Abstract Superconducting circuits offer a scalable platform for the construction of large-scale quantum networks, where information can be encoded in multiple temporal modes of propagating microwaves. Characterization of such microwave signals with a method extendable to an arbitrary number of temporal modes with a single detector and demonstration of their phase-robust nature are of great interest. Here, we show the on-demand generation and Wigner tomography of a microwave time-bin qubit with superconducting circuit quantum electrodynamics architecture. We perform the tomography with a single heterodyne detector by dynamically switching the measurement quadrature independently for two temporal modes through the pump phase of a phase-sensitive amplifier. We demonstrate that the time-bin encoding scheme relies on the relative phase between the two modes and does not need a shared phase reference between sender and receiver.
Abstract Superconducting circuits offer a scalable platform for the construction of large-scale quantum networks, where information can be encoded in multiple temporal modes of propagating microwaves. Characterization of such microwave signals with a method extendable to an arbitrary number of temporal modes with a single detector and demonstration of their phase-robust nature are of great interest. Here, we show the on-demand generation and Wigner tomography of a microwave time-bin qubit with superconducting circuit quantum electrodynamics architecture. We perform the tomography with a single heterodyne detector by dynamically switching the measurement quadrature independently for two temporal modes through the pump phase of a phase-sensitive amplifier. We demonstrate that the time-bin encoding scheme relies on the relative phase between the two modes and does not need a shared phase reference between sender and receiver.
A nonclassical light source is essential for implementing a wide range of quantum information processing protocols, including quantum computing, networking, communication, and metrology. In the microwave regime, propagating photonic qubits that transfer quantum information between multiple superconducting quantum chips serve as building blocks of large-scale quantum computers. In this context, spectral control of propagating single photons is crucial for interfacing different quantum nodes with varied frequencies and bandwidth. Here we demonstrate a deterministic microwave quantum light source based on superconducting quantum circuits that can generate propagating single photons, time-bin encoded photonic qubits and qudits. In particular, the frequency of the emitted photons can be tuned in situ as large as 200 MHz. Even though the internal quantum efficiency of the light source is sensitive to the working frequency, we show that the fidelity of the propagating photonic qubit can be well preserved with the time-bin encoding scheme. Our work thus demonstrates a versatile approach to realizing a practical quantum light source for future distributed quantum computing.
Summary form only given. One of today's challenges to realize computing based on quantum mechanics is to reliably and scalably encode information in quantum systems. This is particularly crucial when dealing with promising approaches such as linear optics quantum computing (LOQC) in photonic circuits [1,2]. Although this method is in principle scalable, in practice it is limited by the probabilistic nature of spontaneous parametric down-conversion (SPDC) sources used to seed the circuits.Here, we demonstrate a novel tool to deterministically deliver quantum bits of information for LOQC circuits with the help of single photons emitted from a strongly coupled atom-cavity system (Fig. [1a]). Due to the ≈ 500ns coherence time of these photons [3] and our ability to arbitrarily shape their amplitude and phase profile we time-bin encode the information within one photon. Therefore, the preparation of a single photon sub-divided in d time bins allows for the delivery of arbitrary qu-d-its. RCPWe verify the fidelity of the quantum state preparation with time-resolved quantum-homodyne measurements [4], performed by sending single signal and reference photons into an elementary photonic circuit. Photon correlations monitored in a time-resolved manner then allow the state to be partially reconstructed. To obtain more insight into the time-bin encoding of information within one photon, it is helpful to define a `virtual' photonic circuit (Fig. [1b]) where temporal modes are displayed as spatial ones. Due to the photons being much longer than the detector time resolution, the absolute time of each photo-detection is used to associate the event with one of these virtual detectors. In this way, the respective number of cross-correlation signals for qubits and qutrits, respectively, can be found (Fig. [1c]). We obtain photon production efficiencies up to 85% and state-preparation fidelities of F=96% [4]. Due to the timebin encoding taking place upon the photon's generation process, our technique is inherently non-probabilistic, versatile, reconfigurable and not subject to systematic photon losses. Time-bins as an additional degree of freedom in LOQC experiments and an their deterministic photon-generation scheme can be regarded as a big step towards large-scale quantum computing in photonic networks.
