Jürgen Volz

Chiral nanophotonic waveguide interface based on spin-orbit interaction of light

Controlling the flow of light with nanoparticles Light propagating through optic fibers could provide the ultimate in information flow, but controlling the direction of flow is a key requirement. Petersen et al. show that the directional flow of light in a fiber can be controlled by placing a single gold nanoparticle on or near the surface of the fiber. By exploiting the chiral properties of light (the spin-orbit interaction), the authors demonstrate that the “handedness” or polarization state of the light hitting the particle determines in which direction the light flows in the fiber. Science, this issue p. 67 A gold nanoparticle can control the directional flow of light in a fiber. Controlling the flow of light with nanophotonic waveguides has the potential of transforming integrated information processing. Because of the strong transverse confinement of the guided photons, their internal spin and their orbital angular momentum get coupled. Using this spin-orbit interaction of light, we break the mirror symmetry of the scattering of light with a gold nanoparticle on the surface of a nanophotonic waveguide and realize a chiral waveguide coupler in which the handedness of the incident light determines the propagation direction in the waveguide. We control the directionality of the scattering process and can direct up to 94% of the incoupled light into a given direction. Our approach allows for the control and manipulation of light in optical waveguides and new designs of optical sensors.

Chiral nanophotonic waveguide interface based on spin-orbit coupling of light

Controlling the flow of light with nanoparticles Light propagating through optic fibers could provide the ultimate in information flow, but controlling the direction of flow is a key requirement. Petersen et al. show that the directional flow of light in a fiber can be controlled by placing a single gold nanoparticle on or near the surface of the fiber. By exploiting the chiral properties of light (the spin-orbit interaction), the authors demonstrate that the “handedness” or polarization state of the light hitting the particle determines in which direction the light flows in the fiber. Science, this issue p. 67 A gold nanoparticle can control the directional flow of light in a fiber. Controlling the flow of light with nanophotonic waveguides has the potential of transforming integrated information processing. Because of the strong transverse confinement of the guided photons, their internal spin and their orbital angular momentum get coupled. Using this spin-orbit interaction of light, we break the mirror symmetry of the scattering of light with a gold nanoparticle on the surface of a nanophotonic waveguide and realize a chiral waveguide coupler in which the handedness of the incident light determines the propagation direction in the waveguide. We control the directionality of the scattering process and can direct up to 94% of the incoupled light into a given direction. Our approach allows for the control and manipulation of light in optical waveguides and new designs of optical sensors.

Strong coupling between single atoms and non-transversal photons

The interaction between single quantum emitters and non-transversally polarized photons for which the electric field vector amplitude has a significant component in the direction of propagation is investigated. Even though this situation seems to be at odds with the description of light as a transverse wave, it regularly occurs when interfacing or manipulating emitters with non-paraxial, guided, or evanescent light. Here, this phenomenon for the case of single atoms that strongly interact with whispering-gallery-mode (WGM) microresonators is quantitatively investigated. These resonators confine light by continuous total internal reflection and offer the advantage of very long photon lifetimes in conjunction with near-lossless in- and out-coupling of light via tapered fiber couplers. In the setup, a novel type of high-Q WGM microresonator - a so-called bottle microresonator, which has the additional advantage of being fully tunable and provides a mode geometry that enables near-lossless simultaneous coupling of two independent tapered fiber couplers, is employed.

Nanophotonic Optical Isolator Controlled by the Internal State of Cold Atoms

Photons are nonchiral particles: their handedness can be both left and right. However, when light is transversely confined, it can locally exhibit a transverse spin whose orientation is fixed by the propagation direction of the photons. Confined photons thus have chiral character. Here, we employ this to demonstrate nonreciprocal transmission of light at the single-photon level through a silica nanofibre in two experimental schemes. We either use an ensemble of spin-polarised atoms that is weakly coupled to the nanofibre-guided mode or a single spin-polarised atom strongly coupled to the nanofibre via a whispering-gallery-mode resonator. We simultaneously achieve high optical isolation and high forward transmission. Both are controlled by the internal atomic state. The resulting optical diode is the first example of a new class of nonreciprocal nanophotonic devices which exploit the chirality of confined photons and which are, in principle, suitable for quantum information processing and future quantum optical networks.

Nonlinear [pi] phase shift for single fibre-guided photons interacting with a single resonator-enhanced atom

A nonlinear π phase shift is induced by the interaction between a 85Rb atom and a fibre-coupled bottle resonator.

Cavity-Based Single Atom Preparation and High-Fidelity Hyperfine State Readout

We prepare and detect the hyperfine state of a single 87Rb atom coupled to a fiber-based high-finesse cavity on an atom chip. The atom is extracted from a Bose-Einstein condensate and trapped at the maximum of the cavity field, resulting in a reproducibly strong atom-cavity coupling. We use the cavity reflection and transmission signal to infer the atomic hyperfine state with a fidelity exceeding 99.92% in a readout time of 100  μs. The atom is still trapped after detection.

