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To enable two photons to interact, a single atom in an optical resonator is used to build a universal photon–photon quantum gate; this could lead to applications in long-distance quantum communication and scalable quantum computing that require the processing of optical quantum information. A quantum gate between two individual photons Two beams of light sharing the same space tend not to interact with one another. Yet if purely photonic technologies such as quantum communication and scalable quantum computing are to be developed — which require components such as switches and logic gates — it will be important to find conditions that facilitate controllable interactions between two photons. To that end, various single-photon quantum devices have been demonstrated in recent years, typically involving interactions between photons and atoms in a resonator. Here Stephan Ritter and colleagues employ such a system to make a logic component for quantum operations — a universal controlled phase flip photon–photon quantum gate — that involves interaction between two individual input photons mediated by a single atom. That two photons pass each other undisturbed in free space is ideal for the faithful transmission of information, but prohibits an interaction between the photons. Such an interaction is, however, required for a plethora of applications in optical quantum information processing 1 . The long-standing challenge here is to realize a deterministic photon–photon gate, that is, a mutually controlled logic operation on the quantum states of the photons. This requires an interaction so strong that each of the two photons can shift the other’s phase by π radians. For polarization qubits, this amounts to the conditional flipping of one photon’s polarization to an orthogonal state. So far, only probabilistic gates 2 based on linear optics and photon detectors have been realized 3 , because “no known or foreseen material has an optical nonlinearity strong enough to implement this conditional phase shift” 4 . Meanwhile, tremendous progress in the development of quantum-nonlinear systems has opened up new possibilities for single-photon experiments 5 . Platforms range from Rydberg blockade in atomic ensembles 6 to single-atom cavity quantum electrodynamics 7 . Applications such as single-photon switches 8 and transistors 9 , 10 , two-photon gateways 11 , nondestructive photon detectors 12 , photon routers 13 and nonlinear phase shifters 14 , 15 , 16 , 17 , 18 have been demonstrated, but none of them with the ideal information carriers: optical qubits in discriminable modes. Here we use the strong light–matter coupling provided by a single atom in a high-finesse optical resonator to realize the Duan–Kimble protocol 19 of a universal controlled phase flip (π phase shift) photon–photon quantum gate. We achieve an average gate fidelity of (76.2 ± 3.6) per cent and specifically demonstrate the capability of conditional polarization flipping as well as entanglement generation between independent input photons. This photon–photon quantum gate is a universal quantum logic element, and therefore could perform most existing two-photon operations. The demonstrated feasibility of deterministic protocols for the optical processing of quantum information could lead to new applications in which photons are essential, especially long-distance quantum communication and scalable quantum computing.

Quantum gates — in which stationary quantum bits are combined with ‘flying’ quantum bits, that is, photons — will be essential in quantum networks; such a gate, between a laser-trapped atomic quantum bit and a single photon, is now reported. Through the gate to quantum networks The development of a quantum gate between a flying optical photonic qubit (polarization) and a single trapped atomic qubit (spin) has been a long-standing goal in quantum information science. Such gates are required both for quantum computation to be scaled to a large number of qubits and for quantum communication to be scaled to long distances. Now two groups, working independently, report the successful implementation of such gates. Gerhard Rempe and colleagues demonstrate a quantum gate between a laser-trapped atomic qubit and a single photon, where the polarization of the photon is flipped depending exactly on the spin state of the atom. Mikhail Lukin and co-workers describe a similar achievement — a quantum gate effect between a single atom trapped near a photonic crystal and a single photon. The steady increase in control over individual quantum systems supports the promotion of a quantum technology that could provide functionalities beyond those of any classical device. Two particularly promising applications have been explored during the past decade: photon-based quantum communication, which guarantees unbreakable encryption 1 but which still has to be scaled to high rates over large distances, and quantum computation, which will fundamentally enhance computability 2 if it can be scaled to a large number of quantum bits (qubits). It was realized early on that a hybrid system of light qubits and matter qubits 3 could solve the scalability problem of each field—that of communication by use of quantum repeaters 4 , and that of computation by use of an optical interconnect between smaller quantum processors 5 , 6 . To this end, the development of a robust two-qubit gate that allows the linking of distant computational nodes is “a pressing challenge” 6 . Here we demonstrate such a quantum gate between the spin state of a single trapped atom and the polarization state of an optical photon contained in a faint laser pulse. The gate mechanism presented 7 , 8 is deterministic and robust, and is expected to be applicable to almost any matter qubit. It is based on reflection of the photonic qubit from a cavity that provides strong light–matter coupling. To demonstrate its versatility, we use the quantum gate to create atom–photon, atom–photon–photon and photon–photon entangled states from separable input states. We expect our experiment to enable various applications, including the generation of atomic 9 and photonic 10 cluster states and Schrödinger-cat states 11 , deterministic photonic Bell-state measurements 12 , scalable quantum computation 7 and quantum communication using a redundant quantum parity code 13 .

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.An atom–light Schrödinger-cat state is deterministically created by reflecting laser pulses from a high-finesse optical cavity containing a single 87Rb atom. A CNOT-type operation is also demonstrated between the atomic qubit and the optical qubit.

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 in2μs . 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.

All optical detectors to date annihilate photons upon detection, thus excluding repeated measurements. Here, we demonstrate a robust photon detection scheme that does not rely on absorption. Instead, an incoming photon is reflected from an optical resonator containing a single atom prepared in a superposition of two states. The reflection toggles the superposition phase, which is then measured to trace the photon. Characterizing the device with faint laser pulses, a single-photon detection efficiency of 74% and a survival probability of 66% are achieved. The efficiency can be further increased by observing the photon repeatedly. The large single-photon nonlinearity of the experiment should enable the development of photonic quantum gates and the preparation of exotic quantum states of light.