Quantum communication between distant superconducting qubits on separate chips using itinerant microwave photons has been studied to realize distributed quantum information processing. To enhance information capacity and fault tolerance in quantum networks, it is beneficial to encode a large quantity of quantum information using auxiliary degrees of freedom of these photons. In this work, we experimentally investigate the use of temporal modes of photon wave packets. Through the photon-shaping technique with a fixed-frequency transmon qubit, we generate single microwave photons in four orthogonal temporal modes propagating along a waveguide. We demonstrate mode-selective absorption across orthogonal modes via the time-reversed process of emission, achieving absorption efficiencies exceeding 0.89 for mode-matched cases, while remaining below 0.13 for orthogonal modes. Photons rejected by a given receiver mode can remain mutually orthogonal, enabling selective absorption at subsequent receivers in future multi-node architectures. These results highlight the feasibility of temporal-mode engineering for constructing a higher-dimensional orthogonal basis for multiplexed quantum networks.
Scaling up a superconducting quantum computer will likely require quantum communication between remote chips, which can be implemented using an itinerant microwave photon in a transmission line. To realize high-fidelity communication, it is essential to control the frequency and temporal shape of the microwave photon. In this work, we demonstrate the generation of frequency-tunable shaped microwave photons without resorting to any frequency-tunable circuit element. We develop a framework that treats a microwave resonator as a band-pass filter mediating the interaction between a superconducting qubit and the modes in the transmission line. This interpretation allows us to stimulate the photon emission by an off-resonant drive signal. We characterize how the frequency and temporal shape of the generated photon depends on the frequency and amplitude of the drive signal. By modulating the drive amplitude and frequency, we achieve a frequency tunability of 40 MHz while maintaining the photon mode shape time symmetric. Through measurements of the quadrature amplitudes of the emitted photons, we demonstrate a consistently high state and process fidelities around 95% across the tunable frequency range. Our hardware-efficient approach eliminates the need for additional biasing lines typically required for frequency tuning, offering a simplified architecture for scalable quantum communication.
We present a superconducting device that realizes the sequential measurement of a transmon qubit. The device disables common limitations of dispersive readout such as Purcell effect or transients in the cavity mode by turning on and off the coupling to the measurement channel on demand. The qubit measurement begins by loading a readout resonator that is coupled to the qubit. After an optimal interaction time with negligible loss, a microwave pump releases the content of the readout mode by upconversion into a measurement line in a characteristic time as low as 10 ns, which is 400 times shorter than the lifetime of the readout resonator. A direct measurement of the released field quadratures demonstrates a readout fidelity of 97.5% in a total measurement time of 220 ns. The Wigner tomography of the readout mode allows us to characterize the non-Gaussian nature of the readout mode and its dynamics.
A classical switch or router determines which of several inputs to the device passes through the output. In analogy, a quantum switch that coherently links the input to a superposition of several outputs plays an important role in quantum information processing, especially when operating at the fundamental limit where a single photon can be controlled. The controllable path superposition of propagating photonic qubits enabled by the quantum switch provides the basis for numerous quantum applications, including entanglement generation and distribution, enhanced quantum communication, and quantum random access memory. In this work, we realize such a quantum switch for microwave single photons with superconducting quantum circuits. We use a superconducting qubit to coherently switch the output path of single microwave photons and confirm quantum entanglement between the control qubit and the photons' propagation path. We further showcase the generation of quantum entanglement between a time-bin-encoded propagating photonic qubit and a superconducting qubit with the quantum switch, thus providing a versatile approach to generate reconfigurable multinode entanglement in one step. As such, our work provides a prime building block for microwave quantum networks and modular superconducting quantum computing.