We report the first experimental generation and characterization of a six-photon Dicke state. The produced state shows a fidelity of F = 0.56 ± 0.02 with respect to an ideal Dicke state and violates a witness detecting genuine six-qubit entanglement by four standard deviations. We confirm characteristic Dicke properties of our resource and demonstrate its versatility by projecting out four-and five-photon Dicke states, as well as four-photon GHZ and W states. We also show that Dicke states have interesting applications in multiparty quantum networking protocols such as opendestination teleportation, telecloning and quantum secret sharing. In this Letter we report the experimental generation and investigation of a variety of multi-photon entangled states. We present a flexible linear-optics setup that can produce four-, five-and six-photon representatives of the important class of Dicke states, as well as four-photon GHZ states. Information is encoded in the polarization degrees of freedom of entangled photons produced by high-order spontaneous parametric down conversion (SPDC). We show that our generated states are genuinely multipartite entangled by using tailormade and experimentally favorable witness tools. These new characterization methods are important in virtue of the non-ideal nature of the six-photon state: although spurious nonlinear processes affect its quality, quantum features can still be observed and characterized. We also highlight the potential for quantum control in large Hilbert spaces by evaluating protocols such as telecloning, open-destination teleportation and quantum secret sharing [11,12,13,14,15].Experiment.- Fig. 1 (a) shows the setup for the generation of the three-excitation six-photon Dicke state |D (3) 6 = 1 √ 20 P |HHHV V V 123456 . Here, |H/V i are horizontal/vertical polarization states of a photon in spatial mode i = 1, .., 6, which encode the logical states of a qubit, while P denotes the sum over all permutations of logical states [16]. In the setup, six photons are probabilistically distributed among the spatial modes by nonpolarizing beamsplitters (BSs): upon detecting one photon in each mode we post-selectively observe |D (3) 6. We use higher-order emissions of a collinear type-II SPDC process for the simultaneous production of three pairs of photons [17]. A Coherent Inc. Verdi V-18 laser is combined with a mode-locked Mira HP Ti:Sa oscillator to reach the energy necessary to observe third-order SPDC emissions. The pulsed-laser output (τ = 200 fs, λ = 810 nm, 76 MHz) is frequency-doubled using a 2 mmthick Lithium triborate (LBO) crystal, resulting in UV pulses of 1.4 W cw-average. To avoid optical damage to the anti-reflection coating of the LBO, we continuously translate it with a step-motor, achieving a very stable source of UV pulses (power and count-rate fluctuations less than 1 − 2% over 30 h). The UV pulses are focused onto a 2 mm-thick β-barium borate (BBO) type-II crystal, cut for collinear down-conversion emission. Dichroic mirrors then separate the down-converted...

We report on precision resonance spectroscopy measurements of quantum states of ultracold neutrons confined above the surface of a horizontal mirror by the gravity potential of the Earth. Resonant transitions between several of the lowest quantum states are observed for the first time. These measurements demonstrate, that Newton's inverse square law of Gravity is understood at micron distances on an energy scale of 10 −14 eV. At this level of precision we are able to provide constraints on any possible gravity-like interaction. In particular, a dark energy chameleon field is excluded for values of the coupling constant β > 5.8 × 10 8 at 95% confidence level (C.L.), and an attractive (repulsive) dark matter axion-like spin-mass coupling is excluded for the coupling strength gsgp > 3.7 × 10 −16 (5.3 × 10 −16 ) at a Yukawa length of λ = 20 µm (95% (C.L.).PACS numbers: 12.15. Ji,13.30.Ce,14.20.Dh,23.40.Bw Experiments that rely on frequency measurements can be performed with incredibly high precision. One example is Rabi spectroscopy, a resonance spectroscopy technique to measure the energy eigenstates of quantum systems. It was originally developed by I. Rabi to measure the magnetic moment of molecules [1]. Today, resonance spectroscopy techniques are applied in various fields of science and medicine including nuclear magnetic resonance, masers, and atomic clocks. These methods have opened up the field of low-energy particle physics with studies of particle properties and their fundamental interactions and symmetries. In an attempt to investigate gravity at short distances, we applied the concept of resonance spectroscopy to quantum states of very slow neutrons in the Earth's gravity potential [2]. Here, we present the first precision measurements of gravitational quantum states with this method that we refer to as gravity resonance spectroscopy (GRS). The strength of GRS is that it does not rely on electromagnetic interactions. The use of neutrons as test particles bypasses the electromagnetic background induced by van der Waals and Casimir forces and other polarizability effects.Within this work, we link these new measurements to dark matter and dark energy searches. Observational cosmology has determined the dark matter and dark energy density parameters to an accuracy of two significant figures [3]. While dark energy explains the accelerated expansion of the universe, dark matter is needed in order to describe the rotation curves of galaxies and the largescale structure of the universe. The true nature of dark energy and the content of dark matter remain a mystery, however. The two most obvious candidates for dark energy are either Einstein's cosmological constant [4] or quintessence theories [5,6], where the dynamic vacuum energy changes over time. The resonant frequencies of our quantum states are intimately related to these models. If some as yet undiscovered dark matter or dark energy particles interact with neutrons, this should result in a measurable energy shift of the observed quantum states. One prom...

