Quantum transducers convert quantum signals through hybrid interfaces of physical platforms in quantum networks. Modeled as quantum communication channels, performance of unidirectional quantum transduction can be measured by the quantum channel capacity. However, characterizing performance of quantum transducers used for duplex quantum transduction where signals are converted bidirectionally remains an open question. Here, we propose rate regions to characterize the performance of duplex quantum transduction. Using this tool, we find that quantum transducers optimized for simultaneous duplex transduction can outperform strategies based on the standard protocol of time-shared unidirectional transduction. Integrated over the frequency domain, we demonstrate that the rate region can also characterize quantum transducers with finite bandwidth.
Quantum transducers convert quantum signals through hybrid interfaces of physical platforms in quantum networks. Modeled as quantum communication channels, performance of unidirectional quantum transduction can be measured by the quantum channel capacity. However, characterizing performance of quantum transducers used for duplex quantum transduction where signals are converted bidirectionally remains an open question. Here, we propose rate regions to characterize the performance of duplex quantum transduction. Using this tool, we find that quantum transducers optimized for simultaneous duplex transduction can outperform strategies based on the standard protocol of time-shared unidirectional transduction. Integrated over the frequency domain, we demonstrate that the rate region can also characterize quantum transducers with finite bandwidth.
High-performance quantum transducers, which faithfully convert quantum information between disparate physical carriers, are essential in quantum science and technology. Different figures of merit, including efficiency, bandwidth, and added noise, are typically used to characterize the transducers' ability to transfer quantum information. Here we utilize quantum capacity, the highest achievable qubit communication rate through a channel, to define a single metric that unifies various criteria of a desirable transducer. Using the continuous-time quantum capacities of bosonic pure-loss channels as benchmarks, we investigate the optimal designs of generic quantum transduction schemes implemented by transmitting external signals through a coupled bosonic chain. With physical constraints on the maximal coupling rate [Formula: see text], the highest continuous-time quantum capacity [Formula: see text] is achieved by transducers with a maximally flat conversion frequency response, analogous to Butterworth electric filters. We further investigate the effect of thermal noise on the performance of transducers.
Quantum transducers play a crucial role in hybrid quantum networks. A good quantum transducer can faithfully convert quantum signals from one mode to another with minimum decoherence. Most investigations of quantum transduction are based on the protocol of direct mode conversion. However, the direct protocol requires the matching condition, which in practice is not always feasible. Here we propose an adaptive protocol for quantum transducers, which can convert quantum signals without requiring the matching condition. The adaptive protocol only consists of Gaussian operations, feasible in various physical platforms. Moreover, we show that the adaptive protocol can be robust against imperfections associated with finite squeezing, thermal noise, and homodyne detection, and it can be implemented to realize quantum state transfer between microwave and optical modes.
Abstract Recent technical advances have sparked renewed interest in physical systems that couple simultaneously to different parts of the electromagnetic spectrum, thus enabling transduction of signals between vastly different frequencies at the level of single quanta. Such hybrid systems have demonstrated frequency conversion of classical signals and have the potential of enabling quantum state transfer, e.g., between superconducting circuits and traveling optical signals. This article describes a simple approach for the theoretical characterization of the performance of quantum transducers. Given that, in practice, one cannot attain ideal one-to-one quantum conversion, we explore how imperfections impact the performance of the transducer in various scenarios. We quantify how knowledge of the well-established transducer parameters signal transfer efficiency η and added noise N suffices to assess its performance in a variety of transduction schemes ranging from classical signal detection to applications for quantum information processing.
Quantum transducers are critical for quantum interconnect, enabling coherent signal transfer across disparate frequency domains. Beyond material and device advances, protocol design has become a powerful means to improve transduction. We introduce a variational quantum transduction (VQT) framework that employs variational tools from near-term quantum computing to systematically optimize protocol performance. As a variational quantum circuit framework, VQT is not plagued by known training issues such as barren plateau, because a small-scale problem is sufficient for substantial advantage and training only needs to be done once to configure a VQT system. Maximizing the quantum information rate within this framework yields protocols that surpass all known schemes in their respective classes. For non-adaptive protocols, VQT exceeds the performance envelopes of Gottesman-Kitaev-Preskill (GKP)-based and entanglement-assisted approaches. In the adaptive setting, VQT provides only a marginal improvement over Gaussian feedforward strategies, indicating that Gaussian adaptive transduction is already close to optimal. With increasingly universal quantum control, VQT provides a systematic path toward optimal quantum transduction.
A microwave-optical transducer of sufficiently low noise and high signal transfer rate would allow entanglement to be distributed between superconducting quantum processors reliably within the lifetimes of their quantum memories. We report the continuous operation of an efficient transducer at high enough bandwidth to realize kHz signal transfer rates, in downconversion as well as upconversion, with noise levels approaching that required for quantum information transfer. To define the significance of our advance and clarify the multifaceted performance required for quantum networking tasks, we derive a broadband quantum channel capacity that bounds the maximum rate at which quantum information can be sent through realistic finite-bandwidth thermal-loss channels. This capacity serves as a comprehensive measure of transducer capability and provides insight into the relative importance of disparate performance metrics. We find that the broadband capacity depends on the throughput---defined as the product of efficiency, bandwidth, and duty cycle---and on the added noise. We present measurements of a membrane-based opto-electromechanical transducer with high throughput of 7 kHz and at an input-referred added noise of 3 photons in both upconversion and downconversion, demonstrating that bidirectional transducer capacities comparable to superconducting qubit decay rates are within reach. In downconversion, throughput of this magnitude at the few-photon noise level is unprecedented, marking an improvement of nearly four orders of magnitude over previous work.
The complementary features of different qubit platforms for computing and communicating impose an intrinsic hardware heterogeneity in any quantum network, where nodes, while processing and storing quantum information, must also communicate through quantum links. Indeed, one of the most promising hardware platforms at quantum node scale for scalable and fast quantum computing is the superconducting technology, which operates at microwave frequencies. Whereas, for communicating at distances of practical interest beyond few meters, quantum links should operate at optical frequencies. Therefore, to allow the interaction between superconducting and photonic technologies, a quantum interface, known asquantum transducer, able to convert one type of qubit to another is required. This paper aims to provide a tutorial treatise on the fundamental research challenges of quantum transduction. The tutorial is structured around a communications engineering framework, thereby shedding light on its fundamental role in quantum network design and deployment—a perspective often overlooked in existing literature. This framework allows us to categorize different transduction modalities and to reveal an unorthodox one where the transducer itself can act as an entanglement source. From this standpoint, it is possible to conceive different source-destination link archetypes, where transduction plays a crucial role in the communication performances. The analysis also translates the quantum transduction process into a proper functional block within a new communication system model for a quantum network.