Frequency-tunable microwave quantum light source based on superconducting quantum circuits

  1. Yan Li,
  2. Zhiling Wang,
  3. Zenghui Bao,
  4. Yukai Wu,
  5. Jiahui Wang,
  6. Jize Yang,
  7. Haonan Xiong,
  8. Yipu Song,
  9. Hongyi Zhang,
  10. and Luming Duan
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 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.

An ultra-high gain single-photon transistor in the microwave regime

  1. Zhiling Wang,
  2. Zenghui Bao,
  3. Yan Li,
  4. Yukai Wu,
  5. Weizhou Cai,
  6. Weiting Wang,
  7. Xiyue Han,
  8. Jiahui Wang,
  9. Yipu Song,
  10. Luyan Sun,
  11. Hongyi Zhang,
  12. and Luming Duan
A photonic transistor that can switch or amplify an optical signal with a single gate photon requires strong non-linear interaction at the single-photon level. Circuit quantum electrodynamics
provides great flexibility to generate such an interaction, and thus could serve as an effective platform to realize a high-performance single-photon transistor. Here we demonstrate such a photonic transistor in the microwave regime. Our device consists of two microwave cavities dispersively coupled to a superconducting qubit. A single gate photon imprints a phase shift on the qubit state through one cavity, and further shifts the resonance frequency of the other cavity. In this way, we realize a gain of the transistor up to 53.4 dB, with an extinction ratio better than 20 dB. Our device outperforms previous devices in the optical regime by several orders in terms of optical gain, which indicates a great potential for application in the field of microwave quantum photonics and quantum information processing.

Experimental preparation of generalized cat states for itinerant microwave photons

  1. Zenghui Bao,
  2. Zhiling Wang,
  3. Yukai Wu,
  4. Yan Li,
  5. Weizhou Cai,
  6. Weiting Wang,
  7. Yuwei Ma,
  8. Tianqi Cai,
  9. Xiyue Han,
  10. Jiahui Wang,
  11. Yipu Song,
  12. Luyan Sun,
  13. Hongyi Zhang,
  14. and Luming Duan
Generalized cat states represent arbitrary superpositions of coherent states, which are of great importance in various quantum information processing protocols. Here we demonstrate
a versatile approach to creating generalized itinerant cat states in the microwave domain, by reflecting coherent state photons from a microwave cavity containing a superconducting qubit. We show that, with a coherent control of the qubit state, a full control over the coherent state superposition can be realized. The prepared cat states are verified through quantum state tomography of the qubit state dependent reflection photon field. We further quantify quantum coherence in the prepared cat states based on the resource theory, revealing a good experimental control on the coherent state superpositions. The photon number statistic and the squeezing properties are also analyzed. Remarkably, fourth-order squeezing is observed in the experimental states. Those results open up new possibilities of applying generalized cat states for the purpose of quantum information processing.

A flying Schrödinger cat in multipartite entangled states

  1. Zhiling Wang,
  2. Zenghui Bao,
  3. Yukai Wu,
  4. Yan Li,
  5. Weizhou Cai,
  6. Weiting Wang,
  7. Yuwei Ma,
  8. Tianqi Cai,
  9. Xiyue Han,
  10. Jiahui Wang,
  11. Yipu Song,
  12. Luyan Sun,
  13. Hongyi Zhang,
  14. and Luming Duan
Schrödinger’s cat originates from the famous thought experiment querying the counterintuitive quantum superposition of macroscopic objects. As a natural extension, several „cats“
(quasi-classical objects) can be prepared into coherent quantum superposition states, which is known as multipartite cat states demonstrating quantum entanglement among macroscopically distinct objects. Here we present a highly scalable approach to deterministically create flying multipartite Schrödinger cat states, by reflecting coherent state photons from a microwave cavity containing a superconducting qubit. We perform full quantum state tomography on the cat states with up to four photonic modes and confirm the existence of quantum entanglement among them. We also witness the hybrid entanglement between discrete-variable states (the qubit) and continuous-variable states (the flying multipartite cat) through a joint quantum state tomography. Our work demonstrates an important experimental control method in the microwave region and provides an enabling step for implementing a series of quantum metrology and quantum information processing protocols based on cat states.

On-Demand Storage and Retrieval of Microwave Photons Using a Superconducting Multiresonator Quantum Memory

  1. Zenghui Bao,
  2. Zhiling Wang,
  3. Yukai Wu,
  4. Yan Li,
  5. Cheng Ma,
  6. Yipu Song,
  7. Hongyi Zhang,
  8. and Luming Duan
A quantum memory that can store quantum states faithfully and retrieve them on demand has wide applications in quantum information science. An efficient quantum memory in the microwave
regime working alongside quantum processors based on superconducting quantum circuits may serve as an important architecture for quantum computers. Here we realize on-demand storage and retrieval of weak coherent microwave photon pulses at the single-photon level. We implement a superconducting multi-resonator quantum memory which is composed of a set of frequency-tunable coplanar transmission line (CPW) resonators. By dynamically tuning the resonant frequencies of the resonators, we achieve tunable memory bandwidth from 10 MHz to 55 MHz, with an overall storage efficiency up to 12 % with well preserved phase coherence. We further demonstrate on-demand storage and retrieval of a time-bin flying qubit. This result opens up a prospect to integrate our chip-based quantum memory with the state-of-the-art superconducting quantum circuit technology for quantum information processing.

Improved superconducting qubit state readout by path interference

  1. Zhiling Wang,
  2. Zenghui Bao,
  3. Yukai Wu,
  4. Yan Li,
  5. Cheng Ma,
  6. Tianqi Cai,
  7. Yipu Song,
  8. Hongyi Zhang,
  9. and Luming Duan
High fidelity single shot qubit state readout is essential for many quantum information processing protocols. In superconducting quantum circuit, the qubit state is usually determined
by detecting the dispersive frequency shift of a microwave cavity from either transmission or reflection. In this paper, we demonstrate the use of constructive interference between the transmitted and reflected signal to optimize the qubit state readout, with which we find a better resolved state discrimination and an improved qubit readout fidelity. As a simple and convenient approach, our scheme can be combined with other qubit readout methods based on the discrimination of cavity photon states to further improve the qubit state readout.

Perturbation impact of spectators and spurious qubit interactions on a cross-resonance gate in a tunable coupling superconducting circuit

  1. Tianqi Cai,
  2. Xiyue Han,
  3. Yukai Wu,
  4. Yulin Ma,
  5. Jiahui Wang,
  6. Hongyi Zhang,
  7. Haiyan Wang,
  8. Yipu Song,
  9. and Luming Duan
Cross-resonance (CR) gate has proved to be a promising scheme for implementing fault-tolerant quantum computation with fixed-frequency qubits. In this work, we experimentally implement
an entangling cross-resonance gate by using a microwave-only control in a tunable coupling superconducting circuit. The flux-controlled tunable coupler allows us to continuously vary adjacent qubit coupling from positive to negative values, and thus providing an extra degree of freedom to verify optimal condition for constructing the CR gate. Based on three-qubit CR Hamiltonian tomography, we systematically investigate the dependency of gate fidelities on spurious interaction components and present the first experimental approach to evaluate the perturbation impact arising from the spectator qubits. Our results reveal that the spectator qubits can lead to reductions in the CR gate fidelity relying on the particular frequency resonance poles and the induced ZZ interaction between the spectator and gate qubits, while an improvement in the gate fidelity to 98.5% can be achieved by optimally tuning the inter-qubit detuning and flux bias on the coupler. Our experiments uncover an optimal CR operation regime and provide a guiding principle to improve the CR gate fidelity by suppression of unwanted qubit interactions.