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Humboldt-Universität zu Berlin - Mathematisch-Naturwissen­schaft­liche Fakultät - Nanooptik

Optical Quantum Information


Staff members:  Matthias Leifgen, Jasper Rödiger, Tim Kroh, Andreas AhlrichsChris Müller


Fundings through:



BMBF Q.com “Quanten-Repeater-Plattform mit Methoden der Quantenoptik und auf Basis von Halbleitern“

BMBF KEPHOSI Project “Kompakte Einzelphotonquellen im Sichtbaren“


SFB 787


Collaborative Research Centre (CRC) 787 "Semiconductor Nanophotonics: Materials, Models, Devices"


1. Quantum Dots as Single Photon Sources


Semiconductor quantum dots are widely investigated as single photon sources for quantum information networks. These so called “artificial atoms” may be used as quantum interfaces between stationary and flying qubits.

We investigate the optical properties of quantum dot samples in a Helium liquid-flow cryostat. The samples are fully characterized in a confocal high NA microscope setup using different excitation schemes to determine the photon statistics, coherence time of the emitted photons, and the lifetime of different excitonic states in the quantum dot (Fig. 1) [1].


Figure 1: Implemented characterization setup for single photon emission from semiconductor quantum dots. Non-resonant cw and pulsed as well as resonant cw excitation sources are available. A Michelson interferometer and a Hanbury Brown - Twiss correlator are used to determine the amplitude and intensity correlation functions, g(1)(t) and g(2)(t), respectively.


Different tuning mechanisms such as DC or AC electrical fields or strain in the semiconductor lattice are investigated to prepare the stabilization of single quantum dots to the atomic Cesium D1 transition. Resonant photon scattering at the two-level system of a quantum dot exciton allows us to accurately set the quantum dot emission to one of the hyperfine-split Cesium D1 transitions (Fig. 2).


Figure 2: Left: Detailed scheme of the resonant excitation. Right: A quantum dot emission line (colored spectra) is tuned by using a piezoelectric material to induce strain into the semiconductor lattice. Such a sample can be used to lock the quantum dot single photon emission to the atomic standard of the Cesium D1 transition. The thin blue line is the measured absorption spectrum of the hyperfine-split states that are used in atomic Cesium clocks.


[1] M. Benyoucef, V. Zuerbig, J. P. Reithmaier, T. Kroh, A. W. Schell, T. Aichele, and O. Benson; “Single-photon emission from single InGaAs/GaAs quantum dots grown by droplet epitaxy at high substrate temperature”; Nanoscale. Res. Lett. 7, 493 (2012)



Tim Kroh


2. A Tunable Entangled Photon Pair Source as Quantum Interface


Today many integrated quantum optical devices work at visible to near infrared wavelengths. For example, most of the quantum dot based setups are restricted to wavelength below 1000nm However, quantum communication between such devices is best performed at Telecom wavelength around 1.3 mu. Flexible photon pair sources that emit one photon at each of these two wavelength can bridge this gap.


Figure 1:  Entangled photon pair source in the modified folded sandwich configuration. The Fresnel rhomb acts as a geometrical quarter-wave plate. The concave mirror (CM) reflects the light back into the crystal. The first dichroic mirror (DM1) separates pump and down-converted photons.  Several YVO 4 slabs are used to control the dispersion for a broad range of wavelengths. Finaly, the second dichroic mirror (DM2) separates signal and idler photons.


In quantum communication  usually entangled photons are used to transfer information between two locations.  It is currently possible to build entangled photon sources but usually they are designed for a special pair of wavelengths. Especially quantum dots vary considerably in their emitting wavelength. This is why we want to eliminate the wavelength dependence of our entangled photon source. We are building an entangled photon source in a folded sandwich configuration [1] with several enhancements (see Fig. 1). We  exchange the wavelength dependent components of the setup with geometric components, which are wavelength independent.  A wavelength independent setup allows us to create entangled photon with different wavelengths. To create the entangled photons we are using spontaneous parametric down conversion. The non-linear crystal in this setup is tunable over a broad spectrum.  That means that our setup enables a quantum communication between setups with different wavelength.

Furthermore, to enable fast and flexible experiments with our collaborators,we designed our setup to be portable.


[1] F. Steinlechner et al., Phase-stable source of polarization-entangled photons in a

linear double-pass configuration, Optics Express, 21 11943 (2013)


Contact: Chris Müller


3. Narrow-Band Single Photon Source


Many applications in quantum optics require the generation of single photons with well-defined frequency and bandwidth. We are using an optical parametric oscillator (Fig. 1) pumped far below the threshold as an ideal tool to tune the frequency and bandwidth of photons generated by spontaneous parametric down-conversion [1,2].





