Ghost imaging with correlated photon pairs has been applied to remote sensing without the spatially detected photons interacting with the object: here we demonstrate for the first time ghost imaging using correlated atom pairs.

Ghost imaging is demonstrated using beams of correlated pairs of ultracold helium atoms, rather than photons, yielding a reconstructed image with submillimetre resolution. Ghost imaging achieves a feat that sounds impossible: the reconstruction of an image of an object using a beam of light that has never interacted with the object. The trick that makes it possible involves the use of two beams of correlated photons. One beam passes through the object to a bucket (single-pixel) detector, while the spatial profile of the second beam is measured by a high-resolution (multi-pixel) detector; but, this second beam never interacts with the object. Until now ghost imaging has been achieved only with photons, but here Andrew Truscott and colleagues report a technique for producing ghost images with massive particles—specifically, with ultracold helium atoms. Substituting photons in quantum mechanical experiments for massive particles could shed light on fundamental questions such as the quantum-to-classical transition. In addition, this methodology may facilitate applications such as real-time control of atom lithography while imaging the deposition remotely via the ghost imaging technique. Ghost imaging is a counter-intuitive phenomenon—first realized in quantum optics1,2—that enables the image of a two-dimensional object (mask) to be reconstructed using the spatio-temporal properties of a beam of particles with which it never interacts. Typically, two beams of correlated photons are used: one passes through the mask to a single-pixel (bucket) detector while the spatial profile of the other is measured by a high-resolution (multi-pixel) detector. The second beam never interacts with the mask. Neither detector can reconstruct the mask independently, but temporal cross-correlation between the two beams can be used to recover a ‘ghost’ image. Here we report the realization of ghost imaging using massive particles instead of photons. In our experiment, the two beams are formed by correlated pairs of ultracold, metastable helium atoms3, which originate from s-wave scattering of two colliding Bose–Einstein condensates4,5. We use higher-order Kapitza–Dirac scattering6,7,8 to generate a large number of correlated atom pairs, enabling the creation of a clear ghost image with submillimetre resolution. Future extensions of our technique could lead to the realization of ghost interference9, and enable tests of Einstein–Podolsky–Rosen entanglement9 and Bell’s inequalities10 with atoms.

In non-Hermitian systems, spectral degeneracies can arise that can cause unusual, counter-intuitive effects; here exciton-polaritons—hybrid light–matter particles—within a semiconductor microcavity are found to display non-trivial topological modal structure exclusive to such systems. In non-Hermitian systems, which are open and subject to gain and loss, exceptional points can arise, spectral degeneracies that can cause unusual, counter-intuitive effects. Recent efforts to observe non-Hermitian physics have concentrated on various optical systems, but not yet on exciton-polaritons. These are hybrid light–matter particles, formed by strongly interacting photons and excitons (electron–hole pairs) in semiconductor microcavities. Such systems require constant pumping of energy and continuously decays releasing coherent radiation, so are a profoundly open quantum system. In a striking experiment involving a chaotic exciton-polariton billiard —a two-dimensional area enclosed by a curved potential barrier — these authors demonstrate this non-Hermitian nature for the first time. The experiment reveals the non-trivial topological modal structure exclusive to non-Hermitian systems. These findings open the way for novel types of operating principles for polariton-based optoelectronic devices. Exciton-polaritons are hybrid light–matter quasiparticles formed by strongly interacting photons and excitons (electron–hole pairs) in semiconductor microcavities1,2,3. They have emerged as a robust solid-state platform for next-generation optoelectronic applications as well as for fundamental studies of quantum many-body physics. Importantly, exciton-polaritons are a profoundly open (that is, non-Hermitian4,5) quantum system, which requires constant pumping of energy and continuously decays, releasing coherent radiation6. Thus, the exciton-polaritons always exist in a balanced potential landscape of gain and loss. However, the inherent non-Hermitian nature of this potential has so far been largely ignored in exciton-polariton physics. Here we demonstrate that non-Hermiticity dramatically modifies the structure of modes and spectral degeneracies in exciton-polariton systems, and, therefore, will affect their quantum transport, localization and dynamical properties7,8,9. Using a spatially structured optical pump10,11,12, we create a chaotic exciton-polariton billiard—a two-dimensional area enclosed by a curved potential barrier. Eigenmodes of this billiard exhibit multiple non-Hermitian spectral degeneracies, known as exceptional points13,14. Such points can cause remarkable wave phenomena, such as unidirectional transport15, anomalous lasing/absorption16,17 and chiral modes18. By varying parameters of the billiard, we observe crossing and anti-crossing of energy levels and reveal the non-trivial topological modal structure exclusive to non-Hermitian systems9,13,14,15,16,17,18,19,20,21,22. We also observe mode switching and a topological Berry phase for a parameter loop encircling the exceptional point23,24. Our findings pave the way to studies of non-Hermitian quantum dynamics of exciton-polaritons, which may uncover novel operating principles for polariton-based devices.

In a delayed-choice experiment the decision to measure the particle or wave nature comes after the photon enters the interferometer. An atomic version of the experiment provides the same outcome despite the mass and internal structure of the atoms. The wave–particle dual nature of light and matter and the fact that the choice of measurement determines which one of these two seemingly incompatible behaviours we observe are examples of the counterintuitive features of quantum mechanics. They are illustrated by Wheeler’s famous ‘delayed-choice’ experiment1, recently demonstrated in a single-photon experiment2. Here, we use a single ultracold metastable helium atom in a Mach–Zehnder interferometer to create an atomic analogue of Wheeler’s original proposal. Our experiment confirms Bohr’s view that it does not make sense to ascribe the wave or particle behaviour to a massive particle before the measurement takes place1. This result is encouraging for current work towards entanglement and Bell’s theorem tests in macroscopic systems of massive particles3.

