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.
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.
Scalable quantum technologies may be applied in prospective architectures employing traditional information processing elements, such as transistors, rectifiers, or switches modulated by low-power inputs. In this respect, recently developed quantum processors based, e.g., on superconducting circuits may alternatively be employed as the basic platform for ultra-low-power consumption classical processors, in addition to obvious applications in quantum information processing and quantum computing. Here we propose a single-photon microwave switch based on a circuit quantum electrodynamics setup, in which a single control photon in a transmission line is able to switch on or off the propagation of another single photon in a separate line. The performances of this single-photon switch are quantified in terms of the photon flux through the output channel, providing a direct comparison of our results with available data. Furthermore, we show how the design of this microwave switch enables the recovery of the single control photon after the switching process. This proposal may be readily realized in state-of-art superconducting circuit technology.
A quantum router is an elementary building block for quantum-information processing. It directs the incoming quantum signal to different routes with phase coherence. To realize scalable quantum routing, a single particle needs to deterministically and coherently route the paths of a flying qubit, which requires nonlinear interaction at the single quantum level and complicated path-data qubit control so far beyond the experimental reach. Here we demonstrate a deterministic quantum router working at microwave frequencies based on superconducting circuits, which can naturally integrate with a leading platform for quantum computing. We encode the quantum signal into a weak coherent pulse and use an entanglement witness to rigorously verify the generated entanglement between the control qubit and the path of the data qubit, thus proving quantum coherence of the routing operation. Our experiment lays the foundation for useful applications of quantum routers in quantum networks and quantum random-access memories.
For a quantum computer, switching a signal carried by single microwave photons is desirable in scaling up superconducting circuitry. The authors design and build such a switch, integrate it on a chip with a single-photon source, and prove that it works with nonclassical microwave input. Their device offers negligible heating, relatively large bandwidth, low nonlinearity, and single-pole double-throw switching in mere nanoseconds. It is well suited for applications where signal routing must be controlled by real-time feedback, as in multiplexing qubit control and readout, or distributing entanglement in quantum networks.
The controlled transfer of quantum entangled states is a fascinating topic in quantum physics, and has potential applications in fields such as quantum network communication, distributed quantum information processing, and development of quantum functional devices. This study focuses on a circuit quantum electrodynamics (QED) system, comprising an input port, two output ports, and a quantum switch. The input port or each output port contains n microwave cavities. The switch is a superconducting transmon qutrit (three-level quantum system). The vacuum and single-photon states in each cavity encode a photonic qubit. We demonstrate that the single-excitation symmetric (SES) entangled state of n photonic qubits can be transferred from the n cavities at the input port to the n cavities at either of the two output ports, controlled by the switch. Remarkably, the operational time decreases as the number n increases. The only use of qutrit-cavity resonant interaction enables rapid quantum state transfer while effectively suppressing system decoherence. This proposal can be used to achieve the controlled transfer of both Bell states and W states of photonic qubits via a quantum switch. To validate the experimental feasibility, we numerically simulate two scenarios: (i) transferring the Bell state of two photonic qubits, and (ii) transferring the W state of three photonic qubits. This proposal may be extended to accomplish the same task in other physical systems, where the switch is implemented with a three-level artificial atom of different types while each cavity is a microwave or optical cavity.
Quantum emitters coupled to nanophotonic structures are an excellent platform for controllable single-photon scattering. The tunable light-matter interaction enables the construction of a single-photon switch -- a device that can route a single photon from an input port to a selected output port. Such single-photon switching devices can be integrated into reconfigurable photonic circuits to actively control the photon propagation direction in a quantum network. Ideally, a single-photon switch should operate with high speed, efficiency, and fidelity, preserving the state of the input photon in the routing process. This review brings together key input-output methods from quantum optics, theoretical proposals of emitter-based single-photon routing mechanisms, and experimental demonstrations of single-photon switching devices across different physical platforms, including semiconductor quantum dots, neutral atoms, superconducting qubits, and color centers. We highlight the need for reporting the key figures of merit (speed/efficiency/fidelity) in future single-photon switch demonstrations to support further developments in the field.
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.