Quantum technologies

We report on vibrational ground-state cooling of a single neutral atom coupled to a high-bandwidth Fabry-Pérot cavity. The cooling process relies on degenerate Raman sideband transitions driven by dipole trap beams, which confine the atoms in three dimensions. We infer a one-dimensional motional ground-state population close to $90\%$ by means of Raman spectroscopy. Moreover, lifetime measurements of a cavity-coupled atom exceeding 40 s imply three-dimensional cooling of the atomic motion, which makes this resource-efficient technique particularly interesting for cavity experiments with limited optical access.

We demonstrate the storage of 5 ns light pulses in a single rubidium atom coupled to a fiber-based optical resonator. Our storage protocol addresses a regime beyond the conventional adiabatic limit and approaches the theoretical bandwidth limit. We extract the optimal control laser pulse properties from a numerical simulation of our system and measure storage efficiencies of (8.1±1.1)%, in close agreement with the maximum expected efficiency. Such well-controlled and high-bandwidth atom-photon interfaces are key components for future hybrid quantum networks.

We employ active feedback to stabilize the frequency of single photons emitted by two separate quantum dots to an atomic standard. The transmission of a rubidium-based Faraday filter serves as the error signal for frequency stabilization. We achieve a residual frequency deviation of <30 MHz, which is less than 1.5% of the quantum dot linewidth. Long-term stability is demonstrated by Hong-Ou-Mandel interference between photons from the two quantum dots. Their internal dephasing limits the expected visibility to V = 40%. We observe V_lock = (41±5)% for frequency-stabilized dots as opposed to V_free = (31±7)% for free-running emission. Our technique reaches the maximally expected visibility for the given system and therefore facilitates quantum networks with indistinguishable photons from distributed sources.

We observe a sixfold Purcell broadening of the D₂ line of an optically trapped ⁸⁷Rb atom strongly coupled to a fiber cavity. Under external illumination by a near-resonant laser, up to 90% of the atom's fluorescence is emitted into the resonant cavity mode. The sub-Poissonian statistics of the cavity output and the Purcell enhancement of the atomic decay rate are confirmed by the observation of a strongly narrowed antibunching dip in the photon autocorrelation function. The photon leakage through the higher-transmission mirror of the single-sided resonator is the dominant contribution to the field decay (κ≈2π×50 MHz), thus offering a high-bandwidth, fiber-coupled channel for photonic interfaces such as quantum memories and single-photon sources.

In this work I theoretically investigate and experimentally realize the storage of short light-pulses in a fiber-based atom-cavity system. Our miniaturized optical resonator – with seven times the natural atomic linewidth and a small mode volume – simultaneously ensures a high bandwidth and operation in the strong-coupling regime. In particular, it enables the storage of light pulses with on average one photon and a temporal extent of less than 10 ns, which is more than a factor of two shorter than the atomic excited state lifetime of rubidium. We obtain a storage efficiency of 8%, consistent with both cavity losses and the employed level scheme.
 In order to improve the coupling and number of measurements for which a single atom can be recycled, we use dipole-trap assisted, degenerate Raman sideband cooling and a further development of our carrier-free Raman sideband cooling scheme, which permits a three-dimensional ground state population of 70%. The new techniques increase the measurement repetition rate by two orders of magnitude to ∼ 2 kHz. Moreover, for the first time we achieve a Zeeman state preparation fidelity above 95% in our experiment.
 On this basis, I present the deterministic generation of single photons in the near-adiabatic limit. By shaping the control laser pulse, we do not only show that we can control the temporal waveform of retrieved photons, but also reach a faster extraction from the cavity-coupled atom than possible in free-space. The quantum nature of the retrieved light is verified by measuring a second-order correlation function, which yields the expected antibunching. Moreover, the generation of photons in the cavity mode with an efficiency exceeding 66% is used as a fast hyperfine-state detection method, since our traditional, non-destructive state detection via a probe laser is no longer applicable in a Raman configuration due to the absence of a cycling transition. In order to realize Raman coupling between the two hyperfine ground states, we develop a scheme for shifting the cavity resonance frequency between two hyperfine transitions. During the scan, we are furthermore able to determine the atom-cavity coupling strength via the vacuum Rabi splitting in each individual measurement – a useful tool for post-selection of acquired data sets.
 By employing a numerical simulation based on a full quantum-mechanical master equation, I find the strategy to store a coherent laser pulse with the maximum possible efficiency for a given system. Although the cavity input field is treated classically, our simulation model is able to calculate efficiencies for a pure single-photon Fock-state input. Moreover, numerical optimal control methods enable us to find control pulses with storage efficiencies slightly above those achieved for temporally-scaled adiabatic control pulses. For our specific system, we finally demonstrate the non-adiabatic storage of a short, coherent light pulse.
 The ability to interact with pulses of high bandwidths encourages quantum hybrid experiments with quantum dots as single-photon sources. In this context, the stabilization of their emission frequency to an atomic transition is required. In collaboration with the IFW Dresden, I present a technique to counteract long-term frequency drifts by applying rate-based feedback to a strain-tunable quantum dot, which results in frequency deviations smaller than 1.5% of its emission linewidth. By simultaneously stabilizing the emission frequency of two quantum dots in separate cryostats, we enhance their two-photon interference visibility in a Hong-Ou-Mandel measurement from 31% to 41%, which corresponds to the maximum reachable visibility for the given emitters. Frequency-stable, efficient photon sources together with atom-cavity based quantum memories may facilitate the realization of quantum networks.

We report on the observation of cooperative radiation of exactly two neutral atoms strongly coupled to the single mode field of an optical cavity, which is close to the lossless-cavity limit. Monitoring the cavity output power, we observe constructive and destructive interference of collective Rayleigh scattering for certain relative distances between the two atoms. Because of cavity backaction onto the atoms, the cavity output power for the constructive two-atom case (N=2) is almost equal to the single-emitter case (N=1), which is in contrast to free-space where one would expect an N^2 scaling of the power. These effects are quantitatively explained by a classical model as well as by a quantum mechanical model based on Dicke states. We extract information on the relative phases of the light fields at the atom positions and employ advanced cooling to reduce the jump rate between the constructive and destructive atom configurations. Thereby we improve the control over the system to a level where the implementation of two-atom entanglement schemes involving optical cavities becomes realistic.

We experimentally realize an enhanced Raman control scheme for neutral atoms that features an intrinsic suppression of the two-photon carrier transition, but retains the sidebands which couple to the external degrees of freedom of the trapped atoms. This is achieved by trapping the atom at the node of a blue detuned standing wave dipole trap, that acts as one field for the two-photon Raman coupling. The improved ratio between cooling and heating processes in this configuration enables a five times lower fundamental temperature limit for resolved sideband cooling. We apply this method to perform Raman cooling to the two-dimensional vibrational ground state and to coherently manipulate the atomic motion. The presented scheme requires minimal additional resources and can be applied to experiments with challenging optical access, as we demonstrate by our implementation for atoms strongly coupled to an optical cavity.