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Deterministic creation of entangled atom–light Schrödinger-cat states


Quantum physics allows for entanglement between microscopic and macroscopic objects, described by discrete and continuous variables, respectively. As in Schrödinger’s famous cat gedanken experiment, a box enclosing the objects can keep the entanglement alive. For applications in quantum information processing, however, it is essential to access the objects and manipulate them with suitable quantum tools. Here we reach this goal and deterministically generate entangled light–matter states by reflecting a coherent light pulse with up to four photons on average from an optical cavity containing one atom. The quantum light propagates freely and reaches a remote receiver for quantum state tomography. We produce a plethora of quantum states and observe negative-valued Wigner functions, a characteristic sign of non-classicality. As a first application, we demonstrate a quantum-logic gate between an atom and a light pulse, with the photonic qubit encoded in the phase of the light field.

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The authors thank J.I. Cirac, S. Dürr and O. Morin for valuable ideas and discussions. This work was supported by the Deutsche Forschungsgemeinschaft via the excellence cluster Nanosystems Initiative Munich (NIM) and the EU flagship project Quantum Internet Alliance (QIA). S.W. was supported by Elitenetzwerk Bayern (ENB) through the doctoral program Exploring Quantum Matter (ExQM).

Author information

Experimental data were taken and analysed by B.H., S.W., S.D. and L.L. The homodyne detection set-up was built by B.H., S.W., S.D., A.S., S.R. and L.L. The manuscript was written by B.H., S.W. and G.R., with input from all authors.

Competing interests

The authors declare no competing interests.

Correspondence to Bastian Hacker.

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Fig. 1: Experimental set-up.
Fig. 2: Cat state generation protocol and Wigner functions of experimentally measured cat states.
Fig. 3: Non-classical properties of measured cat states.
Fig. 4: Control over cat state degrees of freedom.
Fig. 5: Entanglement between atom and cat state.
Fig. 6: Truth table of the atom–cat gate.