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.