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.As early as 1935, Schrödinger formulated a gedanken experiment 1 with a living cat and a radioactive atom placed inside a box, which is then closed. When the atom decays, it triggers a death mechanism that kills the cat. According to the laws of quantum physics, the decay of the atom occurs at some random time. Consequently, the time of death of the cat is unknown. Mathematically, the situation inside the box is described by an entangled superposition state that is known as a 'Schrödinger-cat state'. Most remarkable, this state offers a unique access to the atom-cat system, at least in principle. For example, a measurement apparatus that is capable of measuring the atom in a superposition of 'not decayed' and 'decayed' immediately projects the cat into a superposition of 'alive' and 'dead'. Such observation thus transfers the superposition state of the microscopic quantum object into the macroscopic classical world, something weird for a cat. In contrast to the entangled Schrödinger-cat state, the coherent superposition state of the cat is here denoted as a 'cat state'.While notoriously difficult to create 2 , several implementations of Schrödinger-cat states or just cat states have emerged during recent decades. In all these experiments, coherent states with distinguishable phases mimic the two cat states 'dead' and 'alive'. Most prominently, Schrödinger-cat states were explored using a trapped ion 3,4 , with the vibrational state in the trap taking the role of the cat, and coherent microwave fields confined to superconducting boxes were used in combination with Rydberg atoms 5,6 and superconducting qubits 7 . In the latter experiment, the cat state was also released from the microwave resonator 8 .Modern applications in an open quantum-communication and distributed quantum-networking architecture could benefit from cat states that propagate over some distances. As long as superconducting transmission lines exist for short distances only, optical fields propagating through low-loss optical fibres in the (near) visible part of the electrom...

Quantum logic gates are fundamental building blocks of quantum computers. Their integration into quantum networks requires strong qubit coupling to network channels, as can be realized with neutral atoms and optical photons in cavity quantum electrodynamics. Here we demonstrate that the long-range interaction mediated by a flying photon performs a gate between two stationary atoms inside an optical cavity from which the photon is reflected. This single step executes the gate in 2 µs. We show an entangling operation between the two atoms by generating a Bell state with 76(2)% fidelity. The gate also operates as a CNOT. We demonstrate 74.1(1.6)% overlap between the observed and the ideal gate output, limited by the state preparation fidelity of 80.2(0.8)%. As the atoms are efficiently connected to a photonic channel, our gate paves the way towards quantum networking with multiqubit nodes and the distribution of entanglement in repeater-based long-distance quantum networks.

The big challenge in quantum computing is to realize scalable multi-qubit systems with cross-talk–free addressability and efficient coupling of arbitrarily selected qubits. Quantum networks promise a solution by integrating smaller qubit modules to a larger computing cluster. Such a distributed architecture, however, requires the capability to execute quantum-logic gates between distant qubits. Here we experimentally realize such a gate over a distance of 60 meters. We employ an ancillary photon that we successively reflect from two remote qubit modules, followed by a heralding photon detection, which triggers a final qubit rotation. We use the gate for remote entanglement creation of all four Bell states. Our nonlocal quantum-logic gate could be extended both to multiple qubits and many modules for a tailor-made multi-qubit computing register.

We demonstrate entanglement generation of two neutral atoms trapped inside an optical cavity. Entanglement is created from initially separable two-atom states through carving with weak photon pulses reflected from the cavity. A polarization rotation of the photons heralds the entanglement. We show the successful implementation of two different protocols and the generation of all four Bell states with a maximum fidelity of (90 ± 2)%. The protocol works for any distance between cavitycoupled atoms, and no individual addressing is required. Our result constitutes an important step towards applications in quantum networks, e.g. for entanglement swapping in a quantum repeater.Entanglement is a central ingredient of quantum physics. It was long debated until groundbreaking experiments with entangled photons [1,2] Here we follow this proposal and show entanglement of two atomic qubits within one cavity QED node from which photons are reflected and detected with polarization-sensitive counters. As the scheme is insensitive to fluctuating photon numbers, we employ weak coherent laser pulses and use photon detection as a herald. Combined with atomic state rotations, we produce entangled states from an initially separable atom pair state, and show the creation of all four maximally entangled Bell states. Our scheme features several distinct advantages that distinguish it from other entanglement schemes [8,9,18,19]. Most notably, the interaction strength between two atoms coupled to the optical cavity does not depend on distance. Also, individual addressing of the two atoms is not required, rendering the technique robust, e.g., against focusing and pointing errors of the laser used for atomic state rotations. Moreover, the entangling protocol is fast, limited only by the duration of the atomic state rotation and the light pulses. The minimum pulse duration is determined by the cavity linewidth.Following [20], we call our technique carving. An initially separable state of two atoms undergoes a common projective measurement with probabilistic outcome. For an appropriately chosen projection subspace, the part of a two-atom wavefunction in that subspace can be entangled. If the measurement yields the orthogonal outcome, the atoms are not entangled and the attempt will cos (θ/2)|↑ j − e iφ sin (θ/2)|↓ j are coherent spin states defined by the spherical coordinates θ and φ on the generalized Bloch sphere. The initial distribution (left sphere) is rotated around the y-axis (middle sphere) before the |↓↓ and |↑↑ components are carved out. After the carving step, the poles are not populated anymore and the maximally entangled Bell-state |Ψ + is prepared (right sphere). The color code is normalized on each sphere and Q increases from dark to bright. be discarded. Specifically, each of the two atoms carries a qubit encoded in the states |↑ and |↓ . Starting with an initially separable two-atom state |↓↓ , a global π/2 rotation prepares 1 2 (|↑↑ − |↑↓ − |↓↑ + |↓↓ ). A projective measurement ("carving") allows us to probabilistically re...

Quantum teleportation enables the deterministic exchange of qubits via lossy channels. While it is commonly believed that unconditional teleportation requires a preshared entangled qubit pair, here we demonstrate a protocol that is in principle unconditional and requires only a single photon as an ex-ante prepared resource. The photon successively interacts, first, with the receiver and then with the sender qubit memory. Its detection, followed by classical communication, heralds a successful teleportation. We teleport six mutually unbiased qubit states with average fidelity F = (88.3 ± 1.3)% at a rate of 6 Hz over 60 m.

Single photons with tailored temporal profiles are a vital resource for future quantum networks. Here we distill them out of custom-shaped laser pulses that reflect from a single atom strongly coupled to an optical resonator. A subsequent measurement on the atom is employed to herald a successful distillation. Out of vacuum-dominated light pulses, we create single photons with fidelity 66(1)%, two-and-more-photon suppression 95.5(6)%, and a Wigner function with negative value −0.125(6). Our scheme applied to state-of-the-art fiber resonators could boost the single-photon fidelity to up to 96%.

One of the most fascinating aspects of quantum networks is their capability to distribute entanglement as a nonlocal communication resource1. In a first step, this requires network-ready devices that can generate and store entangled states2. Another crucial step, however, is to develop measurement techniques that allow for entanglement detection. Demonstrations for different platforms3–13 suffer from being not complete, destructive or local. Here, we demonstrate a complete and nondestructive measurement scheme14–16 that always projects any initial state of two spatially separated network nodes onto a maximally entangled state. Each node consists of an atom trapped inside an optical resonator from which two photons are successively reflected. Polarization measurements on the photons discriminate between the four maximally entangled states. Remarkably, such states are not destroyed by our measurement. In the future, our technique might serve to probe the decay of entanglement and to stabilize it against dephasing via repeated measurements17,18.

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