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Cold atom-semiconductor quantum interface

Periodic Reporting for period 1 - 3-5-FIRST (Cold atom-semiconductor quantum interface)

Reporting period: 2016-03-01 to 2018-02-28

The interdisciplinary project 3-5-FIRST aimed at the realization of a atom-semiconductor quantum interface. A group III-V semiconductor quantum dot (QD) is interfaced with an ensemble of atoms from the first group (alkali metal). This innovative approach allows for combining efficient single photon generation in semiconductor QDs with the excellent properties of atomic quantum memories to build a so far missing key element for high-speed quantum networks. Thus, 3-5-FIRST promises a plethora of radically new applications and novel insights. For example, high-speed quantum cryptography networks will be used for unconditional secure communication in metropolitan areas. Memory enhanced quantum computers and simulators will allow for exponential speed-up in solving complex problems. Both applications can be implemented in high-speed quantum networks for which the envisioned storage and retrieval of a single QD photon in a cold atom memory is a major breakthrough.
In 3-5-First important steps towards this ambitious goal were taken. Most relevant are the development of a quantum dot single photon source with temporally shaped emission, spectrally matched to Rb atoms and the development of a Rb quantum memory that combines high efficiency with high bandwidth ( ~1 GHz).
Since the project 3-5-FIRST was initiated in March 2016, we have been working hard to implement the prime objectives and work was performed in both involved disciplines, atomic physics and semiconductor physics.

On the atomic physics side, we have demonstrated a quantum memory in warm Rb vapor with on-demand storage and retrieval, based on electromagnetically induced transparency, and with an acceptance bandwidth of δf = 0.66 GHz. This memory is suitable for single photons emitted by semiconductor quantum dots. In this regime, vapor cell memories offer an excellent compromise between storage efficiency, storage time, noise level, and experimental complexity.
On the semiconductor physics side we have demonstrated a fast, bandwidth-tunable single-photon source based on an epitaxial GaAs quantum dot. Exploiting spontaneous spin-flip Raman transitions, single photons at Rb compatible wavelengths are generated on-demand and with tailored spectral and temporal profiles. This allowed the generation of quantum dot photons with linewidths as low as 0.2 GHz, narrow compared to the 1.1 GHz natural of the quantum dot and well within the acceptance bandwidth of the atomic quantum memory. These experiments are crucial step towards the development of an optimized hybrid semiconductor-atom interface as needed for high-speed quantum networks.
Despite these efforts and successes, the brightness of the developed single photon source is still too small to be combined with the developed atomic quantum memory at its present level of performance. Until brighter quantum dot sources are available, the memory can be tested with single photons generated by an alternative single photon source, namely a source based on cavity-enhanced spontaneous parametric down conversion (SPDC). During a secondment of the ER to Humboldt-Universität Berlin first steps towards the construction of such a source were carried out and first results are expected soon. In parallel, efforts are ongoing to improve the performance of the atomic quantum memory.

Results dissemination

The work on the “simple atomic quantum memory suitable for semiconductor quantum dot single photons” was published in Physical Review Letters 19, 060502 (2017) and discussed by an article in the Basel University News (Uni News) in 9/2017.

The work on the “on-demand semiconductor source of 780 nm single photons with controlled temporal wave packets” has been accepted for publication in Physical Review B and is available as preprint arXiv:1710.02490.

The results were presented at several international conferences and workshops, most prominent are:

1. Presentation – J. Wolters, “Cold atom-semiconductor hybrid quantum system,” Spring meeting of the German physical society, Hannover, February/March 2016.

2. Presentation ¬– J. Wolters, “Towards an atom-semiconductor quantum interface,” SPS Annual Meeting, Lugano, August 2016

3. Presentation – J. Wolters, “Towards an atom-semiconductor quantum interface,” International Conference on Quantum Communication, Measurement and Computing (QCMC), Singapore, July 2016.

4. Presentation – J. Wolters, “Towards an atom-semiconductor quantum interface,” Single Photons Single Spins (SPSS) Meeting, Oxford, September 2016.

5. Presentation – J. Wolters, “Generation of single photons with tailored waveforms using a quantum dot emitting near the Rb D2 line,” Spring meeting of the German physical society, Mainz, March 2017.

6. Presentation – J. Wolters, “An atomic memory suitable for semiconductor quantum dot single photons,” Spring meeting of the German physical society, Mainz, March 2017.

7. Presentation – J. Wolters, “An atomic memory suitable for semiconductor quantum dot single photons,” Spring meeting of the German physical society, Dresden, March 2017.

8. Presentation – J. Wolters, “An atomic memory suitable for semiconductor quantum dot single photons,” CLEO Europe, Munich, June 2017.

9. Presentation – J. Wolters, “Towards storage of quantum dot single photons in a rubidium quantum memory for quantum networks,” Single Photons Single Spins (SPSS) Meeting, Troyes, August/September 2017.
The combination of efficient single photon generation in semiconductor quantum dots with the excellent properties of atomic quantum memories in high-speed quantum networks promises a plethora of radically new applications and novel insights. Prominent examples are quantum cryptography that makes communication unconditional secure and quantum computing and simulation that allows for solving complex problems arising for example in life- and neuroscience.
However, before the project 3-5-FIRST no feasible route towards the realization of the required atom-semiconductor quantum interface was clearly visible. The two main hurdles were a) the immense bandwidth mismatch between atomic transitions and quantum dot transitions by two orders of magnitude; b) the excess noise in atomic quantum memories impeding the storage of single photons. On the atomic physics side, both were overcome by the development of simple atomic quantum memory suitable for semiconductor quantum dot single photons. In parallel, on the semiconductor quantum dot side a new method for the on-demand generation of single photons with controlled temporal wave packets was established to overcome the bandwidth mismatch.
These results pave the way towards the realization of future ultrafast quantum networks based on semiconductor quantum dot single photon sources and atomic quantum memories. Thereby 3-5-FIRST helped to strengthen European leadership in quantum optics. In future the obtained results will help to exploit the potential of quantum science and develop a range of emerging technologies with the potential to benefit the European economies and societies.