Optical atomic clocks represent the state of the art in the frontier of modern measurement science. In this article a detailed review on the development of optical atomic clocks that are based on trapped single ions and many neutral atoms is provided. Important technical ingredients for optical clocks are discussed and measurement precision and systematic uncertainty associated with some of the best clocks to date are presented. An outlook on the exciting prospect for clock applications is given in conclusion.

Time has always had a special status in physics because of its fundamental role in specifying the regularities of nature and because of the extraordinary precision with which it can be measured. This precision enables tests of fundamental physics and cosmology, as well as practical applications such as satellite navigation. Recently, a regime of operation for atomic clocks based on optical transitions has become possible, promising even higher performance. We report the frequency ratio of two optical atomic clocks with a fractional uncertainty of 5.2 x 10(-17). The ratio of aluminum and mercury single-ion optical clock frequencies nuAl+/nuHg+ is 1.052871833148990438(55), where the uncertainty comprises a statistical measurement uncertainty of 4.3 x 10(-17), and systematic uncertainties of 1.9 x 10(-17) and 2.3 x 10(-17) in the mercury and aluminum frequency standards, respectively. Repeated measurements during the past year yield a preliminary constraint on the temporal variation of the fine-structure constant alpha of alpha/alpha = (-1.6+/-2.3) x 10(-17)/year.

Entanglement, its generation, manipulation and fundamental understanding is at the very heart of quantum mechanics. The phrase entanglement was coined by Erwin Schrödinger in 1935 for particles that are described by a common wave function where individual particles are not independent of each other but where their quantum properties are inextricably interwoven 1 . Entanglement properties of two and three particles have been studied extensively and are very well understood. Entanglement of four 2 and five 3 particles was demonstrated experimentally. However, both creation and characterization of entanglement become exceedingly difficult for multi-particle systems. Thus the availability of such multiparticle entangled states together with the full information on these states in form of their 1

We present a general technique for precision spectroscopy of atoms that lack suitable transitions for efficient laser cooling, internal state preparation, and detection. In our implementation with trapped atomic ions, an auxiliary "logic" ion provides sympathetic laser cooling, state initialization, and detection for a simultaneously trapped "spectroscopy" ion. Detection is achieved by applying a mapping operation to each ion, which results in a coherent transfer of the spectroscopy ion's internal state onto the logic ion, where it is then measured with high efficiency. Experimental realization, by using 9Be+ as the logic ion and 27Al+ as the spectroscopy ion, indicates the feasibility of applying this technique to make accurate optical clocks based on single ions.

Within the last two decades, quantum technologies (QT) have made tremendous progress, moving from Nobel Prize award-winning experiments on quantum physics (1997: Chu, Cohen-Tanoudji, Phillips; 2001: Cornell, Ketterle, Wieman; 2005: Hall, Hänsch-, Glauber; 2012: Haroche, Wineland) into a cross-disciplinary field of applied research. Technologies are being developed now that explicitly address individual quantum states and make use of the 'strange' quantum properties, such as superposition and entanglement. The field comprises four domains: quantum communication, where individual or entangled photons are used to transmit data in a provably secure way; quantum simulation, where well-controlled quantum systems are used to reproduce the behaviour of other, less accessible quantum systems; quantum computation, which employs quantum effects to dramatically speed up certain calculations, such as number factoring; and quantum sensing and metrology, where the high sensitivity of coherent quantum systems to external perturbations is exploited to enhance the performance of measurements of physical quantities. In Europe, the QT community has profited from several EC funded coordination projects, which, among other things, have coordinated the creation of a 150-page QT Roadmap (http://qurope.eu/h2020/qtflagship/roadmap2016). This article presents an updated summary of this roadmap.

