Dominique Elser

Naturally phase matched second harmonic generation in a whispering gallery mode resonator

We observed conversion efficiencies of 9% at 30µW pump power in LiNbO<inf>3</inf>, as well as self-limiting effects at high powers. The continuous-wave pump at a wavelength of 1064nm and the second-harmonic feature Q > 10⁷.

Device calibration impacts security of quantum key distribution.

Characterizing the physical channel and calibrating the cryptosystem hardware are prerequisites for establishing a quantum channel for quantum key distribution (QKD). Moreover, an inappropriately implemented calibration routine can open a fatal security loophole. We propose and experimentally demonstrate a method to induce a large temporal detector efficiency mismatch in a commercial QKD system by deceiving a channel length calibration routine. We then devise an optimal and realistic strategy using faked states to break the security of the cryptosystem. A fix for this loophole is also suggested.

Quantum light from a whispering-gallery-mode disk resonator.

Optical parametric down-conversion has proven to be a valuable source of nonclassical light. The process is inherently able to produce twin-beam correlations along with individual intensity squeezing of either parametric beam, when pumped far above threshold. Here, we present for the first time the direct observation of intensity squeezing of -1.2  dB of each of the individual parametric beams in parametric down-conversion by use of a high quality whispering-gallery-mode disk resonator. In addition, we observed twin-beam quantum correlations of -2.7  dB with this cavity. Such resonators feature strong optical confinement and offer tunable coupling to an external optical field. This work exemplifies the potential of crystalline whispering-gallery-mode resonators for the generation of quantum light. The simplicity of this device makes the application of quantum light in various fields highly feasible.

Low-threshold optical parametric oscillations in a whispering gallery mode resonator

Whispering gallery mode (WGM) resonators feature strong optical confinement, small mode volume, and offer tunable coupling to an external optical field. Fabricating WGM resonators from lithium niobate one can take advantage of these properties to achieve very strong optical nonlinear response, e. g. parametric downconversion (PDC). This process offers a highly wavelength tunable light source and is used as a state of the art source for nonclassical light. Driving PDC in cavities, also referred to as an optical parametric oscillator (OPO), provides efficient wavelength conversion. Thus, it is intruiging to investigate PDC in a WGM resonator, although phase matching conditions become involved in spherical geometry. In the process of higher harmonic generation, quasi phase-matching has already been demonstrated in a lithium niobate WGM cavity [1, 2], showing the potential of these resonators. Here we present a highly efficient OPO with a WGM resonator using natural temperature phase matching, where the individual optical fields have narrow optical linewidth [3].

After-gate attack on a quantum cryptosystem

We present a method to control the detection events in quantum key distribution systems that use gated single-photon detectors. We employ bright pulses as faked states, timed to arrive at the avalanche photodiodes outside the activation time. The attack can remain unnoticed, since the faked states do not increase the error rate per se. This allows for an intercept-resend attack, where an eavesdropper transfers her detection events to the legitimate receiver without causing any errors. As a side effect, afterpulses, originating from accumulated charge carriers in the detectors, increase the error rate. We have experimentally tested detectors of the system id3110 (Clavis2) from ID Quantique. We identify the parameter regime in which the attack is feasible despite the side effect. Furthermore, we outline how simple modifications in the implementation can make the device immune to this attack.

Thermal blinding of gated detectors in quantum cryptography.

It has previously been shown that the gated detectors of two commercially available quantum key distribution (QKD) systems are blindable and controllable by an eavesdropper using continuous-wave illumination and short bright trigger pulses, manipulating voltages in the circuit [Nat. Photonics 4, 686 (2010)]. This allows for an attack eavesdropping the full raw and secret key without increasing the quantum bit error rate (QBER). Here we show how thermal effects in detectors under bright illumination can lead to the same outcome. We demonstrate that the detectors in a commercial QKD system Clavis2 can be blinded by heating the avalanche photo diodes (APDs) using bright illumination, so-called thermal blinding. Further, the detectors can be triggered using short bright pulses once they are blind. For systems with pauses between packet transmission such as the plug-and-play systems, thermal inertia enables Eve to apply the bright blinding illumination before eavesdropping, making her more difficult to catch.

Reduction of Guided Acoustic Wave Brillouin Scattering in Photonic Crystal Fibers

Thermally excited transverse phonons in glass fibers generate guided acoustic wave Brillouin scattering (GAWBS) which afflicts the propagating light with phase and polarization noise. This excess noise is a major limitation for fiber-based squeezing sources as well as for the transmission of quantum information through fibers. In order to achieve quantum states of high quality it is therefore important to reduce the harmful effect of GAWBS. The thermal nature of GAWBS naturally allows for a reduction by cooling the fiber. Alternatively, subtracting the correlated noise of two closely spaced pulses leads to a cancellation. These methods, however, are inconvenient for fiber-based squeezing sources and inapplicable in practical quantum information transmission systems.

Entangling different degrees of freedom by quadrature squeezing cylindrically polarized modes.

C. Gabriel1,2,∗, A. Aiello1,2, W. Zhong1,2, T. G. Euser1, N.Y. Joly2,1, P. Banzer1,2, M. Förtsch1,2, D. Elser1,2, U. L. Andersen1,2,3, Ch. Marquardt1,2, P. St.J. Russell1,2 and G. Leuchs1,2 1 Max Planck Institute for the Science of Light, Guenther-Scharowsky-Str. 1, D-91058 Erlangen, Germany 2 Institute for Optics, Information and Photonics, University Erlangen-Nuremberg, Staudtstr. 7/B2, D-91058 Erlangen, Germany 3 Department of Physics, Technical University of Denmark, 2800 Kongens Lyngby, Denmark ∗ Christian.Gabriel@mpl.mpg.de

Feasibility of free space quantum key distribution with coherent polarization states

Free space QKD over an atmospheric channel was demonstrated in 1996 for the first time [1]. Since then, several prepare-and-measure and entanglement-based schemes have been implemented (for a detailed overview, see [2]). All of these systems use single-photon detectors, and therefore have to employ spatial, spectral and/or temporal filtering in order to reduce background light. In our system, we use an alternative approach: with the help of a bright local oscillator (LO), we perform homodyne measurements on weak coherent polarization states [3].

Avoiding the blinding attack in QKD

This is a reply to the comment by Yuan et al. [arXiv:1009.6130v1] on our publication [arXiv:1008.4593].