Session F67: Transduction for Quantum Networks

Optimization and Comparison of Two-node Gaussian Quantum Networks with Optical-Microwave Transducers

Achieving quantum communication between two remote microwave nodes connected via an optical link will require quantum transducers, many of which can be modeled as two-mode Gaussian quantum channels between optical and microwave modes. We propose a method for finding the optimal network for distributing microwave-microwave entanglement under certain restrictions, such allowing only Gaussian resources in the optical domain. We show that networks which utilize the two-mode nature of the transducer via ancillary states/measurements outperform those where the transducer is reduced to an effective single mode operation (e.g. upconversion). We are then able to upper-bound the achievable microwave-microwave entanglement rates for experimentally relevant transducers, such as electro-opto-mechanical devices.   

Microwave to optical transduction via engineered dissipation

Quantum transduction, coherent conversion of quantum information between disparate systems, is an essential element in quantum technology.   One crucial application of quantum transduction is interconnection of a hybrid quantum network consisting of microwave-frequency quantum processors and optical-frequency quantum communication links.  Microwave to optical conversion has been proposed and realized in various atomic platforms using coherent interactions.  Here we present a quantum transduction scheme between microwave and optical frequencies via engineering suitable interactions with dissipative atomic levels.  With atomic relaxation as the resource, the proposed quantum transduction process is completed autonomously by dissipation and is immune from noises at the receiver port.  We consider protocols for both microwave-to-optical and optical-to-microwave directional transduction.  Finally, we compare the performance of the proposed dissipation-based transduction protocols with other transduction schemes.

    

Pump 'n' Dump - High Fidelity Communication with Lossy Microwave Links: Part 1

Short-distance modular quantum networks allow us to build more complex quantum devices from simpler ones, provided we are not limited by our interconnects. Superconducting cavity modules within a single dilution refrigerator can be networked together over centimeter to meter scale distances with microwave transmission line (or waveguide) links. However, quantum communication is typically limited by loss in the links connecting the modules. Given that the modes of our cavities and link exist in the 1-10 GHz regime, we can actuate the coupling between our cavities and a single standing wave mode of our link. This provides us a new way to generate heralded entanglement between the modules which we call ‘Pump ‘n’ Dump’. Our scheme not only completely avoids loss in the link, but has a loss-independent success probability of 50%.

    

Pump 'n' Dump - High Fidelity Communication with Lossy Microwave Links: Part 2

We realize a new scheme for entangling two superconducting cavity qubits in separate modules while avoiding link loss. The cavities are connected via a 5cm long superconducting coaxial cable with an FSR of 2 GHz. When we switch on parametric coupling between each module and a single standing mode in the cable, we can treat the cable mode as a 'dump mode' and use interference between the two couplings to generate entanglement. We prepare Bell states in the coherent state basis with 93% state fidelity and 37% success probability. This is possible even though the cable loss rate is 4x faster than our cavity-cable coupling rate, demonstrating our scheme's utility even in the presence of substantial link loss. Finally, we use this Bell state as a resource to deterministically teleport a qubit from one module to another with 90% transfer fidelity. Our work shows standing wave mode communication channels are a useful and still largely unexplored paradigm for near-term modular devices.   

Deterministic generation of shaped single microwave photons using a parametrically driven coupler

A distributed quantum computing system requires to share information between spatially separated quantum processors by creating a quantum channel, which can be implemented by emitting a microwave photon from a sender processor and absorbing it by a receiver processor. For this purpose, deterministically encoding the state of a stationary qubit into a travelling photon is of great interest. Here we introduce a superconducting circuit consisting of a data qubit and an emitter qubit coupled to a quantum channel, with a coupler between them to parametrically exchange quantum state. With this structure, we demonstrate the deterministic transfer of a qubit state into a propagating microwave photon with a single-rail encoding, with a process fidelity in excess of 90%. We use a time-dependent parametric drive to shape the temporal profile of the propagating mode to be time-symmetric and with a constant phase so that the absorption process by the receiver can be implemented as a time-reversed version of the emission. Compared with previous works which utilize a second-order transition to control the emission, the presented technique uses a first-order parametric process, which requires no strong pump and can therefore help eliminate AC Stark shifts and heating of the data qubit. It offers ease of calibration and flexibility in fine-tuning photon emission and absorption, and can therefore facilitate high-fidelity quantum state transfer and remote entanglement operations in a distributed quantum computing system.    

Teleportation-based microwave-optical quantum transduction enhanced by squeezing

Quantum transduction is an important building blockfor quantum networking. Although various platformshave been proposed, the transduction effi ciency of the-state-of-the-art systems is still way below the thresholdto provide robust quantum information transductionvia a direct conversion approach. In [Phys. Rev.Applied 16, 064044 (2021)], we propose a differenttransduction paradigm based on teleportation thatshows a much higher rate in the low cooperativitiyregion. While more recently, [arXiv:2204.05521]proposes to utilize microwave squeezing to help withdirect conversion. In this work, we explore the role ofsqueezing in teleportation-based transduction protocoland identify a signifi cant performance boost viaevaluating quantum capacity lower and upper bounds.Our analyses includes both microwave squeezing andoptical squeezing, and provides a systematicalbenchmark between teleportation-based approach anddirect-conversion approach.   

