We demonstrate efficient (>30%) quantum frequency conversion of visible single photons (711 nm) emitted by a quantum dot to a telecom wavelength (1313 nm). Analysis of the first- and second-order coherence before and after wavelength conversion clearly proves that pivotal properties, such as the coherence time and photon antibunching, are fully conserved during the frequency translation process. Our findings underline the great potential of single photon sources on demand in combination with quantum frequency conversion as a promising technique that may pave the way for a number of new applications in quantum technology.

Abstract. 232 hours of data were accumulated from 1997 to 1999, using the HEGRA Stereoscopic Cherenkov Telescope System to observe the supernova remnant Cassiopeia A. TeV γ-ray emission was detected at the 5 σ level, and a flux of (5.8 ± 1.2stat ± 1.2syst) 10 −9 ph m −2 s −1 above 1 TeV was derived. The spectral distribution is consistent with a power law with a differential spectral index of −2.5 ± 0.4stat ± 0.1syst between 1 and 10 TeV. As this is the first report of the detection of a TeV γ-ray source on the "centi-Crab" scale, we present the analysis in some detail. Implications for the acceleration of cosmic rays depend on the details of the source modeling. We discuss some important aspects in this paper.

Abstract. Deep observation (∼113 hrs) of the Cygnus region at TeV energies using the HEGRA stereoscopic system of aiř Cerenkov telescopes has serendipitously revealed a signal positionally inside the core of the OB association Cygnus OB2, at the edge of the 95% error circle of the EGRET source 3EG J2033+4118, and ∼0.5• north of Cyg X-3. The source centre of gravity is RA α J2000 : 20 hr 32 m 07 s ± 9.2 s stat ± 2.2 s sys , Dec δ J2000 : +41• 30 30 ± 2.0 stat ± 0.4 sys . The source is steady, has a post-trial significance of +4.6σ, indication for extension with radius 5.6 at the ∼3σ level, and has a differential power-law flux with hard photon index of −1.9 ± 0.3 stat ± 0.3 sys . The integral flux above 1 TeV amounts ∼3% that of the Crab. No counterpart for the TeV source at other wavelengths is presently identified, and its extension would disfavour an exclusive pulsar or AGN origin. If associated with Cygnus OB2, this dense concentration of young, massive stars provides an environment conducive to multi-TeV particle acceleration and likely subsequent interaction with a nearby gas cloud. Alternatively, one could envisage γ-ray production via a jet-driven termination shock.

We demonstrate pronounced single-photon emission from InAs/AlGaInAs/InP quantum dots (QDs) at wavelengths above 1.5 μm that are compatible with standard long-distance fiber communication. The QDs are grown by molecular beam epitaxy on distributed Bragg reflectors. A low QD density of about 5 × 108 cm−2 was obtained using optimized growth conditions. Low-temperature micro-photoluminescence spectroscopy exhibits sharp excitonic emission lines from single QDs without the necessity of further processing steps. The combination of excitation power-dependent and polarization-resolved photoluminescence measurements reveal a characteristic exciton-biexciton behavior with biexciton binding energies that range from 3.5 to 4 meV and fine-structure splitting values down to 20 μeV.

By metal-organic vapor-phase epitaxy, we have fabricated InAs quantum dots (QDs) on InGaAs/GaAs metamorphic buffer layers on a GaAs substrate with area densities that allow addressing single quantum dots. The photoluminescence emission from the quantum dots is shifted to the telecom C-band at 1.55 μm with a high yield due to the reduced stress in the quantum dots. The lowered residual strain at the surface of the metamorphic buffer layer results in a reduced lattice mismatch between the quantum dot material and growth surface. The quantum dots exhibit resolution-limited linewidths (mean value: 59 μeV) and low fine-structure splittings. Furthermore, we demonstrate single-photon emission (g(2)(0)=0.003) at 1.55 μm and decay times on the order of 1.4 ns comparable to InAs QDs directly deposited on GaAs substrates. Our results suggest that these quantum dots can not only compete with their counterparts deposited on InP substrates but also constitute an InAs/GaAs-only approach for the development of non-classical light sources in the telecom C-band.

Efficient fiber-based long-distance quantum communication via quantum repeaters relies on deterministic single-photon sources at telecom wavelengths, with the potential to exploit the existing world-wide infrastructures. For upscaling the experimental complexity in quantum networking, two-photon interference (TPI) of remote non-classical emitters in the low-loss telecom bands is of utmost importance. With respect to TPI of distinct emitters, several experiments have been conducted, e.g., using trapped atoms [1], ions [2], NV-centers [3, 4], SiV-centers [5], organic molecules[6] and semiconductor quantum dots (QDs) [7][8][9][10][11][12][13][14]; however, the spectral range was far from the highly desirable telecom C-band. Here, we report on TPI at 1550 nm between down-converted single photons from remote QDs [15], demonstrating quantum frequency conversion [16][17][18] as precise and stable mechanism to erase the frequency difference between independent emitters. On resonance, a TPI-visibility of (29 ± 3) % has been observed, being only limited by spectral diffusion processes of the individual QDs [19,20]. Up to 2-km of additional fiber channel has been introduced in both or individual signal paths with no influence on TPI-visibility, proving negligible photon wave-packet distortion. The present experiment is conducted within a local fiber network covering several rooms between two floors of the building. Our studies pave the way to establish long-distance entanglement distribution between remote solid-state emitters including interfaces with various quantum hybrid systems [21][22][23][24].

(MJD 51989-51991, March 21-23, 2001), this correlation of spectral hardness and change in flux has been observed within a few hours. The cut-off energy for the Mkn 421 TeV spectrum remains within the errors constant for the different flux levels and differs by ∆E = 2.6 ± 0.6 stat ± 0.6 sys TeV from the value determined for Mkn 501. This indicates that the observed exponential cut-off in the energy spectrum of Mkn 421 is not solely caused by absorption of multi-TeV photons by pair-production processes with photons of the extragalactic near/mid infrared background radiation.

In this paper we present the results of simultaneous observations of the TeV blazar Markarian 421 (Mrk 421) at X-ray and TeV gamma-ray energies with the Rossi X-Ray Timing Explorer (RXTE) and the stereoscopic Cherenkov Telescope system of the HEGRA (High Energy Gamma Ray Astronomy) experiment, respectively. The source was monitored from February 2nd to February 16th and from