The standard model of cosmology provides a robust description of the evolution of the universe. Nevertheless, the small magnitude of the vacuum energy is troubling from a theoretical point of view 9 . An appealing resolution to this problem is to introduce additional scalar fields. However, these have so far escaped experimental detection, suggesting some kind of screening mechanism may be at play. Although extensive exclusion regions in parameter space have been established for one screening candidate -chameleon fields 10,17 -another natural screening mechanism based on spontaneous symmetry breaking has also been proposed, in the form of symmetrons 11 . Such fields would change the energy of quantum states of ultra-cold neutrons in the gravitational potential of the earth. Here we demonstrate a spectroscopic approach based on the Rabi resonance method that probes these quantum states with a resolution of Δ = 2 × 10 −15 eV. This allows us to exclude the symmetron as the origin of Dark Energy for a large volume of the three-dimensional parameter space.Resonance spectroscopy -originally introduced by I. Rabi 1 as a "molecular beam resonance method" -has evolved into an indispensable method for precision experiments with two-levelsystems. Here, the energy difference between two quantum states translates via the Planck-Einstein relation = ℎ into a frequency, which can be measured with unprecedented accuracy. Rabi's experiment has led to new insights into physics, chemistry and biology providing detailed information about the structure, dynamics, reaction state, and chemical environment of molecules. This work, realized by the qBounce collaboration, extends the quantum technique of Rabi spectroscopy to the gravitational sector. The experiment measures transition frequencies of bound quantum states of neutrons in the gravitational field of the earth. These discrete quantum states occur when very slow ultra-cold neutrons (UCN) totally reflect on perfectly polished horizontal surfaces. The typical spatial extent of the corresponding wave functions is of order of ten microns. The eigenenergies are in the pico-eV range and depend on the local acceleration g, the neutron mass m, and the reduced Planck constant ℏ. Furthermore, any two states can be treated as an effective two-level system. This offers the possibility of applying resonance spectroscopy techniques to test gravity at short distances.

Entangled photons are a crucial resource for quantum communication and linear optical quantum computation. Unfortunately, the applicability of many photon-based schemes is limited due to the stochastic character of the photon sources. Therefore, a worldwide effort has focused in overcoming the limitation of probabilistic emission by generating two-photon entangled states conditioned on the detection of auxiliary photons. Here we present the first heralded generation of photon states that are maximally entangled in polarization with linear optics and standard photon detection from spontaneous parametric down-conversion. We utilize the down-conversion state corresponding to the generation of three photon pairs, where the coincident detection of four auxiliary photons unambiguously heralds the successful preparation of the entangled state. This controlled generation of entangled photon states is a significant step towards the applicability of a linear optics quantum network, in particular for entanglement swapping, quantum teleportation, quantum cryptography and scalable approaches towards photonics-based quantum computing