Bidirectional multi-photon communication between remote superconducting resonators

Quantum communication protocols based on multi-photon states can support larger transmitted information density than those relying on single photon states. Quantum networks based on superconducting circuits have been used to synthesize and transmit arbitrary qubit [1,2] and multi-photon resonator states [3] between nodes. Itinerant communication between nodes has been limited to single photons [4]. Here we demonstrate bidirectional multiphoton transfers between two remote tunable resonators in a superconducting system, enabling single-pass on-demand transfers of photon superposition states.

[1] P. Kurpiers, et al, Nature, 558, 264-267 (2018)

[2] P. Campagne-Ibarcq, et al, Phys. Rev. Lett., 120, 200501 (2018)

[3] C. J. Axline, et al, Nature Physics, 14, 705-710 (2018)

[4] Y. Zhong, et al, Nature Physics, (2019)

    

Optimal input states for quantifying the performance of continuous-variable unidirectional and bidirectional teleportation

Continuous-variable (CV) teleportation is a foundational protocol in quantum information science. A number of experiments have been designed to simulate ideal teleportation under realistic conditions. In this paper, we detail an analytical approach for determining optimal input states for quantifying the performance of CV unidirectional and bidirectional teleportation. The metric that we consider for quantifying performance is the energy-constrained channel fidelity between ideal teleportation and its experimental implementation, and along with this, our focus is on determining optimal input states for distinguishing the ideal process from the experimental one. We prove that, under certain energy constraints, the optimal input state in unidirectional, as well as bidirectional, teleportation is a finite entangled superposition of twin-Fock states saturating the energy constraint. Moreover, we also prove that, under the same constraints, the optimal states are unique; that is, there is no other optimal finite entangled superposition of twin-Fock states.   

Cavity Piezomechanics for Microwave-Optical Quantum Transduction and Network

Connecting the two quantum worlds in microwave and optics is a long-pursing goal in quantum information science. It would allow the integration of state-of-the-art microwave quantum processors, such as superconducting qubits, with long-distance quantum communication channels through fiber optics, hence establish a future quantum network. High-fidelity quantum state transfer between microwave and optical frequencies will also enable new quantum technologies such as distributed quantum computing and sensing. Here, we discuss microwave-optical quantum transduction with a focus on cavity piezomechanics as an integrated platform for providing efficient interaction between superconducting and nanophotonic circuits. Importantly, the high operation frequency of piezomechanics at several GHz can substantially suppress thermal noise, making it an appealing candidate for quantum applications. We present experimental efforts on integrated superconducting piezo-optomechanical transducers with simultaneous electromechanical and optomechanical cavity enhancement and demonstrate bidirectional linear photon conversion between microwave and optics. We also discuss the entanglement-based quantum transduction and compare with the direct quantum transduction scheme in the framework of quantum channel theory. Based on practical piezo-optomechanical system parameters, we show that the entanglement-based scheme can indeed admit a positive transduction rate when the direct quantum transduction has zero quantum capacity. Using a pair of piezo-optomechanical transducers, we also propose protocols for remote microwave-microwave entanglement within both continuous variable and discrete variable settings, showing the potential of connecting distant microwave quantum processors via quantum teleportation.   

Transfer of squeezing and entanglement near the exceptional points of three coupled superconducting resonators

Continuous-variable systems have a broad range of applications in quantum information, metrology, and thermodynamics. In all these fields, dissipative dynamics can contribute to state synthesis, which motivates the incorporation of tunable elements exploring the potential benefits of open-system degeneracies, referred to as exceptional points (EPs). Here we show theoretical results for the exceptional-point engineering in a continuous-variable Gaussian system which consistis of a lossy chain of three superconducting resonators. We observe that the rich structure of branches yielding EPs can be used to identify regions in the parameter space that favor a fast and stable transfer of squeezing, entanglement, and the reset of the system. Our results shed light on the role of EPs in multimode Gaussian systems and pave the way for optimized distribution of squeezing and entanglement between different nodes of a photonic network.

    

On-demand generation of quantum photonic resource states using a phononic memory

Fusion-based linear optical quantum computing using dual-rail qubits typically relies on spontaneous parametric down-conversion sources, which act as probabilistic single photon sources. Although the probability of generation is low, a successful event can be heralded. Successfully generated photons pass through a complex network which multiplexes them into definite spatiotemporal slots for further processing. In this work, we propose a scheme for on-demand single photon generation through optomechanical readout of phononic resonator arrays which buffer single-phonon states. First, the opto-mechanical coupling is used to probabilistically generate and herald a single phonon. Due to the long phononic lifetime, the single quantum excitation can be stored in the phonon mode until needed, at which time it can be deterministically converted to an optical photon and integrated into a linear-optic network. Our proposal for linear-optic quantum computing based on phononic quantum memories greatly simplifies optical routing and resource requirements by displacing the redundancy necessary whenever using probabilistic state generation from space into time.   

Ultrahigh-Q on-chip silicon-germanium microresonators for quantum transduction

As an entirely monocrystalline photonics platform, Si_1-xGe_x/Si has exceptionally low losses in both microwave [1] optical [2] and domains, with internal optical quality factors reaching 170M at room temperature. At 4 K, Si_1-xGe_x/Si waveguides still have high quality factors but also a low threshold for bistability and pulsation, which are nonlinear optical effects deriving from the interplay between optically generated free carrier dispersion and thermo-optics. While instabilities deriving from them can be detrimental, they can be mitigated by applying DC electric fields. Together with silicon’s intrinsically high third-order nonlinear optical suscetability, this active control over nonlinearities could lead silicon germanium to applications like electro-optic modulation [3], optical logic, Kerr comb generation, and quantum transduction [4].