Figure 1: Left:  Setup of the optical parametric oscillator. Signal and idler photons are generated by spontaneous parametric down-conversion in a periodically poled KTP crystal (PPKTP) which is placed inside an optical resonator. Right: Comb-like spectral structure of the generated photons. The width of the resonances can be tuned by changing the mirror reflectivities.


To obtain only photons with the desired frequency, spectral filtering of the generated photons is required. Therefore we developed a fiber-coupled two-stage filter system (Fig. 1) based on monolithic Fabry-Perot cavities [3]. These are extremely stable, polarization independent and can conveniently be tuned by changing the substrate temperature.


Figure 2: Filter system based on monolithic Fabry-Perot cavities with high transmission. By combining two resonators with a different free spectral ranges a band-pass filter with an effective free spectral range of more than 400 GHz and a line-width of about 200 MHz can be realized.


[1] M. Scholz, L. Koch, and O. Benson, Phys. Rev. Lett. 102, 063603 (2009).

[2] M. Wahl, T. Röhlicke, H.-J. Rahn, R. Erdmann, G. Kell, A. Ahlrichs, M. Kernbach, A.W. Schell, and O. Benson, Review of Scientific Instruments 84, 043102 (2013).

[3] A. Ahlrichs, C. Berkemeier, B. Sprenger, and O. Benson, Applied Physics Letters 103, 241110 (2013).


Contact: Andreas Ahlrichs


4. Conversion of Single Photons


To combine the advantages of entirely different quantum systems – such as quantum dots, atoms, and photon pair sources – it can be important to make them interact by the exchange of photons. Furthermore, long distance quantum communication works most efficiently in the telecommunications wavelength range of about 1550 nm.

Inter-conversion of wavelengths can bridge these gaps. We use a nonlinear crystal to perform Difference Frequency Generation in a periodically-poled lithium niobate waveguide. This technique has been shown to be very efficient at converting single photons from the near-infrared to the telecom range, while preserving the quantum properties [1].


Fig. 1: Periodically-poled lithium niobate waveguide. Single photons at 894 nm and a strong pump laser at 2100 nm couple into the waveguide on the left, and single 1557 nm photons can be extracted on the right.

Fig. 2: Blue dots: coincidences between photons from the optical parametric oscillator (see section above). Red dots: coincidences between one 894 nm photon and one that has been converted to 1557 nm. Time-correlation is preserved.

[1] S. Zaske et al., Phys. Rev. Lett. 109, 147404 (2012).


Contact:                  Andreas AhlrichsTim KrohChris Müller


5. Quantum Key Distribution


In QKD, two parties, usually called Alice and Bob, want to communicate in a secured way. To do this, they both have to possess a key, which is used by Alice to encrypt a message and by Bob to decrypt it afterwards. To exchange their key, they can use quantum systems, e.g. single photons and exploit the rules of quantum mechanics to ensure secure communication, explicitly the two following rules: a quantum system cannot be measured without perturbing it and a quantum system cannot be copied with arbitrarily high accuracy. A potential eavesdropper (generally called Eve) who tries to pick up the bit sent to Bob will inevitably introduce errors while measuring the system, thus can be detected. If such errors reveal the presence of Eve, Alice and Bob will reject the key and restart communication until a secure key is transmitted (figure 1).


Figure 1: Principle of quantum cryptography in the BB84 protocol with polarization encoding


Today, quantum key distribution (QKD) is the only safe way to communicate secretly, secured by the laws of quantum physics. On the one hand it is already a mature field of research, on the other hand, there is still need for improvement when it comes to transmission rates and distances. Because it is an important application in the field of quantum technology, it is interesting to investigate new light sources and protocols for QKD.

We have used defect centres as single photon sources for a BB84 QKD experiment and evaluated their utility in comparison with other light sources [1]. For this experiment, a post-processing algorithm based on CASCADE  was developed (https://github.com/rriemann/privacy-amplification/releases).


Figure 2: The setup of our QKD experiment. A single photon source (SPS) in the form of a confocal setup which is able to host different defect centres emits photons which are modulated in polarization with an electro optic modulator (EOM) on Alice’s side. On Bob’s side the polarization of the photons is measured with a polarizing beam splitter (PBS) and two avalanche photodiodes (APDs). The basis choice on Bob’s side is implemented with an EOM.


As a byproduct of our research, a quantum random number generator (QRNG) was developed and tested in cooperation with PicoQuant (http://www.picoquant.com) [2].

Currently, a new kind of QKD-protocol is under investigation, where the quantum information is encoded in arrival time and frequency of a photon and the security comes from the time-frequency uncertainty relation. This protocol is especially well suited for free space links, thus a 500m testbed is under development at the moment. The project is conducted in cooperation with the Fraunhofer Heinrich Hertz Institute.


[1] M. Leifgen et al., New J. Phys. 16, 023021 (2014)

[2] M. Wahl et al., Appl. Phys. Lett. 98, 171105 (2011)


Contact:                  Matthias Leifgen, Jasper Rödiger