Explore the vast range of titles available.

Multiparticle entangled quantum states, a key resource in quantum-enhanced metrology and computing, are usually generated by coherent operations exclusively. However, unusual forms of quantum dynamics can be obtained when environment coupling is used as part of the state generation. In this work, we used quantum Zeno dynamics (QZD), based on nondestructive measurement with an optical microcavity, to deterministically generate different multiparticle entangled states in an ensemble of 36 qubit atoms in less than 5 microseconds. We characterized the resulting states by performing quantum tomography, yielding a time-resolved account of the entanglement generation. In addition, we studied the dependence of quantum states on measurement strength and quantified the depth of entanglement. Our results show that QZD is a versatile tool for fast and deterministic entanglement generation in quantum engineering applications.

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.

Imaging the optical properties of individual nanosystems beyond fluorescence can provide a wealth of information. However, the minute signals for absorption and dispersion are challenging to observe, and only specialized techniques requiring sophisticated noise rejection are available. Here we use signal enhancement in a high-finesse scanning optical microcavity to demonstrate ultra-sensitive imaging. Harnessing multiple interactions of probe light with a sample within an optical resonator, we achieve a 1,700-fold signal enhancement compared with diffraction-limited microscopy. We demonstrate quantitative imaging of the extinction cross-section of gold nanoparticles with a sensitivity less than 1 nm 2 ; we show a method to improve the spatial resolution potentially below the diffraction limit by using higher order cavity modes, and we present measurements of the birefringence and extinction contrast of gold nanorods. The demonstrated simultaneous enhancement of absorptive and dispersive signals promises intriguing potential for optical studies of nanomaterials, molecules and biological nanosystems. Fluorescence from nanoparticles enables high-resolution optical imaging, but this approach is limited to those structures that emit light. Here, the authors demonstrate a microscope that uses a cavity to enhance the measurement of the alternative optical properties of absorption and dispersion.

An optical cavity enhances the interaction between atoms and light, and the rate of coherent atom-photon coupling can be made larger than all decoherence rates of the system. For single atoms, this 'strong coupling regime' of cavity quantum electrodynamics has been the subject of many experimental advances. Efforts have been made to control the coupling rate by trapping the atom and cooling it towards the motional ground state; the latter has been achieved in one dimension so far. For systems of many atoms, the three-dimensional ground state of motion is routinely achieved in atomic Bose-Einstein condensates (BECs). Although experiments combining BECs and optical cavities have been reported recently, coupling BECs to cavities that are in the strong-coupling regime for single atoms has remained an elusive goal. Here we report such an experiment, made possible by combining a fibre-based cavity with atom-chip technology. This enables single-atom cavity quantum electrodynamics experiments with a simplified set-up and realizes the situation of many atoms in a cavity, each of which is identically and strongly coupled to the cavity mode. Moreover, the BEC can be positioned deterministically anywhere within the cavity and localized entirely within a single antinode of the standing-wave cavity field; we demonstrate that this gives rise to a controlled, tunable coupling rate. We study the heating rate caused by a cavity transmission measurement as a function of the coupling rate and find no measurable heating for strongly coupled BECs. The spectrum of the coupled atoms-cavity system, which we map out over a wide range of atom numbers and cavity-atom detunings, shows vacuum Rabi splittings exceeding 20 gigahertz, as well as an unpredicted additional splitting, which we attribute to the atomic hyperfine structure. We anticipate that the system will be suitable as a light-matter quantum interface for quantum information.

In canonical optomechanical systems, mechanical vibrations are dynamically encoded on an optical probe field, which reciprocally exerts a backaction force. Because of the weak single-photon coupling strength achieved with macroscopic oscillators, most of the existing experiments were conducted with large photon numbers to achieve sizable effects, thereby hiding the original optomechanical nonlinearity. To increase the optomechanical interaction, we make use of subwavelength-sized ultrasensitive suspended nanowires inserted in the mode volume of a fiber-based microcavity. By scanning the nanowire within the cavity mode volume and measuring its impact on the cavity mode, we obtain a map of the 2D optomechanical interaction. Then, by using the toolbox of nanowire-based force-sensing protocols, we explore the backaction of the optomechanical interaction and map the optical force field experienced by the nanowire. These measurements also allow us to demonstrate the possibility to detect variations of the mean intracavity photon number smaller than unity. This implementation should also allow us to enter the promising regime of cavity optomechanics, where a single intracavity photon can displace the oscillator by more than its zero-point fluctuations, which will open novel perspectives in the field.

Light-touch quantum measurement Methods that use light to detect single atoms or ions usually involve some spontaneous scattering of photons, which exchange energy with the measured object, changing its state. The heat produced in such interactions is problematic for atom-based quantum information processing. However, energy exchange of this type is not a fundamental requirement of quantum mechanics, and Jakob Reichel and colleagues now show that, by confining an atom in an optical cavity, they can circumvent the problem of spontaneous scattering. The strong coupling of the atom to the cavity allows the state of the atom to be probed by sensing the interaction of light with the cavity. In effect, the photons measure the atom without entering the cavity. Besides its fundamental interest, this technique could simplify proposed neutral atom quantum computation schemes, and may enable sensitive detection of molecules and atoms lacking closed transitions. 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 1 , 2 , 3 , 4 , 5 , 6 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 7 , a sensitive and well-developed method) inevitably require spontaneous scattering, even in the dispersive regime 8 . 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 9 . 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 10 , 11 , 12 , 13 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 14 , and may enable sensitive detection of molecules and atoms lacking closed transitions.

We report on measurements and modeling of the mode structure of tunable Fabry-Pérot optical microcavities with imperfect mirrors. We find that non-spherical mirror shape and finite mirror size leave the fundamental mode mostly unaffected, but lead to loss, mode deformation, and shifted resonance frequencies at particular mirror separations. For small mirror diameters, the useful cavity length is limited to values significantly below the expected stability range. We explain the observations by resonant coupling between different transverse modes of the cavity and mode-dependent diffraction loss. A model based on resonant state expansion that takes into account the measured mirror profile can reproduce the measurements and identify the parameter regime where detrimental effects of mode mixing are avoided.

We implement a simple method for fast and precise delivery of ultracold atoms to a microscopic device, i.e. a Fabry–Perot microcavity. By moving a single beam optical dipole trap in a direction perpendicular to the beam axis with an acousto-optical deflector, we transport up to 1 million atoms within 100 ms over 1 cm. Under these conditions, a transport efficiency above 95% is achieved with only minimal heating. The atomic cloud is accurately positioned within the microcavity and transferred into an intra-cavity optical lattice. With the addition of a secondary guiding beam, we show how residual sloshing motion along the shallow axis of the trap can be minimized.

We describe an experiment where spin squeezing occurs spontaneously within a standard Ramsey sequence driving a two-component Bose-Einstein condensate (BEC) of 87Rb atoms trapped in an elongated magnetic trap. The squeezing is generated by state-dependent collisional interactions, despite the near-identical scattering lengths of the spin states in 87Rb. In our proof-of-principle experiment, we observe a metrological spin squeezing that reaches 1.3 0.4 dB for 5000 atoms, with a contrast of 90 1%. The method may be applied to realize spin-squeezed BEC sources for atom interferometry without the need for cavities, state-dependent potentials or Feshbach resonances.

The field of cold atoms was born fourty years ago and today remains a theme regularly awarded Nobel Prizes and at the forefront of physics research. The book presents the most recent developments and traces the exceptional growth of the field over the last ten years.