Ana Predojević

Squeezed-light optical magnetometry.

We demonstrate a light-shot-noise-limited magnetometer based on the Faraday effect in a hot unpolarized ensemble of rubidium atoms. By using off-resonant, polarization-squeezed probe light, we improve the sensitivity of the magnetometer by 3.2 dB. The technique could improve the sensitivity of the most advanced magnetometers and quantum nondemolition measurements of atomic spin ensembles.

Deterministic photon pairs and coherent optical control of a single quantum dot.

The strong confinement of semiconductor excitons in a quantum dot gives rise to atomlike behavior. The full benefit of such a structure is best observed in resonant excitation where the excited state can be deterministically populated and coherently manipulated. Because of the large refractive index and device geometry it remains challenging to observe resonantly excited emission that is free from laser scattering in III/V self-assembled quantum dots. Here we exploit the biexciton binding energy to create an extremely clean single photon source via two-photon resonant excitation of an InAs/GaAs quantum dot. We observe complete suppression of the excitation laser and multiphoton emissions. Additionally, we perform full coherent control of the ground-biexciton state qubit and observe an extended coherence time using an all-optical echo technique. The deterministic coherent photon pair creation makes this system suitable for the generation of time-bin entanglement and experiments on the interaction of photons from dissimilar sources.

Bright filter-free source of indistinguishable photon pairs

We demonstrate a high-brightness source of pairs of indistinguishable photons based on a type-II phase-matched doubly-resonant optical parametric oscillator operated far below threshold. The cavityenhanced down-conversion output of a PPKTP crystal is coupled into two single-mode fibers with a mode coupling efficiency of 58%. The high degree of indistinguishability between the photons of a pair is demonstrated by a Hong-Ou-Mandel interference visibility of higher than 90% without any filtering at an instantaneous coincidence rate of 450,000 pairs/s per mW of pump power per nm of down-conversion bandwidth. For the degenerate spectral mode with a linewidth of 7 MHz at 795 nm a rate of 70 pairs/(s mW MHz) is estimated, increasing the spectral brightness for indistinguishable photons by two orders of magnitude compared to similar previous sources.

Time-bin entangled photons from a quantum dot

Long distance quantum communication is one of the prime goals in the field of quantum information science. With information encoded in the quantum state of photons, existing telecommunication fibre networks can be effectively used as a transport medium. To achieve this goal, a source of robust entangled single photon pairs is required. Here, we report the realization of a source of time-bin entangled photon pairs utilizing the biexciton-exciton cascade in a III/V self-assembled quantum dot. We analyse the generated photon pairs by an inherently phase-stable interferometry technique, facilitating uninterrupted long integration times. We confirm the entanglement by performing quantum state tomography of the emitted photons, which yields a fidelity of 0.69(3) and a concurrence of 0.41(6) for our realization of time-energy entanglement from a single quantum emitter.

Narrowband tunable filter based on velocity-selective optical pumping in an atomic vapor

We demonstrate a tunable narrowband filter based on optical-pumping-induced circular dichroism in rubidium vapor. The filter achieves a peak transmission of 14.6%, a linewidth of 80 MHz, and an out-of-band extinction of >or=35 dB. The transmission peak can be tuned within the range of the Doppler linewidth of the D1 line of atomic rubidium at 795 nm. While other atomic filters work at frequencies far from absorption, the presented technique provides light resonant with atomic media, useful for atom-photon interaction experiments. The technique could readily be extended to other alkali atoms.

Polarization entangled photons from quantum dots embedded in nanowires.

In this Letter, we present entanglement generated from a novel structure: a single InAsP quantum dot embedded in an InP nanowire. These structures can grow in a site-controlled way and exhibit high collection efficiency; we detect 0.5 million biexciton counts per second coupled into a single mode fiber with a standard commercial avalanche photo diode. If we correct for the known setup losses and detector efficiency, we get an extraction efficiency of 15(3) %. For the measured polarization entanglement, we observe a fidelity of 0.76(2) to a reference maximally entangled state as well as a concurrence of 0.57(6).

Pulsed Sagnac source of polarization entangled photon pairs

Entangled photon pairs are used in several important quantum information applications including quantum networks [1] and quantum computing [2]. Moreover, they are a crucial component for multi-photon experiments like entanglement swapping [3].

Efficiency vs. multi-photon contribution test for quantum dots

The development of linear quantum computing within integrated circuits demands high quality semiconductor single photon sources. In particular, for a reliable single photon source it is not sufficient to have a low multi-photon component, but also to possess high efficiency. We investigate the photon statistics of the emission from a single quantum dot with a method that is able to sensitively detect the trade-off between the efficiency and the multi-photon contribution. Our measurements show, that the light emitted from the quantum dot when it is resonantly excited possess a very low multi-photon content. Additionally, we demonstrated, for the first time, the non-Gaussian nature of the quantum state emitted from a single quantum dot.

Coherence and degree of time-bin entanglement from quantum dots

We report on the generation of time-bin entangled photon pairs from a semiconductor quantum dot via pulsed resonant biexciton generation. Based on theoretical modeling we optimized the duration of the excitation pulse to minimize the laser-induced dephasing and increase the biexciton-to-background single exciton occupation probability. This results in a high degree of entanglement with a concurrence of up to 0.78(6) and a 0.88(3) overlap with a maximally entangled state. Theoretical simulations also indicate a power dependent nature of the dephasing during the laser excitation that limits the coherence of the excitation process.

Optimal excitation conditions for indistinguishable photons from quantum dots

In this letter, we present a detailed, all optical study of the influence of different excitation schemes on the indistinguishability of single photons from a single InAs quantum dot. For this study, we measure the Hong-Ou-Mandel interference of consecutive photons from the spontaneous emission of an InAs quantum dot state under various excitation schemes and different excitation conditions and give a comparison.