We demonstrate the realization of a quantum register using string single neutral atoms which are trapped in an optical dipole trap. The selectively and coherently manipulated magnetic field gradient microwave radiation. Our addressing scheme operates with high spatial resolution qubit rotations on individual performed 99% contrast. In final read-out operation we analyze each atomic state. Finally, have measured coherence time identified predominant dephasing mechanism for our register.

Quantum simulation can provide insight into physical systems that are too complex for traditional computing techniques. A new proposal describes how a quantum simulator could be realized using laser-trapped circular Rydberg atoms, whose long lifetimes and stability beneficial simulations lasting up to seconds.

Fock states with photon numbers $n$ up to 7 are prepared on demand in a microwave superconducting cavity by quantum feedback procedure that reverses decoherence-induced jumps. Circular Rydberg atoms used as nondemolition sensors or single-photon emitter absorber actuators. The nature of these actuators matches the correction jumps due relaxation. flexibility this method is suited generation arbitrary sequences states.

We study in detail the mechanisms causing dephasing of hyperfine coherences cesium atoms confined by a far-off-resonant standing-wave optical dipole trap [S. Kuhr et al., Phys. Rev. Lett. 91, 213002 (2003)]. Using Ramsey-spectroscopy and spin-echo techniques, we measure reversible irreversible times ground-state coherences. present an analytical model to interpret experimental data identify homogeneous inhomogeneous mechanisms. Our scheme prepare detect atomic state is applied at level...

We have prepared and detected quantum coherences of trapped cesium atoms with long dephasing times. Controlled transport by an "optical conveyor belt" over macroscopic distances preserves the atomic coherence slight reduction time. The limiting effects are experimentally identified, we present analytical model reversible irreversible mechanisms. Our experimental methods applicable at single-atom level. Coherent bit operations along state open route towards a "quantum shift register"...

We have frozen the coherent evolution of a field in cavity by repeated measurements its photon number. use circular Rydberg atoms dispersively coupled to mode for an absorption-free counting. These inhibit growth injected classical source. This manifestation quantum Zeno effect illustrates backaction number determination onto phase. The residual can be seen as random walk amplitude two-dimensional phase space. experiment sheds light measurement process and opens perspectives active feedback.

We analyze the quantum Zeno dynamics that takes place when a field stored in cavity undergoes frequent interactions with atoms. show repeated measurements or unitary operations performed on atoms probing state confine evolution to tailored subspaces of total Hilbert space. This confinement leads non-trivial evolutions and generation interesting non-classical states, including mesoscopic superpositions. elucidate main features mechanism context state-of-the-art electrodynamics experiment. A...

Superconducting atom chips and Rydberg atoms are promising tools for quantum information processing operations based on the dipole blockade effect. Nevertheless, one has to face severe problem of stray electric fields in vicinity chip. We demonstrate a simple method circumventing this problem. Microwave spectroscopy reveals extremely long coherence lifetimes (in millisecond range) qubit stored level superposition close chip surface. This is an essential step development simulations with...

The relaxation of a quantum field stored in high-$Q$ superconducting cavity is monitored by nonresonant Rydberg atoms. field, subjected to repetitive nondemolition photon counting, undergoes jumps between number states. We select ensembles realizations evolving from given Fock state and reconstruct the subsequent evolution their distributions. realize this way tomography process yielding all jump rates damping $n$ states ($0\ensuremath{\le}n\ensuremath{\le}7$) are found increase linearly...

We discuss an implementation of quantum Zeno dynamics in a cavity electrodynamics experiment. By performing repeated unitary operations on atoms coupled to the field, we restrict field evolution chosen subspaces total Hilbert space. This procedure leads promising methods for tailoring nonclassical states. propose realize "tweezers" picking coherent at point phase space and moving it towards arbitrary final position without affecting other nonoverlapping components. These effects could be...

We propose a quantum feedback scheme for the preparation and protection of photon-number states light trapped in high-$Q$ microwave cavity. A nondemolition measurement cavity field provides information on distribution. The loop is closed by injecting into coherent pulse adjusted to increase probability target photon number. efficiency reliability closed-loop state stabilization assessed Monte Carlo simulations. show that, realistic experimental conditions, Fock are efficiently produced...

A quantum system can be monitored through repeated interactions with meter systems. The state of the at time $t$, represented by density matrix $\ensuremath{\rho}(t)$, then becomes conditioned on information obtained meters until that time. More insight in any $t$ is provided, however, taking into account full detection all interacting both past and future $t$. We present experiments use near-resonant atomic probes to monitor evolution quantized field a cavity. application forward-backward...

We report a quantum measurement beyond the standard limit (SQL) for amplitude of small displacement acting on cavity field. This uses as resource an entangled mesoscopic state, prepared by resonant interaction circular Rydberg atom with field stored in superconducting cavity. analyse process terms Fisher information and prove that it is, principle, optimal. The achieved experimental precision, 2.4 dB below SQL, is well understood imperfections.This method could be transposed to other...

We present an autonomous Maxwell's demon scheme. It is first analysed theoretically in term of information exchange a closed system and then implemented experimentally with single Rydberg atom high-quality microwave resonator. The simulates both qubit interacting the cavity, carrying on state. While cold crosses hot prevents energy absorption from cavity mode, apparently violating second law thermodynamics. Taking into account change mutual between qubit-cavity gives rise to generalized...

We optically detect the positions of single neutral cesium atoms stored in a standing wave dipole trap with subwavelength resolution 143 nm rms. The distance between two simultaneously trapped is measured an even higher precision 36 resolve discreteness interatomic distances due to 532 spatial period potential and infer exact number trapping wells separating atoms. Finally, combining initial position detection controlled transport, we place at predetermined along axis within 300

We experimentally investigate the interaction between one and two atoms field of a high-finesse optical resonator. Laser-cooled caesium are transported into cavity using an dipole trap. monitor dynamics single atom strongly coupled to resonator mode for several hundred milliseconds by observing transmission. Moreover, we position-dependent coupling shuttling them through mode. demonstrate alternative method, which suppresses heating effects, analyse atom–field retrieving from measuring its...

The ubiquitous decoherence phenomenon is responsible for the lack of quantum superpositions at macroscopic scale. It increasingly difficult to isolate a system from its environment when size increases. Making use weird properties mesoscopic states thus requires efficient means combat decoherence. One option real-time feedback. features components conventional feedback: measurement system's state (sensor), analysis (controller), and feedback action (actuator) aiming target state. random...

The efficient initialization of a quantum system is prerequisite for technological applications. Here we show that several classes states harmonic oscillator can be efficiently prepared by means Jaynes-Cummings interaction with single two-level system. This achieved suitably tailoring external fields which drive the dipole and/or oscillator. time-dependent dynamics leads to target state identified optimal control theory (OCT) based on Krotov's method. Infidelities below...

The simple resonant Rabi oscillation of a two-level system in single-mode coherent field reveals complex features at the mesoscopic scale, with collapses and revivals. Using slow circular Rydberg atoms interacting superconducting microwave cavity, we explore this phenomenon an unprecedented range interaction times photon numbers. We demonstrate efficient production cat states, which are quantum superposition components nearly opposite phases sizes few tens photons. measure cuts their Wigner...

Tomography of a quantum state is usually based on positive-operator-valued measure (POVM) and their experimental statistics. Among the available reconstructions, maximum-likelihood (MaxLike) technique an efficient one. We propose extension this when measurement process cannot be simply described by instantaneous POVM. Instead, tomography relies set trajectories records. This model includes fact that, in practice, each could corrupted imperfections decoherence, also associated with record...