Barry Jack

Quantum correlations in optical angle-orbital angular momentum variables

Entanglement in a Twist The strong correlations observed in quantum mechanically entangled particles, such as photons, offer potential for secure communication and quantum information processing. Leach et al. (p. 662) now show such strong quantum correlations between the complementary variables—angular position and orbital angular momentum—of two photons created during the parametric down-conversion process in a nonlinear crystal. This demonstration of entanglement in an angular basis establishes that angles are genuine quantum observables and can therefore be considered a resource for quantum information processing, capable of secure, high-dimension, key distribution. Strong quantum correlations are induced between the angular position and angular momentum of two photons. Entanglement of the properties of two separated particles constitutes a fundamental signature of quantum mechanics and is a key resource for quantum information science. We demonstrate strong Einstein, Podolsky, and Rosen correlations between the angular position and orbital angular momentum of two photons created by the nonlinear optical process of spontaneous parametric down-conversion. The discrete nature of orbital angular momentum and the continuous but periodic nature of angular position give rise to a special sort of entanglement between these two variables. The resulting correlations are found to be an order of magnitude stronger than those allowed by the uncertainty principle for independent (nonentangled) particles. Our results suggest that angular position and orbital angular momentum may find important applications in quantum information science.

Violation of a Bell inequality in two-dimensional orbital angular momentum state-spaces

We observe entanglement between photons in controlled super-position states of orbital angular momentum (OAM). By drawing a direct analogy between OAM and polarization states of light, we demonstrate the entangled nature of high order OAM states generated by spontaneous downconversion through violation of a suitable Clauser Horne Shimony Holt (CHSH)-Bell inequality. We demonstrate this violation in a number of two-dimensional subspaces of the higher dimensional OAM Hilbert space.

Precise quantum tomography of photon pairs with entangled orbital angular momentum

We report a high fidelity tomographic reconstruction of the quantum state of photon pairs generated by parametric down-conversion with orbital angular momentum (OAM) entanglement. Our tomography method allows us to estimate an upper and lower bound for the entanglement between the down-converted photons. We investigate the two-dimensional state subspace defined by the OAM states ±l and superpositions thereof, with l=1, 2, ..., 30. We find that the reconstructed density matrix, even for OAMs up to around l=20, is close to that of a maximally entangled Bell state with a fidelity in the range between F=0.979 and F=0.814. This demonstrates that, although the single count-rate diminishes with increasing l, entanglement persists in a large dimensional state space.

Violation of Leggett inequalities in orbital angular momentum subspaces

We report an experimental test of Leggetts non-local hidden variable theory in an orbital angular momentum (OAM) state space of light. We show that the correlations we observe are in conflict with Leggetts model, thus excluding a particular class of non-local hidden variable theories for the first time in a non-polarization state space. It is known that the violation of the Leggett inequality becomes stronger as more detection settings are used. The required measurements become feasible in an OAM subspace, and we demonstrate this by testing the inequality using three and four settings. We observe excellent agreement with quantum predictions and a violation of five and six standard deviations, respectively, compared to Leggetts non-local hidden variable theory.

Demonstration of the angular uncertainty principle for single photons

We present an experimental demonstration of a form of the angular uncertainty principle for single photons. Producing light from type I down-conversion, we use spatial light modulators to perform measurements on signal and idler photons. By measuring states in the angle and orbital angular momentum basis, we demonstrate the uncertainty relation of Franke-Arnold et al (2004 New J. Phys. 6 103). We consider two manifestations of the uncertainty relation. In the first we herald the presence of a photon by detection of its paired partner and demonstrate the uncertainty relation on this single photon. In the second, we perform orbital angular momentum measurements on one photon and angular measurements on its correlated partner exploring, in this way, the uncertainty relation through non-local measurements.

New Twist on Light Beams for Quantum Information Science

Quantum information science continues to drive work in quantum optics. New research makes fundamental measurements showing parity-time symmetry breaking using an optical system.

Full-field quantum measurements and holographic ghost imaging

Using fiber arrays we measure the strength of position and momentum correlations, enabling a full-field demonstration of EPR. We also demonstrate contrast enhancement of images within a ghost-imaging system by use of non-local phase filters.

Quantum imaging and orbital angular momentum

We report a violation of the CHSH inequality for ghost-images. This is achieved by using two spatially separated phase modulators within the context of a two-photon parametric down-conversion experiment. We obtain edge enhanced images as a direct consequence of the quantum correlations in the orbital angular momentum (OAM) of the down-converted photon pairs. For phase objects, with differently orientated edges, we show a violation of the CHSH Bell-type inequality for an OAM subspace, thereby unambiguously revealing the quantum nature of our ghost-imaging arrangement.

Continuous Variable EPR Paradox for Angle and Orbital Angular Momentum

We demonstrate the Einstein-Podolsky-Rosen paradox for angle and orbital angular momentum states of light. We show strong angular position and orbital angular momentum correlations therefore demonstrating the quantum nature of the entangled light field.