Measurement of the internal state of a single atom without energy exchange

A measurement necessarily changes the quantum state being measured, a phenomenon known as back-action. Real measurements, however, almost always cause a much stronger back-action than is required by the laws of quantum mechanics. Quantum non-demolition measurements have been devised that keep the additional back-action entirely within observables other than the one being measured. However, this back-action on other observables often imposes its own constraints. In particular, free-space optical detection methods for single atoms and ions (such as the shelving technique, a sensitive and well-developed method) inevitably require spontaneous scattering, even in the dispersive regime. This causes irreversible energy exchange (heating), which is a limitation in atom-based quantum information processing, where it obviates straightforward reuse of the qubit. No such energy exchange is required by quantum mechanics. Here we experimentally demonstrate optical detection of an atomic qubit with significantly less than one spontaneous scattering event. We measure the transmission and reflection of an optical cavity containing the atom. In addition to the qubit detection itself, we quantitatively measure how much spontaneous scattering has occurred. This allows us to relate the information gained to the amount of spontaneous emission, and we obtain a detection error below 10 per cent while scattering less than 0.2 photons on average. Furthermore, we perform a quantum Zeno-type experiment to quantify the measurement back-action, and find that every incident photon leads to an almost complete state collapse. Together, these results constitute a full experimental characterization of a quantum measurement in the ‘energy exchange-free’ regime below a single spontaneous emission event. Besides its fundamental interest, this approach could significantly simplify proposed neutral-atom quantum computation schemes, and may enable sensitive detection of molecules and atoms lacking closed transitions.

Entangled States of More Than 40 Atoms in an Optical Fiber Cavity

All Together Now In quantum entanglement, correlations between particles mean that the measurement of one determines the outcome of the other(s). Generally, when trying to exploit quantum entanglement, the larger the number of entangled particles, the better. However, the size of entangled systems has been limited. Haas et al. (p. 180, published online 27 March; see the Perspective by Widera) prepared a small ensemble of ultracold atoms into a collective entangled state. Starting from one internal quantum state, the system of cold atoms was excited with a weak microwave pulse leading to a small excitation probability. Because it is not known which atom is promoted into the excited state, the detection of one quantum of excitation projects the system into an entangled quantum state, called a W-state. A fast repeat-until-success scheme produced such W-states quasi-deterministically. Using such a technique was able to yield entangled states of more than 40 particles. The relatively large ensemble-entangled states could potentially in the future find use in quantum sensing or enhanced quantum metrology applications. A small ensemble of ultracold atoms in a chip trap has been used to realize a collective entangled state. [Also see Perspective by Widera] Multiparticle entanglement enables quantum simulations, quantum computing, and quantum-enhanced metrology. Yet, there are few methods to produce and measure such entanglement while maintaining single-qubit resolution as the number of qubits is scaled up. Using atom chips and fiber-optical cavities, we have developed a method based on nondestructive collective measurement and conditional evolution to create symmetric entangled states and perform their tomography. We demonstrate creation and analysis of entangled states with mean atom numbers up to 41 and experimentally prove multiparticle entanglement. Our method is independent of atom number and should allow generalization to other entangled states and other physical implementations, including circuit quantum electrodynamics.

Quantum optical circulator controlled by a single chirally coupled atom

A quantum optical circulator A circulator is a passive three- or four-port device that routes signals according to a simple protocol: If the ports are numbered in ascending order, a signal that enters the circulator through port 1, 2, 3, or 4 exits it through port 2, 3, 4, or 1, respectively. Scheucher et al. demonstrate an integrated optical circulator that operates by using the internal quantum state of a single atom (see the Perspective by Munro and Nemoto). Moreover, the routing can be reversed by flipping the atomic spin. Such an integrated optical device may be important for routing and processing quantum information in scalable integrated optical circuits. Science, this issue p. 1577; see also p. 1532 The internal state of a single atom is used to route single photons in an optical circulator. Integrated nonreciprocal optical components, which have an inherent asymmetry between their forward and backward propagation direction, are key for routing signals in photonic circuits. Here, we demonstrate a fiber-integrated quantum optical circulator operated by a single atom. Its nonreciprocal behavior arises from the chiral interaction between the atom and the transversally confined light. We demonstrate that the internal quantum state of the atom controls the operation direction of the circulator and that it features a strongly nonlinear response at the single-photon level. This enables, for example, photon number–dependent routing and novel quantum simulation protocols. Furthermore, such a circulator can in principle be prepared in a coherent superposition of its operational states and may become a key element for quantum information processing in scalable integrated optical circuits.

Fiber ring resonator with a nanofiber section for chiral cavity quantum electrodynamics and multimode strong coupling

We experimentally realize an optical fiber ring resonator that includes a tapered section with a subwavelength-diameter waist. In this section, the guided light exhibits a significant evanescent field which allows for efficient interfacing with optical emitters. A commercial tunable fiber beam splitter provides simple and robust coupling to the resonator. Key parameters of the resonator such as the out-coupling rate, free spectral range, and birefringence can be adjusted. Thanks to the low taper- and coupling-losses, the resonator exhibits an unloaded finesse of F=75±1, sufficient for reaching the regime of strong coupling for emitters placed in the evanescent field. The system is ideally suited for trapping ensembles of laser-cooled atoms along the nanofiber section. Based on measured parameters, we estimate that the system can serve as a platform for optical multimode strong coupling experiments. Finally, we discuss the possibilities of using the resonator for applications based on chiral quantum optics.