are currently testing the microwave resonators. In the near future, we will incorporate an electro-optic material such as lithium niobate or an electro-optic polymer into the structure to form a high efficiency microwave – optical transducer, suitable for transduction at the level of individual quanta.   

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Millimeter-wave frequencies offer many advantages for hybrid experiments with cold atoms, spin defects, and superconducting circuits. Simultaneously, the millimeter-wave band could be a powerful independent platform for quantum information technology at 1 K, enabling lower cost, more flexibility, and viable transfer of information. In this talk, I will present new developments from our hybrid experiment for interfacing single optical and mm-wave photons using Rydberg atoms, including measurement of the hybrid cavity and observation of mm-wave and optical hybridization through coupling to Rydberg states of atoms. I will also outline the prospects of this platform for transduction and nonlinear photonics experiments.   

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We investigate the bulk properties of InAs nanowires proximitized by thin-film aluminium, which are a potential platform for topological quantum computing. We measure the frequency response of a quarter-wave coplanar wave guide resonator shunted by a hybrid Al-InAs nanowire as a function of gate voltage and axial magnetic field. From the detected shift in resonance frequency and quality factor, we extract the complex impedance of the nanowire. Our results show a monotonic dependence of the nanowire inductance and resistance on the gate voltage and an oscillating behaviour as a function of the axial magnetic field. This measurement approach complements existing characterization techniques and may facilitate the identification of the topological phase transition predicted for these hybrid structures.   

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Microwave spectroscopy of Andreev bound states (ABS) in hybrid superconducting circuit-QED has so far been limited to magnetic fields of a few tens of mT. In contrast to ABS transitions involving pairs of quasiparticles, transitions involving a single spin-carrying quasiparticle to higher lying ABS have been observed and manipulated recently. Here, we expand on these results by performing microwave spectroscopy of a gate-tunable InAs-Al hybrid weak link in presence of spin-orbit coupling and Zeeman fields. In parallel fields up to 250 mT we track both even and odd fermion parity branches of the spectrum. Additionally, in junctions with broken chiral and time-reversal symmetry, through spin-orbit coupling and magnetic field respectively, an anomalous Josephson current is expected to appear. In contrast to measurements of this aggregate effect, we show a field induced anomalous phase shift in transitions between individual Andreev bound states. Finally, the ability to perform microwave spectroscopy of ABS in magnetic fields is requisite for probing signatures of the 4-pi periodic Josephson effect directly in the ABS spectrum and can serve as a platform for Andreev spin based qubits.   

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We explore the interaction between quantized electromagnetic fields and 1D atomic chains under the framework of cavity QED, by studying the cavity transmission. When the coupling between subsystems is small, the transmission can be used as a way to probe the properties of the fermonic system in a non-invasive way, while at large coupling rates, these properties are highly modified and hybrid excitations of the total system appear.
When considering systems with topological properties, such as topological insulators (TIs), the topological and trivial phase can be distinguished by the presence of in-gap edge states in the former case, topologically protected against local perturbations.
In this work, we propose a general method to compute the cavity transmission through the photonic Green’s function, which allows to go beyond the usual linear response regime, while studying the interplay between the photonic field and non-trivial topology, captured by the well-known SSH model for TIs in 1D. We find that the cavity transmission can be used to distinguish between both phases at different coupling rates.   

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An efficient quantum transducer that faithfully and reversibly converts optical quantum signals to microwave quantum signals could be used as an interface between superconducting-based quantum devices, such as transmon qubits, and optical photons. Despite the great potential for quantum sensors and heterogeneous quantum networks, and many theoretical studies dedicated to transduction protocols, there are relatively few demonstrations of microwave-to-optical transduction at the quantum level. Most demonstrations have demonstrate either low conversion efficiency or high noise, and thus are not suitable for quantum signals. One way to improve the efficiency of a transducer is to use high quality factor (Q) microwave cavities. Fermilab has developed bulk Nb superconducting cavities with record-high 2 second photon lifetime (Q=1011), a significant improvement compared to previous efforts. We will present preliminary studies and solutions to determine an optimum transducer design for these cavities.   

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Building the first large-scale quantum network is a highly challenging endeavor. Not only is it a highly contested question of what the most promising hardware platform might be, but even if we had selected one, it is unknown what the precise requirements for its realization would be.

In this talk, we will present a series of methods that can be used in order to determine minimal requirements of creating such a network on an existing fiber network infrastructure. We start by presenting an algorithm to perform a pre-selection of where quantum repeaters may be located on an existing fiber grid. We present a purpose built discrete event simulator that can be used to validate candidate architectures, and a matching machine learning method that can be used to determine minimal hardware (or software) requirements necessary to achieve a specific target functionality of the network. Finally, we provide an initial case study of a resulting Blueprint for a network architecture on the real world fiber grid of SURF in the Netherlands.