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A simple approach to preparing high-dimensional entangled states by quantum interference.

Nearly 30 years ago, two-photon interference was observed, marking the beginning of a new quantum era. Indeed, two-photon interference has no classical analogue, giving it a distinct advantage for a range of applications. The peculiarities of quantum physics may now be used to our advantage to outperform classical computations, securely communicate information, simulate highly complex physical systems and increase the sensitivity of precise measurements. This separation from classical to quantum physics has motivated physicists to study two-particle interference for both fermionic and bosonic quantum objects. So far, two-particle interference has been observed with massive particles, among others, such as electrons and atoms, in addition to plasmons, demonstrating the extent of this effect to larger and more complex quantum systems. A wide array of novel applications to this quantum effect is to be expected in the future. This review will thus cover the progress and applications of two-photon (two-particle) interference over the last three decades.

We present a detailed study of the pulsation of α Circini, the brightest of the rapidly oscillating Ap stars. We have obtained 84 d of high‐precision photometry from four runs with the star tracker on the WIRE satellite. Simultaneously, we collected ground‐based Johnson B observations on 16 nights at the South African Astronomical Observatory. In addition to the dominant oscillation mode at 2442 μHz, we detect two new modes that lie symmetrically around the principal mode to form a triplet. The average separation between these modes is Δf= 30.173 ± 0.004 μHz and they are nearly equidistant with the separations differing by only 3.9 nHz. We compare the observed frequencies with theoretical pulsation models based on constraints from the recently determined interferometric radius and effective temperature, and the recently updated Hipparcos parallax. We show that the theoretical large separations for models of α Cir with global parameters within the 1σ observational uncertainties vary between 59 and 65 μHz. This is consistent with the large separation being twice the observed value of Δf, indicating that the three main modes are of alternating even and odd degrees. The frequency differences in the triplet are significantly smaller than those predicted from our models, for all possible combinations of mode degrees, and may indicate that the effects of magnetic perturbations need to be taken into account. The WIRE light curves are modulated by a double wave with a period of 4.479 d, and a peak‐to‐peak amplitude of 4 mmag. This variation is due to the rotation of the star and is a new discovery, made possible by the high precision of the WIRE photometry. The rotational modulation confirms an earlier indirect determination of the rotation period. The main pulsation mode at 2442 μHz has two sidelobes split by exactly the rotation frequency. From the amplitude ratio of the sidelobes to the central peak, we show that the principal mode is consistent with an oblique axisymmetric dipole mode (l= 1, m= 0) or with a magnetically distorted mode of higher degree with a dominant dipolar component.

Thermal Hilbert moment QCD sum rules are used to obtain the temperature dependence of the hadronic parameters of charmonium in the vector channel, i.e. the J/ψ resonance mass, coupling (leptonic decay constant), total width, and continuum threshold. The continuum threshold s 0 , which signals the end of the resonance region and the onset of perturbative QCD (PQCD), behaves as in all other hadronic channels, i.e. it decreases with increasing temperature until it reaches the PQCD threshold s 0 = 4 m 2 Q , with m Q the charm quark mass, at T ≃ 1.22 T c . The rest of the hadronic parameters behave very differently from those of light-light and heavy-light quark systems. The J/ψ mass is essentially constant in a wide range of temperatures, while the total width grows with temperature up to T ≃ 1.04 T c beyond which it decreases sharply with increasing T. The resonance coupling is also initially constant and then begins to increase monotonically around T ≃ T c . This behaviour of the total width and of the leptonic decay constant provides a strong indication that the J/ψ resonance might survive beyond the critical temperature for deconfinement. 1 Supported in part by FONDECYT 1060653, 1095217 and 7080120 (Chile), Centro de Estudios Subatomicos (Chile), and NRF (South Africa).A successful quantum field theory framework to extract hadronic information from QCD analytically is that of QCD sum rules [1]. This technique is based on the Operator Product Expansion (OPE) of current correlators beyond perturbation theory, and on Cauchy's theorem in the complex energy plane (quark-hadron duality). This program was first extended to finite temperature in [2]. It is based on two basic assumptions, (a) that the OPE continues to be valid, with the vacuum condensates developing a temperature dependence, and (b) that no thermal singularities appear in the complex energy plane, other than on the real axis, i.e. the notion of quark-hadron duality also remains valid. Field theory evidence in support of these assumptions was provided later in [3]. Numerous applications of QCD sum rules at finite temperature have been made over the years [4]-[6], leading to the following scenario for light-light and heavy-light quark hadrons. (i) As the temperature increases, hadronically stable particles develop a non-zero width, and resonances become broader, diverging at a critical temperature interpreted as the deconfinement temperature (T c ). This width is a result of particle absorption in the thermal bath, and resonance broadening at finite temperature was first proposed in [7]. (ii) Above the resonance region the continuum threshold in hadronic spectral functions, i.e. the onset of perturbative QCD (PQCD), decreases monotonically with increasing temperature. In other words, as T → T c hadrons melt disappearing from the spectrum, which then becomes smooth. (iii) Additional support for this picture is provided by the behaviour of hadronic couplings, or leptonic decay constants, which approach zero as T → T c . Also, hadronic and electromagnetic mean...

Quantum communication has been successfully implemented in optical fibres and through free-space. Fibre systems, though capable of fast key and low error rates, are impractical in communicating with destinations without an established fibre link. Free-space quantum channels can overcome such limitations and reach long distances with the advent of satellite-to-ground links. However, turbulence, resulting from local fluctuations in refractive index, becomes a major challenge by adding errors and losses. Recently, an interest in investigating the possibility of underwater quantum channels has arisen. Here, we investigate the effect of turbulence on an underwater quantum channel using twisted photons in outdoor conditions. We study the effect of turbulence on transmitted error rates, and compare different quantum cryptographic protocols in an underwater quantum channel, showing the feasibility of high-dimensional encoding schemes. Our work may open the way for secure high-dimensional quantum communication between submersibles, and provides important input for potential submersibles-to-satellite quantum communication.

High-bit-rate long-distance quantum communication is a proposed technology for future communication networks and relies on high-dimensional quantum entanglement as a core resource. While it is known that spatial modes of light provide an avenue for high-dimensional entanglement, the ability to transport such quantum states robustly over long distances remains challenging. To overcome this, entanglement swapping may be used to generate remote quantum correlations between particles that have not interacted; this is the core ingredient of a quantum repeater, akin to repeaters in optical fibre networks. Here we demonstrate entanglement swapping of multiple orbital angular momentum states of light. Our approach does not distinguish between different anti-symmetric states, and thus entanglement swapping occurs for several thousand pairs of spatial light modes simultaneously. This work represents the first step towards a quantum network for high-dimensional entangled states and provides a test bed for fundamental tests of quantum science.