Optical Quantum Information: Narrow-band Single Photons and Photon Storage
Towards Quantum Repeaters
Conversion of Single and Entangled Photons and Photon Storage
With optical quantum communication unconditionally secure transmission of information is possible in principle. Although there are many successful experiments in the lab and even commercial products for quantum key distribution, the maximum distance for secure communication over an optical fiber is limited to several 100 km by losses. To overcome this limitation Briegel, Dür, Cirac and Zoller  proposed the concept of a quantum repeater.
The prerequisite for a working quantum repeater is an extensive degree of control over the (quantum-) optical system which includes:
- generation of single photons with well defined spectro-temporal characteristics
- detection of single photons
- entanglement of photon pairs
- swapping of entanglement from one physical system to another with high fidelity
- transmission of single photons with low loss
- storage of photons (quantum memory)
All of these processes have to be very efficient. Within the joint project QuaHl-Rep (semiconductor quantum repeater platforms) funded by the Federal Ministry for Education and Research (BMBF) our current research tackles several of these issues:
Single Photon Sources
Recently we studied the generation of heralded single photons in a parametric downconversion process. We built an optical parametric oscillator (OPO) which produces narrow-band (MHz) single photon pairs at a high rate . The OPO is also suitable as a source of entangled photon pairs.
FIG. 1:OPO setup for a narrow-band single photon source .
We will set up a new downconversion source with a larger bandwidth (several 100 MHz) to generate photons which are indistinguishable from single photons generated by quantum dots. With such a source one can swap the photon ↔ photon entanglement for photon ↔ quantum dot entanglement or in a further step entangle two quantum dots.
Other sources of single photons which are studied in our group are nitrogen vacancies in nano diamonds. These sources are very bright even at room temperature without bleaching or blinking and emit single photons at 640 nm. To use these photons for quantum communication we want to coherently convert them to the telecommunication wavelength of 1550 nm using either a waveguide structure  or a downconversion setup similar to our optic parametric oscillator.
FIG. 2: Conversion of single photons to telecom wavelength in a nonlinear crystal.
A crucial step for quantum communication and especially for communication via a quantum repeater is the storage of the quantum information carried by a photon. A device capable of this kind of photon storage is often called a quantum memory.
A quantum memory writes the information of an incoming photon to a stationary qubit. At a later point in time this information can be retrieved by sending out a new photon on demand. (Therefore a quantum memory can act as a single photon source, too.)
Today there are many different approaches to quantum memories, each of them having it's own advantages and drawbacks. (See for a review ) The most important parameters are the maximum storage time, the efficiency (probability for a full cycle of storage and retrieval without photon loss) and the fidelity (overlap of the wave functions of incoming and outgoing photons) of the memory. If one wants to interface a quantum memory with a given quantum information setup the memory has also to be matched with the frequency and bandwidth of the photons to be stored.
In our current research (See for example ) we focus on quantum memories where the information of an incoming photon is stored in a collective excitation of atoms in alkali-gases. This approach is advantageous since the atoms can be held at room temperature and there is no need for a sophisticated UHV setup. With this kind of memory we want to store photons generated by our downconversion source or by quantum dots.
 H.-J. Briegel, W. Dür, J.I. Cirac, P. Zoller: Quantum Repeaters: The Role of Imperfect Local Operations in Quantum Communication, Phys. Rev. Lett., 81, no. 26, 5932 (1998)
 M. Scholz, L. Koch, and O. Benson: Statistics of Narrow-Band Single Photons for Quantum Memories Generated by Ultrabright Cavity-Enhanced Parametric Down-Conversion, Phys. Rev. Lett. 102, no. 6 (2009)
 C. Simon, et al.: Quantum Memories, Eur. Phys. J. D 58, 1 (2010)
 S. Zaske, A. Lenhard, and C. Becher: Efficient frequecy downconverion at the single photon level from the red spectral range to the telecommunications C-band, Opt. Express, 19, 12825 (2011)
 D. Höckel and O. Benson: Electromagnetically induced transparency in cesium vapor with probe pulses on the single photon level, Phys. Rev. Lett. 105, 153605 (2010)