Scientific publications

November 4, 2020

Fiber-integrated microcavities for efficient generation of coherent acoustic phonons


Coherent phonon generation by optical pump-probe experiments has enabled the study of acoustic properties at the nanoscale in planar heterostructures, plasmonic resonators, micropillars, and nanowires. Focalizing both the pump and the probe on the same spot of the sample is a critical part of pump-probe experiments. This is particularly relevant in the case of small objects. The main practical challenges for the actual implementation of this technique are stability of the spatiotemporal overlap, reproducibility of the focalization, and optical mode matching conditions. In this work, we solve these three challenges for the case of planar and micropillar optophononic cavities. We integrate the studied samples to single mode fibers lifting the need for focusing optics to excite and detect coherent acoustic phonons. The resulting reflectivity contrast of at least 66% achieved in our samples allows us to observe stable coherent phonon signals over at least a full day and signals at an extremely low excitation power of 1 μW. The monolithic sample structure is transportable and could provide a means to perform reproducible plug-and-play experiments.

July 8, 2020

Efficient Source of Indistinguishable Single Photons based on Phonon-Assisted Excitation


Semiconductor quantum dots in cavities are high-performance single-photon sources. Thus far the most efficient sources utilise resonant excitation of an unpolarized quantum emitter coupled to a highly birefringent cavity. However, this demands very high polarization extinction, and challenging experimental operation. Here, we remove these requirements by using off-resonant phonon-assisted excitation of a linear exciton dipole, exploiting the quantum dot’s vibrational environment and natural asymmetry. This allows the collection of single photons that are spectrally separated from the excitation laser, and intrinsically present a very high degree of linear polarization up to 0.994 ± 0.007. This phonon-assisted excitation scheme enables very high single-photon purity and indistinguishability, and only reduces the emitter population by (15 ± 1) %, as compared to resonant excitation. Overall, we simultaneously demonstrate a polarized first lens brightness of 0.51 ± 0.01, with a single-photon purity of 0.939 ± 0.001 and corrected single-photon indistinguishability of 0.915 ± 0.003.

May 4, 2020

Hong-Ou-Mandel Interference with imperfect single photon sources


Hong-Ou-Mandel interference is a cornerstone of optical quantum technologies. We explore both theoretically and experimentally how the nature of unwanted multi-photon components of single photon sources affect the interference visibility. We apply our approach to quantum dot single photon sources in order to access the mean wavepacket overlap of the single-photon component – an important metric to understand the limitations of current sources. We find that the impact of multi-photon events has thus far been underestimated, and that the effect of pure dephasing is even milder than previously expected.

March 2, 2020

Reproducibility of High-Performance Quantum Dot Single-Photon Sources


Single-photon sources based on semiconductor quantum dots have emerged as an excellent platform for high efficiency quantum light generation. However, scalability remains a challenge since quantum dots generally present inhomogeneous characteristics. Here we benchmark the performance of 15 deterministically fabricated single-photon sources. They display an average single-photon purity of 95.4% ± 1.5% with an average mean wavepacket overlap of 88.0% ± 3.1% corresponding to a single-photon indistinguishability of 92.2% ± 2.6% and high homogeneity in operation wavelength and temporal profile. Each source also has state-of-the-art brightness with an average first lens brightness value of 13.6% ± 4.4%. While the highest brightness is obtained with charged quantum dots, the highest quantum purity is obtained with neutral ones. We also introduce various techniques to identify the nature of the emitting state. Our study sets the groundwork for large-scale fabrication of identical sources.

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December 9, 2019

Sequential generation of linear cluster states from a single photon emitter


Light states composed of multiple entangled photons – such as cluster states – are essential for developing and scaling-up quantum computing networks. Photonic cluster states with discrete variables can be obtained from single-photon sources and entangling gates, but so far this has only been done with probabilistic sources constrained to intrinsically-low efficiencies, and an increasing hardware overhead. Here, we report the resource-efficient generation of polarization-encoded, individually-addressable, photons in linear cluster states occupying a single spatial mode. We employ a single entangling-gate in a fiber loop configuration to sequentially entangle an ever-growing stream of photons originating from the currently most efficient single-photon source technology – a semiconductor quantum dot. With this apparatus, we demonstrate the generation of linear cluster states up to four photons in a single-mode fiber. The reported architecture can be programmed to generate linear-cluster states of any number of photons with record scaling ratios, potentially enabling practical implementation of photonic quantum computing schemes.

November 27, 2019

Interfacing scalable photonic platforms: solid-state based multi-photon interference in a reconfigurable glass chip


Scaling-up optical quantum technologies requires a combination of highly efficient multi-photon sources and integrated waveguide components. Here, we interface these scalable platforms, demonstrating high-rate three-photon interference with a quantum dot based multi-photon source and a reconfigurable photonic chip on glass. We actively demultiplex the temporal train of single photons obtained from a quantum emitter to generate a 3.8×103  s−1three-photon source, which is then sent to the input of a tunable tritter circuit, demonstrating the on-chip quantum interference of three indistinguishable single photons. We show via pseudo number-resolving photon detection characterizing the output distribution that this first combination of scalable sources and reconfigurable photonic circuits compares favorably in performance with respect to previous implementations. Our detailed loss-budget shows that merging solid-state multi-photon sources and reconfigurable photonic chips could allow 10-photon experiments on chip at ∼40  s−1 rate in a foreseeable future.

August 19, 2019

Generation of non-classical light in a photon-number superposition


Generating light in a pure quantum state is essential for advancing optical quantum technologies. However, controlling its photon number remains elusive. Optical fields with zero and one photon can be produced by single atoms, but, so far, this has been limited to generating incoherent mixtures or coherent superpositions with a very small one-photon term. Here, we report the on-demand generation of quantum superpositions of zero, one and two photons via coherent control of an artificial atom. Driving the system up to full atomic inversion leads to quantum superpositions of vacuum and one photon, with their relative populations controlled by the driving laser intensity. A stronger driving of the system, with 2π pulses, results in a coherent superposition of vacuum, one and two photons, with the two-photon term exceeding the one-photon component, a state allowing phase super-resolving interferometry. Our results open new paths for optical quantum technologies with access to the photon-number degree of freedom.


November 21, 2019

June 4 2019

October 14, 2019

April 29, 2019