We study the frequency splitting of the polarization eigenmodes of the fundamental transverse mode in CO2 laser-machined, high-finesse optical Fabry-Perot cavities and investigate the influence of the geometry of the cavity mirrors. Their highly reflective surfaces are typically not rotationally symmetric but have slightly different radii of curvature along two principal axes. We observe that the eccentricity of such elliptical mirrors lifts the degeneracy of the polarization eigenmodes. The impact of the eccentricity increases for smaller radii of curvature. A model derived from corrections to the paraxial resonator theory is in excellent agreement with the measurements, showing that geometric effects are the main source of the frequency splitting of polarization modes for the type of microscopic cavity studied here. By rotating one of the mirrors around the cavity axis, the splitting can be tuned. In the case of an identical differential phase shift per mirror, it can even be eliminated, despite a nonvanishing eccentricity of each mirror. We expect our results to have important implications for many experiments in cavity quantum electrodynamics, where Fabry-Perot cavities with small mode volumes are required.

We theoretically analyze the efficiency of a quantum memory for single photons. The photons propagate along a transmission line and impinge on one of the mirrors of a high-finesse cavity. The quantum memory is constituted by a single atom within the optical resonator. Photon storage is realized by the controlled transfer of the photonic excitation into a metastable state of the atom and occurs via a Raman transition with a suitably tailored laser pulse, which drives the atom. Our study is supported by numerical simulations, in which we include the modes of the transmission line and we use the experimental parameters of existing experimental setups. It reproduces the results derived using input-output theory in the corresponding regimes and can be extended to compute dynamics where the input-output formalism cannot be straightforwardly applied. Our analysis determines the maximal storage efficiency, namely, the maximal probability to store the photon in a stable atomic excitation, in the presence of spontaneous decay and cavity parasitic losses. It further delivers the form of the laser pulse that achieves the maximal efficiency by partially compensating parasitic losses. We numerically assess the conditions under which storage based on adiabatic dynamics is preferable to non-adiabatic pulses. Moreover, we systematically determine the shortest photon pulse that can be efficiently stored as a function of the system parameters.

Cavity optomechanics studies the coupling between a mechanical oscillator and the electromagnetic field in a cavity. We report on a cavity optomechanical system in which a collective density excitation of a Bose-Einstein condensate serves as the mechanical oscillator coupled to the cavity field. A few photons inside the ultrahigh-finesse cavity trigger strongly driven back-action dynamics, in quantitative agreement with a cavity optomechanical model. We approach the strong coupling regime of cavity optomechanics, where a single excitation of the mechanical oscillator substantially influences the cavity field. The results open up new directions for investigating mechanical oscillators in the quantum regime and the border between classical and quantum physics.

Cavity quantum electrodynamics (cavity QED) describes the coherent interaction between matter and an electromagnetic field confined within a resonator structure, and is providing a useful platform for developing concepts in quantum information processing. By using high-quality resonators, a strong coupling regime can be reached experimentally in which atoms coherently exchange a photon with a single light-field mode many times before dissipation sets in. This has led to fundamental studies with both microwave and optical resonators. To meet the challenges posed by quantum state engineering and quantum information processing, recent experiments have focused on laser cooling and trapping of atoms inside an optical cavity. However, the tremendous degree of control over atomic gases achieved with Bose-Einstein condensation has so far not been used for cavity QED. Here we achieve the strong coupling of a Bose-Einstein condensate to the quantized field of an ultrahigh-finesse optical cavity and present a measurement of its eigenenergy spectrum. This is a conceptually new regime of cavity QED, in which all atoms occupy a single mode of a matter-wave field and couple identically to the light field, sharing a single excitation. This opens possibilities ranging from quantum communication to a wealth of new phenomena that can be expected in the many-body physics of quantum gases with cavity-mediated interactions.

Single-atom memory Efficient, high-fidelity storage and exchange of quantum information between light and an optical quantum memory is essential for long-distance quantum communication, quantum networking and distributed quantum computing. Stephan Ritter and colleagues demonstrate the most fundamental implementation of such a quantum memory, mapping arbitrary polarization states of light into and out of single atoms trapped inside an optical cavity. The high fidelity (93%) and relatively long qubit coherence time of this atomic memory make it a versatile quantum node, with excellent prospects for applications in optical quantum gates and quantum repeaters. The faithful storage of a quantum bit (qubit) of light is essential for long-distance quantum communication, quantum networking and distributed quantum computing 1 . The required optical quantum memory must be able to receive and recreate the photonic qubit; additionally, it must store an unknown quantum state of light better than any classical device. So far, these two requirements have been met only by ensembles of material particles that store the information in collective excitations 2 , 3 , 4 , 5 , 6 , 7 . Recent developments, however, have paved the way for an approach in which the information exchange occurs between single quanta of light and matter 8 , 9 , 10 , 11 , 12 , 13 . This single-particle approach allows the material qubit to be addressed, which has fundamental advantages for realistic implementations. First, it enables a heralding mechanism that signals the successful storage of a photon by means of state detection 14 , 15 , 16 ; this can be used to combat inevitable losses and finite efficiencies. Second, it allows for individual qubit manipulations, opening up avenues for in situ processing of the stored quantum information. Here we demonstrate the most fundamental implementation of such a quantum memory, by mapping arbitrary polarization states of light into and out of a single atom trapped inside an optical cavity. The memory performance is tested with weak coherent pulses and analysed using full quantum process tomography. The average fidelity is measured to be 93%, and low decoherence rates result in qubit coherence times exceeding 180 microseconds. This makes our system a versatile quantum node with excellent prospects for applications in optical quantum gates 17 and quantum repeaters 18 .