Lars S. Madsen

Continuous variable quantum key distribution with modulated entangled states

Quantum key distribution enables two remote parties to grow a shared key, which they can use for unconditionally secure communication over a certain distance. The maximal distance depends on the loss and the excess noise of the connecting quantum channel. Several quantum key distribution schemes based on coherent states and continuous variable measurements are resilient to high loss in the channel, but are strongly affected by small amounts of channel excess noise. Here we propose and experimentally address a continuous variable quantum key distribution protocol that uses modulated fragile entangled states of light to greatly enhance the robustness to channel noise. We experimentally demonstrate that the resulting quantum key distribution protocol can tolerate more noise than the benchmark set by the ideal continuous variable coherent state protocol. Our scheme represents a very promising avenue for extending the distance for which secure communication is possible.

Experimental investigation of the evolution of gaussian quantum discord in an open system.

Gaussian quantum discord is a measure of quantum correlations in gaussian systems. Using gaussian discord, we quantify the quantum correlations of a bipartite entangled state and a separable two-mode mixture of coherent states. We experimentally analyze the effect of noise addition and dissipation on gaussian discord and show that the former noise degrades the discord, while the latter noise for some states leads to an increase of the discord. In particular, we experimentally demonstrate the near death of discord by noisy evolution and its revival through dissipation.

Quantum-enhanced micromechanical displacement sensitivity

We report on a hitherto unexplored application of squeezed light: for quantum-enhancement of mechanical transduction sensitivity in microcavity optomechanics. Using a toroidal silica microcavity, we experimentally demonstrate measurement of the transduced phase modulation signal in the frequency range 4-5.8 MHz with a sensitivity -0.72(±0.01) dB below the shot noise level. This is achieved for resonant probing in the highly undercoupled regime, by preparing the probe in a weak coherent state with phase squeezed vacuum states at sideband frequencies.

Assessing the polarization of a quantum field from stokes fluctuations.

We propose an operational degree of polarization in terms of the variance of the Stokes vector minimized over all the directions of the Poincaré sphere. We examine the properties of this second-order definition and carry out its experimental determination. Quantum states with the same standard (first-order) degree of polarization are correctly discriminated by this new measure. We argue that a comprehensive quantum characterization of polarization properties requires a whole hierarchy of higher-order degrees.

Gaussian error correction of quantum states in a correlated noisy channel.

Noise is the main obstacle for the realization of fault-tolerant quantum information processing and secure communication over long distances. In this work, we propose a communication protocol relying on simple linear optics that optimally protects quantum states from non-Markovian or correlated noise. We implement the protocol experimentally and demonstrate the near-ideal protection of coherent and entangled states in an extremely noisy channel. Since all real-life channels are exhibiting pronounced non-Markovian behavior, the proposed protocol will have immediate implications in improving the performance of various quantum information protocols.

A quantum optomechanical interface beyond the resolved sideband limit

Mechanical oscillators which respond to radiation pressure are a promising means of transferring quantum information between light and matter. Optical--mechanical state swaps are a key operation in this setting. Existing proposals for optomechanical state swap interfaces are only effective in the resolved sideband limit. Here, we show that it is possible to fully and deterministically exchange mechanical and optical states outside of this limit, in the common case that the cavity linewidth is larger than the mechanical resonance frequency. This high-bandwidth interface opens up a significantly larger region of optomechanical parameter space, allowing generation of non-classical motional states of high-quality, low-frequency mechanical oscillators.

Coherent control and feedback cooling in a remotely coupled hybrid atom–optomechanical system

Cooling to the motional ground state is an important first step in the preparation of nonclassical states of mesoscopic mechanical oscillators. Light-mediated coupling to a remote atomic ensemble has been proposed as a method to reach the ground state for low frequency oscillators. The ground state can also be reached using optical measurement followed by feedback control. Here we investigate the possibility of enhanced cooling by combining these two approaches. The combination, in general, outperforms either individual technique, though atomic ensemble-based cooling and feedback cooling each individually dominate over large regions of parameter space.

Parsing polarization squeezing into Fock layers

C. R. Müller,1, 2, 3 L. S. Madsen,3 A. Klimov,4 L. L. Sánchez-Soto,5, 1, 2 G. Leuchs,1, 2 Ch. Marquardt,1, 2, 3 and U. L. Andersen3, 1 1Max-Planck-Institut für die Physik des Lichts, Günther-Scharowsky-Straße 1, Bau 24, 91058 Erlangen, Germany 2Institut für Optik, Information und Photonik, Universität Erlangen-Nürnberg, Staudtstraße 7/B2, 91058 Erlangen, Germany 3Department of Physics, Technical University of Denmark, Fysikvej, 2800 Kgs. Lyngby, Denmark 4Departamento de Fı́sica, Universidad de Guadalajara, 44420 Guadalajara, Jalisco, Mexico 5Departamento de Óptica, Facultad de Fı́sica, Universidad Complutense, 28040 Madrid, Spain

Experimental determination of the degree of polarization of quantum states

We demonstrate experimental excitation-manifold-resolved polarization characterization of quantum states of light ranging from the few-photon to the many-photon level. In contrast to the traditional characterization of polarization that is based on the Stokes parameters, we experimentally determine the Stokes vector of each excitation manifold separately. Only for states with a given photon number do the methods coincide. For states with an indeterminate photon number, for example Gaussian states, the employed method gives a richer and more accurate description. We apply the method both in theory and in experiment to some common states to demonstrate its advantages.

Complete elimination of information leakage in continuous-variable quantum communication channels

In all lossy communication channels realized to date, information is inevitably leaked to a potential eavesdropper. Here we present a communication protocol that does not allow for any information leakage to a potential eavesdropper in a purely lossy channel. By encoding information into a restricted Gaussian alphabet of squeezed states we show, both theoretically and experimentally, that the Holevo information between the eavesdropper and the intended recipient can be exactly zero in a purely lossy channel while minimized in a noisy channel. This result is of fundamental interest, but might also have practical implications in extending the distance of secure quantum key distribution.Quantum communication: no information for the eavesdroppersA new study demonstrates that information leakage from purely lossy quantum channels can be prevented without the need for data post-processing. Christian Jacobsen and colleagues from the Technical University of Denmark and the Palacky University of Czech Republic have shown that communicating quantum information encoded into the squeezed states of a restricted Gaussian alphabet decouples any potential eavesdropper from the communication. The authors experimentally demonstrate complete elimination of information leakage for photonic continuous-variable quantum-key distribution using squeezed states of light and homodyne detection. Through this scheme, security is achieved without the need of burdensome data post-processing such as privacy amplification – common to standard quantum-key distribution protocols. In the presence of noise, information leakage cannot be entirely eliminated, but can still be efficiently minimised. These findings will be important to future developments in secure quantum communications.