Semiconductor quantum dots are excellent artificial atoms that naturally appear when growing InAs on GaAs. Lattice mismatch between the two materials lead to the spontaneous formation of InAs islands with lens shape, that are typically 10-20 nm wide and 3-5 nm high. These nanostructures are excellent artificial atoms to make efficient quantum devices. To that end, they much be inserted in optical microcavities, which is of course a great challenge since the quantum position is random and their energy difficult to control.
A solution is to fabricate many devices, test thousands of them to find one where the quantum dot is well coupled to the cavity. Such technique has been widely used in many groups to obtain proof of principle demonstrations.
In our group, we have invented a technology, called the in-situ lithography that allows fabricating many devices in a fully controlled way, operating either in the weak or strong coupling regime.
The lithography is performed at 10K; we measure optically the quantum dot position and draw a cavity around it.
Recently, the technology was improved to control electrically the devices and suppress sources of charge noise.
To know more on the in-situ lithography technique check our posts here (“A deterministic technique to insert a single quantum dot in a cavity“) and here (“Electrically tunable bright single photon source“).
Light is an excellent system to implement quantum simulation or quantum computation tasks and the natural choice for connecting different nodes of a quantum networks.
Despite spectacular progresses in the last decade, optical quantum technologies today suffer from the low efficiency of currently used sources, most of them based on frequency conversion where only one percent of each pulse does indeed contain a single photon or an entangled photon pair.
We fabricate highly efficient integrated quantum light sources by inserting a single quantum dot in a microcavity.
Ultrabright single photon sources, where 60-80% of the pulse acutally contain a single photon are routinely fabricated using the in-situ lithography technique [Gazzano et. al. Nature Commun. 2013].
To obtain bright sources of entangled photon pairs, the quantum dot is inserted in coupled optical cavities [Dousse et. al, Nature 2010].
Recently, we have added an electrical control to the sources [Nowak et. al. Nature Commun. 2014] that allows tuning the source wavelength and obtaining highly indistinguishable photons [Somaschi et.al, Nature Photon. 2016].
A strong limitation of today’s optical quantum technologies comes from the use of linear optics for implementing logic gates. In the linear regime, the quantum gates are probabilistics, meaning that they operate as expected typically 10-25% of the times. We explore new ways to manipulate the quantum information using non-linear phenomena at the single photon level.
We use the quantum dot as a non-linear medium for which the absorption of one photon conditions the transmission of a second one.
We have demonstrated that such non-linearities can be reached at the very few photon level (n=0.2-8) and that we can fabricate devices that perform as photon filters: when a laser is sent on the device, the reflected light is 80% single photon like.
Further reading: “A solid-state single photon filter“.
The spin of a carrier in a quantum dot is an attractive solid-state quantum bit to store the quantum information. The spin coherence time of an electron or a hole can reach values in the 0.1-1 µs range when it takes typically hundred picoseconds to manipulate a spin.
This coherence to manipulation time ratio above one thousand is highly promising for quantum computation.
To benefit from this possibility, we develop efficient spin-photon interfaces to make sure that every photon sent on the device interacts with a single spin.
We have recently demonstrated that a single spin in a micropillar cavity can rotate the polarization of a photon by +-6°, depending on the spin state.
Such efficient spin-photon interfaces are the backbone for the demonstration of photon deterministic gates, overcoming a strong limitation of today’s optical quantum technologies coming from the probabilistic operations of the gates.
Further reading: “Macroscopic rotation of polarization induced by a single spin“
Indistinguishable single-photons can be used in linear optical schemes to implement quantum logic gates and quantum simulation.
Bright quantum dot single-photon sources that generate highly indistinguishable single-photons are highly interesting in this context: they can speed up the gate operation by orders of magnitude and potentially strongly increase the number of photons involved in a quantum simulation.
Our first results along this research line concern the demonstration of an entangling two photon gate using a 80% brightness single-photon source [Gazzano et al. PRL 110, 250501 (2013)] and a Boson sampling measurement with 3 photons [Loredo et al. PRL 118, 130503 (2017)] .
We explore the possibility of controlling the spontaneous emission of quantum emitters using plasmonic structures.
Although spontaneous emission is now well controlled in dielectric cavities like micropillars, microdisk or photonic crystal cavities, such structures require demanding technological developments.
Moreover, in these cavities the electromagnetic field is confined on the wavelength scale. Large quality factors are needed to obtain large Purcell factors. Although well suited to extract quasi-monochromatic single photons generated by emitters like epitaxial semiconductor quantum dots, this approach is not appropriate for spectrally broad single photon emitters operating at room temperature like N-V centers in diamonds or colloidal quantum dots.
We have studied two metallic cavities to control the spontaneous emission, with the objective of simplifying the technology to obtain bright single photon sources and to develop cavities suitable for spectrally broad emitters, operating at room temperature.
Further readings: “Full control of spontaneous emission in confined Tamm optical modes” and “Controlling spontaneous emission with plasmonic optical patch antennas“.