We present the first measurement for helium atoms of the tune-out wavelength at which the atomic polarizability vanishes. We utilize a novel, highly sensitive technique for precisely measuring the effect of variations in the trapping potential of confined metastable (2^{3}S_{1}) helium atoms illuminated by a perturbing laser light field. The measured tune-out wavelength of 413.0938(9_{stat})(20_{syst}) nm compares well with the value predicted by a theoretical calculation [413.02(9) nm] which is sensitive to finite nuclear mass, relativistic, and quantum electrodynamic effects. This provides motivation for more detailed theoretical investigations to test quantum electrodynamics.

We demonstrate a novel temperature controlled filtered laminar air enclosure composed of primarily off-the-shelf components that can be applied to a broad class of systems to significantly enhance their performance. An air mixing method is employed to provide variable cooling of the incoming filtered air, providing a temperature stability of ± 0.02 °C within the enclosure. The method is inexpensive to implement, and is suitable for a wide range of temperature controlled enclosures, with dimensions in the approximate range from 1 m to 5 m, making it ideal for many scientific applications.

The possibility of investigating macroscopic coherent quantum states in polariton condensates and of engineering polariton landscapes in semiconductors has triggered interest in using polaritonic systems to simulate complexmany-body phenomena. However, advanced experiments require superior trapping techniques that allow for the engineering of periodic and arbitrary potentials with strong on-site localization, clean condensate formation, and nearest-neighbor coupling. Here we establish a technology that meets these demands and enables strong, potentially tunable trapping without affecting the favorable polariton characteristics. The traps are based on a locally elongated microcavity which can be formed by standard lithography. We observe polariton condensation with non-resonant pumping in single traps and photonic crystal square lattice arrays. In the latter structures, we observe pronounced energy bands, complete band gaps, and spontaneous condensation at the M-point of the Brillouin zone.

We establish a technology which allows for deep and tunable polariton trapping. Pronounced band structures with full gaps and condensation at high symmetry points is observed in square lattice arrangements of evanescently coupled potential traps.

An important aspect of the rapidly growing field of quantum atom optics is exploring the behavior of ultracold atoms at a deeper level than the mean field approximation, where the quantum properties of individual atoms becomes important. Major recent advances have been achieved with the creation and detection of reliable single-atom sources, which is a crucial tool for testing fundamental quantum processes. Here, we create a source comprised of a single ultracold metastable helium atom, which enables novel free-space quantum atom optics experiments to be performed with single massive particles with large de Broglie wavelengths.

Controlled transfer of orbital angular momentum to an exciton-polariton Bose-Einstein condensate spontaneously created under incoherent, off resonant excitation conditions is a long-standing challenge in the field of microcavity polaritonics. We demonstrate, experimentally and theoretically, a simple and efficient approach to the generation of nontrivial orbital angular momentum states by using optically induced potentials-chiral polaritonic lenses. These lenses are produced by a structured optical pump with a spatial distribution of intensity that breaks the chiral symmetry of the system.

A fundamental property of a three-dimensional Bose-Einstein condensate is long-range coherence; however, in systems of lower dimensionality, not only is the long-range coherence destroyed but additional states of matter are predicted to exist. One such state is a "transverse condensate," first predicted by van Druten and Ketterle [Phys. Rev. Lett. 79, 549 (1997)], in which the gas condenses in the transverse dimensions of a highly anisotropic trap while remaining thermal in the longitudinal dimension. Here, we detect the transition from a three-dimensional thermal gas to a gas undergoing transverse condensation by probing Hanbury Brown-Twiss correlations.

Quantum coherence has been extensively investigated in quantum optics, but less is known about its properties in massive particles. The higher-order many-body correlation functions have now been measured in an atom optics experiment, validating Wick’s theorem.

We present here the first measurement of the third-order spatial correlation function for atoms, made possible by cooling a metastable helium cloud to create an ultracold thermal ensemble just above the Bose–Einstein condensation point. The resulting large correlation length well exceeds the spatial resolution limit of the single-atom detection system, and enables extension of our earlier temporal measurements to evaluate the third-order correlation function in the spatial plane of the detector. The enhancement of the spatial third-order correlation function above a value of unity demonstrates the presence of spatial three-atom bunching, as expected for an incoherent source.

In direct analogy to the textbook example of light guided in a few-mode fiber (FMF), we report the observation of the first excited mode of an optically guided atomic beam. We selectively excite the atomic analog of the LP₀₁ optical mode by controlling the energy distribution of ultracold atoms loaded into the guide, resulting in a modal structure dominated by a 47(2)% population in the first excited transverse mode. The ability to guide lower-order modes has been essential to demonstrating optical effects such as multimode interferometry, slow light, and entanglement, and an atomic analog to a FMF may lead to similarly useful applications.

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text S.S. Hodgman, R.G. Dall, A.G. Manning, M.T. Johnsson, K.G.H. Baldwin, and A.G. Truscott, "Characterizing Atom Sources with Quantum Coherence," Optics & Photonics News 22(12), 37-37 (2011) Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

Using single atom detection offered by metastable helium atoms (He*), we are able to measure correlations between atoms. With this we have demonstrated second-order coherence of matter waves that emerge from amplified four-wave mixing (FWM). Our results provide the first second-order, or quantum test of coherence for amplified matter waves. Using the same detection scheme, we have investigated the onset of coherence during the formation of a Bose-Einstein condensate (BEC), attempting to reveal how the long-range order of a BEC is established. Our initial results support the existence of quasi-condensates.