Tim Thomay

Dynamics of nonclassical light from a single solid-state quantum emitter.

We measure the dynamics of a nonclassical optical field using two-time second-order correlations in conjunction with pulsed excitation. The technique quantifies single-photon purity and coherence during the excitation-decay cycle of an emitter, illustrated here using a quantum dot. We observe that for certain pump wavelengths, photons detected early in the cycle have reduced single-photon purity and coherence compared to those detected later. A model indicates that the single-photon purity dynamics are due to exciton recapture after initial emission and within the same pulse cycle.

Statistically background-free, phase-preserving parametric up-conversion with faint light.

We demonstrate up-conversion with no statistically significant background photons and a dynamic range of 15 decades. Near-infrared 920 nm photons were converted into the visible at 577 nm using periodically poled lithium niobate waveguides pumped by a 1550 nm laser. In addition to achieving statistically noiseless frequency up-conversion, we report a high degree of phase preservation (with fringe visibilities ≥ 0.97) at the single-photon level using an up-converting Mach-Zehnder interferometer. This background-free process opens a path to single-photon detection with no intrinsic dark count. Combined with a demonstrated photon-number preserving property of an up-converter, this work demonstrates the feasibility of noiseless frequency up-conversion of entangled photon pairs.

Simultaneous, full characterization of a single-photon state

As single-photon sources become more mature and are used more often in quantum information, communications and measurement applications, their characterization becomes more important. Single-photon-like light is often characterized by its brightness, and two quantum properties: the single-photon composition and the photon indistinguishability. While it is desirable to obtain these quantities from a single measurement, currently two or more measurements are required. Here, we simultaneously determine the brightness, the single photon purity, the indistinguishability, and the statistical distribution of Fock states to third order for a quantum light source. The measurement uses a pair of two-photon (n = 2) number-resolving detectors. n > 2 number-resolving detectors provide no additional advantage in the single-photon characterization. The new method extracts more information per experimental trial than a conventional measurement for all input states, and is particularly more e cient for statistical mixtures of photon states. Thus, using this n=2, number- resolving detector scheme will provide advantages in a variety of quantum optics measurements and systems.

Dynamics of a pulsed single photon source

We propose and demonstrate a method for an independent verification of a degree of single photon purity and coherence applicable for all single-photon emitters used in pulsed mode. Using two-time second-order correlation measurements, we reconstruct the dynamics of the nonclassical photon wavepacket. This reveals the temporal evolution of the photon field during the excitation-relaxation cycle of an emitter. The technique allows for the simultaneous measurement of multiphoton content and decoherence. Here we applied this technique to characterize a nonclassical state produced from a single InAs quantum dot (QD). We experimentally observe variations in the degree of multiphoton content and coherence of the wavepacket. A rate equation is introduced to explain multiphoton variations of our source and is found to describe the observed two-dimensional second-order correlation function accurately.

Manipulating single photons from disparate quantum sources to be indistinguishable [Invited]

Quantum information networks will likely require different quantum systems for different functionality within the network. Indistinguishable photons can be used to interconnect these different subsystems. We discuss methods for coherently manipulating the single photons from different quantum systems and experimentally demonstrate spatial, temporal, and frequency matching of single photons using quantum dot and heralded parametric downconversion single photons. The bosonic nature of light insures that when two indistinguishable photons are superimposed on a beam splitter, they will form a single two-photon state, a process we call coalescence. This coalescence property can be used as both a fundamental test of indistinguishability and in quantum networks—connecting and propagating quantum information.

Demonstration of background-free, phase-preserving parametric up-conversion at the single-photon level

We demonstrate single-photon-level phase preservation in an up-converting interferometer. The up-conversion process is background-free to within experimental uncertainty, allowing high fringe contrast even at low photon levels. This enables faithful up-conversion of entangled photon pairs.

Nearly-background-free, phase-preserving parametric up-conversion at the single-photon level

We demonstrate a background-free (to within experimental uncertainty) frequency up-converter. In addition, high fringe contrast at a single-photon-level in an up-converting interferometer is demonstrated, an enabling step towards faithful up-conversion of entangled photon pairs.

Two-photon interference from disparate quantum sources

When photons emitted from different sources are indistinguishable they will interfere when brought together on a beam splitter. Using quantum dots and parametric down conversion, we manipulate dissimilar quantum sources to produce mutually indistinguishable photons.