Characterizing entanglement of an artificial atom and a cavity cat state with Bell's inequality.

Brian Vlastakis,Liang Jiang,Mazyar Mirrahimi,M H Devoret,Zaki Leghtas,Luigi Frunzio,Nissim Ofek,Luyan Sun,Katrina Sliwa,Yehan Liu,R J Schoelkopf,Michael Hatridge,Jacob Blumoff,Andrei Petrenko

doi:10.1038/ncomms9970

Abstract

The Schrodinger's cat thought experiment highlights the counterintuitive concept of entanglement in macroscopically distinguishable systems. The hallmark of entanglement is the detection of strong correlations between systems, most starkly demonstrated by the violation of a Bell inequality. No violation of a Bell inequality has been observed for a system entangled with a superposition of coherent states, known as a cat state. Here we use the Clauser–Horne–Shimony–Holt formulation of a Bell test to characterize entanglement between an artificial atom and a cat state, or a Bell-cat. Using superconducting circuits with high-fidelity measurements and real-time feedback, we detect correlations that surpass the classical maximum of the Bell inequality. We investigate the influence of decoherence with states up to 16 photons in size and characterize the system by introducing joint Wigner tomography. Such techniques demonstrate that information stored in superpositions of coherent states can be extracted efficiently, a crucial requirement for quantum computing with resonators.

Highlights

The Schrodinger’s cat thought experiment highlights the counterintuitive concept of entanglement in macroscopically distinguishable systems
Using a circuit quantum electrodynamics architecture[16], we show efficient, highfidelity measurements of an encoded cat state qubit and demonstrate this technology by detecting a violation of the CHSH Bell inequality between the encoded cat state qubit and a superconducting transmon qubit[17]
We have demonstrated the efficient detection of an artificial atom and a cat state in a cavity mode

Summary

Introduction

The Schrodinger’s cat thought experiment highlights the counterintuitive concept of entanglement in macroscopically distinguishable systems. We investigate the influence of decoherence with states up to 16 photons in size and characterize the system by introducing joint Wigner tomography Such techniques demonstrate that information stored in superpositions of coherent states can be extracted efficiently, a crucial requirement for quantum computing with resonators. Alternative encoding schemes that use coherent state superpositions, known as cat states[12], take advantage of a cavity resonators much larger Hilbert space, as compared with that of a two-level system This architecture allows redundant qubit encodings that can simplify the operations needed to initialize, manipulate and measure the encoded information[13,14,15]. We measure a range in which correlations surpass the Bell inequality threshold and observe its reduction due to decoherence, benchmarking the efficiency of our encoding and detection schemes with cat state qubits

Methods

Results

Conclusion