We demonstrate experimentally a robust quantum memory using a magnetic-field-independent hyperfine transition in 9 Be + atomic ion qubits at a magnetic field B ≃ 0.01194 T. We observe that the single physical qubit memory coherence time is greater than 10 seconds, an improvement of approximately five orders of magnitude from previous experiments with 9 Be + . We also observe long coherence times of decoherence-free subspace logical qubits comprising two entangled physical qubits and discuss the merits of each type of qubit.PACS numbers: 03.67. Pp, 32.60.+i, 03.65.Yz, 03.67.Mn Scalable quantum information processing (QIP) requires physical systems capable of reliably storing coherent superpositions for periods over which quantum error correction can be implemented [1]. Moreover, suppressing memory error rates to very low levels allows for simpler error-correcting algorithms [2,3]. In many current atomic ion QIP experiments, a dominant source of memory error is decoherence induced by fluctuating ambient magnetic fields [4,5]. To address this problem, we investigate creating long-lived qubit memories using a first-order magnetic-field-independent hyperfine transition and logical qubits of a decoherence-free subspace [6].Atomic systems have proven themselves as good candidates for quantum information storage through their use in highly stable atomic clocks [7]. Here, the principle of using first-order magnetic-field-independent transitions is well established. A typical clock transition |F, m F = 0 ↔ |F ′ , m F ′ = 0 between hyperfine states of angular momentum F and F ′ in alkali atoms has no linear Zeeman shift at zero magnetic field, and coherence times exceeding 10 minutes have been observed [8]. Unfortunately, magnetic sublevels in each hyperfine manifold are degenerate at zero magnetic field. This makes it more advantageous to operate at a nonzero field in order to spectrally resolve the levels, thereby inducing a linear field dependence of the transition frequency. However, field-independent transitions between hyperfine states also exist at nonzero magnetic field. In the context of atomic clocks, coherence times exceeding 10 minutes have been observed in 9 Be + ions at a magnetic field B = 0.8194 T [9].In neutral-atom systems suitable for QIP, fieldindependent transitions at nonzero magnetic field have been investigated in rubidium [10,11]. The radio-frequency (RF)/microwave two-photon stimulatedRaman hyperfine transition |F = 1, m F = −1 ↔ |F ′ = 2, m F ′ = 1 is field-independent at approximately 3.23 × 10 −4 T , and coherence times of 2.8 s have been observed [11]. In these and the clock experiments, transitions were driven by microwave fields on large numbers of atoms. Using microwaves, it may be difficult to localize the fields well enough to drive individual qubits unless a means (e.g., a magnetic-field gradient or Stark-shift gradient) is employed to provide spectral selection [12,13], a technique that has the additional overhead of keeping track of the phases induced by these shifts. With transitio...

Proposed quantum networks require both a quantum interface between light and matter and the coherent control of quantum states1,2. A quantum interface can be realized by entangling the state of a single photon with the state of an atomic or solid-state quantum memory, as demonstrated in recent experiments with trapped ions3,4, neutral atoms5,6, atomic ensembles7,8, and nitrogen-vacancy spins9. The entangling interaction couples an initial quantum memory state to two possible light–matter states, and the atomic level structure of the memory determines the available coupling paths. In previous work, these paths’ transition parameters determine the phase and amplitude of the final entangled state, unless the memory is initially prepared in a superposition state4, a step that requires coherent control. Here we report the fully tunable entanglement of a single 40Ca+ ion and the polarization state of a single photon within an optical resonator. Our method, based on a bichromatic, cavity-mediated Raman transition, allows us to select two coupling paths and adjust their relative phase and amplitude. The cavity setting enables intrinsically deterministic, high-fidelity generation of any two-qubit entangled state. This approach is applicable to a broad range of candidate systems and thus presents itself as a promising method for distributing information within quantum networks.

We report, for the first time, laser spectroscopy of the 1S0-->3P0 clock transition in 27Al+. A single aluminum ion and a single beryllium ion are simultaneously confined in a linear Paul trap, coupled by their mutual Coulomb repulsion. This coupling allows the beryllium ion to sympathetically cool the aluminum ion and also enables transfer of the aluminum's electronic state to the beryllium's hyperfine state, which can be measured with high fidelity. These techniques are applied to measure the clock transition frequency nu=1,121,015,393,207,851(6) Hz. They are also used to measure the lifetime of the metastable clock state tau=20.6+/-1.4 s, the ground state 1S0 g factor gS=-0.000,792,48(14), and the excited state 3P0 g factor gP=-0.001,976,86(21), in units of the Bohr magneton.