We always have interesting MSc/BSc/internship projects for enthusiastic students, we always show cutting-edge experiments to teachers, and see opportunities!

New MSc programme Leiden & Delft QIST - Quantum Information Science and Technology!

(Fundamental) research and education is at jeopardy in the Netherlands - at NWO Physics 2025 we organize a focus session “The value of fundamental research”, click here for more information!

On December 10, the quantum light source from the TU Berlin that makes a tour through Europe arrived in Leiden! As the only stop in the Netherlands, a world-class source of single photons will be investigated in the Leiden labs. The source is sent through Europe connecting many researchers, and is measured in more than 12 laboratories. The journey started from Germany to Austria, Italy, Switzerland, Spain, France, England, Ireland, Scotland, before arriving in the Netherlands. In 2025 it will go to Denmark, Sweden, and Poland. In this week, we will explain several aspects of the source, show how we measure the single photons emitted by the source, and some related topics. Like the olympic torch relay, QuanTour will prepare us for the International Year of Quantum Science and Technology 2025!

Links: http://quantour.eu, https://thesciencetalk.com/quantour and https://quantum2025.org/en/

Wolfgang's podcast episode on Under the Microscope with Pranoti: https://www.youtube.com/watch?v=ak9WHrumboI

An article on the Leiden university website: NL and EN

Matteo got his PhD with his thesis “Cavities for light and sound - A cavity-enhanced platform for quantum acoustics”, congratulations!

Petr Steindl very successfully has defended his thesis “Quantum dots in microcavities: From single spins to engineered quantum states of light” - congratulations! Here is an interview with Petr over his work from Leiden University: link.

Light consists of single photons, and the most classical type of light, coherent laser light, actually consists of a large mix of so-called photon number states - packets containing a certain number of photons. This fact makes it very hard to transform laser light into single photons, which are an important resource for photonic quantum information from quantum key distribution to quantum computing. High-quality sources of single photons were only recently demonstrated by several groups including us. In this paper in PRL, we demonstrate that we can turn it around: based on such a high-quality source of single photons, and quantum interference in an optical setup, we are able to engineer artificial states of light that are very similar to coherent laser light, literally, we are making “light from scratch”! There are subtle differences between textbook coherent states and our artificial coherent states, for instance, our artificial light contains some quantum entanglement between photons at different times, in fact it contains so-called photonic cluster states that are also very useful for quantum information, see QLUSTER. In the figure we compare the Wigner functions where quantum entanglement shows up as negative values (the white curves indicate a value of zero). Apart from that, we see that the overlap with perfect coherent states is pretty good!

Phys. Rev. Lett. 126, 143601 (2021), Arxiv:2010.15471

We are happy to announce the start of the ERC-funded QLUSTER project, where we aim to use semiconductor quantum dots in micro-cavities to produce many quantum-entangled photons in form of cluster and graph states!

Read more on https://qluster.eu

All light that we see consists of photons, however, single photons itself show fascinating different properties that enable, for instance, 100% secure communications in quantum cryptography or superresolution in microscopy. But making single photons is not an easy task. In one approach, the “photon blockade” effect, a conventional laser beam is sent to an optical cavity with a single atom (or an artificial atom aka quantum dot). Quantum effects in this device make it possible that only one photon at a time exits the device, like a turnstile for single photons. This device, however, requires very special properties and is extremely hard to fabricate. Now, we have confirmed for the first time experimentally that this can be done much easier: by using the “unconventional photon blockade” effect. This was theoretically conceived by our co-authors Vincenzo Savona and Hugo Flayac from the EPFL Lausanne. In short, we exploit the polarization property of the photons, and by cleverly using quantum interference of different polarizations, we obtain the same as in the photon blockade: a nice stream of single photons. The “unconventional photon blockade” might be very useful for future single photon sources and gives insight in the exciting photon number dependent physics of such devices. Editors suggestion: A Double Take on Unconventional Photon Blockade", Article: Phys. Rev. Lett. 121, 043601 (2018), pre-print Arxiv:1803.10992.

Cyril Vaneph and colleagues at the University of Paris-Sud have at the same time demonstrated the effect in the microwave regime.

An ordered stream of single photons is fundamentally different from conventional light which contains bunches of a random number of photons. Sources of such single-photon light are essential for emerging quantum technologies such as quantum cryptography or computing, but widespread use of recently developed bright semiconductor quantum-dot based single-photon sources was hindered by the need for complex optical setups. Here we show a fiber-integrated source of high-quality single photons; integration with conventional optical fiber technology will enable broad use in quantum photonics but it also might enable a number of new fundamental studies in various fields from microscopy to quantum metrology by reducing the experimental complexity significantly.

Phys. Rev. Applied (Letter) 9, 031002 (2018).

With a single semiconductor quantum dot in a polarization degenerate microcavity, operating in the weak coupling regime, we can transform incident coherent light into a stream of strongly correlated photons with a second-order correlation function g2(0) up to 40! This work is published in Nat. Commun. 7, 12578 (2016).

See cqed for more details.

Our work on the first observation of 4-photon orbital angular momentum entanglement is published in PRL as an Editor's Suggestion. The article is accompanied by a Physics synopsis.

See 4photonoam for more details.

Polarization vortices are singular points in generic light fields, different but closely connected to phase vortices. Their singular nature makes them ideal as a positional marker of light beams, we investigated this by studying how they behave while being reflected at an interface. Apart from the scientific article here that is on the cover of issue 8, you can also find a labtalk article with a bit more background, and see here for more information.

Loosely connected to our workshops on spin-orbit coupling we were asked to make a special issue on beam shifts in the Journal of Optics, as part of their nice special issues collection, see here.

The spatial structure of light can be either pure or in an incoherent superposition, a statistical mixture. A laserpointer reflected from a rough surface shows “speckle”, and if you move the surface very quickly these speckle appear washed out; this is light with reduced spatial coherence. We studied how beam shifts depend on this by theory and experiment, click here for more information.

“Spin-Orbit Interaction for Light and Matter Waves” is a workshop which we have organized at the MPIPKS in Dresden (germany) from 15.-19. April 2013. More information here. The workshop was a great success!

In April 2011, we had arranged a workshop on beam shift phenomena and spin-orbit coupling in optics at the Lorentz Center in Leiden. Click here for further information.