Alexander Ling

Interference of single photons from two separate semiconductor quantum dots

We demonstrate interference between discrete photons emitted by two different semiconductor quantum dots and quantify their degree of indistinguishability. The quantum dot emission energies are tuned into resonance by straining the samples. Upon interference on a beamsplitter, the photons are shown to be 18.1% indistinguishable, resulting in a coincidence detection rate below the classical limit. Post-selecting only those detections occurring within a short time of each other increases the measured indistinguishability to 47%. The photons are partially distinguishable due to dephasing of the exciton states, and post-selection is also affected by the detector response time.

A versatile waveguide source of photon pairs for chip-scale quantum information processing

We demonstrate a bright, bandwidth-tunable, quasi-phase-matched single-waveguide source generating photon pairs near 900 nm and 1300 nm. Two-photon coincidence spectra are measured at a range of operating temperatures of a periodically-poled KTiOPO(4) (PPKTP) waveguide, which supports both type-0 and type-II spontaneous parametric down-conversion. We map out relative contributions of two-photon to one-photon fluorescence for a range of operating parameters. Such a versatile device is highly promising for future chip-scale quantum information processing.

Testing quantum correlations versus single-particle properties within Leggett’s model and beyond

Quantum mechanics enables distant events to be more strongly correlated than is possible classically. The proposal for a new family of experimental tests, and the implementation of one of them, provides further insight into the nature of such non-local correlations.

Absolute rates of Spontaneous Parametric Down Conversion into a single transverse Gaussian mode

We provide expressions for the absolute emission rate of photon pairs produced in SPDC when all interacting fields are in a single transverse Gaussian mode.

Experimental quantum key distribution based on a Bell test

We report on a complete free-space field implementation of a modified Ekert protocol for quantum key distribution using entangled photon pairs. For each photon pair we perform a random choice between key generation and a Bell inequality. The amount of violation is used to determine the possible knowledge of an eavesdropper to ensure security of the distributed final key.

Experimental polarization state tomography using optimal polarimeters

We report on the experimental implementation of a polarimeter based on a scheme known to be optimal for obtaining the polarization vector of ensembles of spin-

Coalescence of Single Photons Emitted by Disparate Single-Photon Sources: The Example of InAs Quantum Dots and Parametric Down-Conversion Sources

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Near-space flight of a correlated photon system

quantum systems and the alignment procedure for this polarimeter. We also show how to use this polarimeter to estimate the polarization state for identically prepared ensembles of single photons and photon pairs and extend the method to obtain the density matrix for generic multiphoton states. State reconstruction and performance of the polarimeter is illustrated by actual measurements on identically prepared ensembles of single photons and polarization entangled photon pairs.

Wigner tomography of two-qubit states and quantum cryptography

Single photons produced by fundamentally dissimilar physical processes will in general not be indistinguishable. We show how photons produced from a quantum dot and by parametric down-conversion in a nonlinear crystal can be manipulated to be indistinguishable. The measured two-photon coalescence probability is 16%, and is limited by quantum-dot decoherence. Temporal filtering to the quantum-dot coherence time and accounting for detector time response increases this to 61% while retaining 25% of the events. This technique can connect different elements in a scalable quantum network.

Quantum optics for space platforms

We report the successful test flight of a device for generating and monitoring correlated photon pairs under near-space conditions up to 35.5 km altitude. Data from ground based qualification tests and the high altitude experiment demonstrate that the device continues to operate even under harsh environmental conditions. The design of the rugged, compact and power-efficient photon pair system is presented. This design enables autonomous photon pair systems to be deployed on low-resource platforms such as nanosatellites hosting remote nodes of a quantum key distribution network. These results pave the way for tests of entangled photon technology in low earth orbit.