Showing papers by "Christoph Marquardt published in 2017"

High-dimensional intracity quantum cryptography with structured photons

Abstract: Quantum key distribution (QKD) promises information-theoretically secure communication and is already on the verge of commercialization. The next step will be to implement high-dimensional protocols in order to improve noise resistance and increase the data rate. Hitherto, no experimental verification of high-dimensional QKD in the single-photon regime has been conducted outside of the laboratory. Here, we report the realization of such a single-photon QKD system in a turbulent free-space link of 0.3 km over the city of Ottawa, taking advantage of both the spin and orbital angular momentum photonic degrees of freedom. This combination of optical angular momenta allows us to create a 4-dimensional quantum state; wherein, using a high-dimensional BB84 protocol, a quantum bit error rate of 11% was attained with a corresponding secret key rate of 0.65 bits per sifted photon. In comparison, an error rate of 5% with a secret key rate of 0.43 bits per sifted photon is achieved for the case of 2-dimensional structured photons. We thus demonstrate that, even through moderate turbulence without active wavefront correction, high-dimensional photon states are advantageous for securely transmitting more information. This opens the way for intracity high-dimensional quantum communications under realistic conditions.

Quantum-limited measurements of optical signals from a geostationary satellite

Abstract: The measurement of quantum signals that travel through long distances is fundamentally and technologically interesting. We present quantum-limited coherent measurements of optical signals that are sent from a satellite in geostationary Earth orbit to an optical ground station. We bound the excess noise that the quantum states could have acquired after having propagated 38,600 km through Earth’s gravitational potential, as well as its turbulent atmosphere. Our results indicate that quantum communication is feasible, in principle, in such a scenario, highlighting the possibility of a global quantum key distribution network for secure communication.

Space QUEST mission proposal: Experimentally testing decoherence due to gravity

Abstract: Models of quantum systems on curved space-times lack sufficient experimental verification. Some speculative theories suggest that quantum properties, such as entanglement, may exhibit entirely different behavior to purely classical systems. By measuring this effect or lack thereof, we can test the hypotheses behind several such models. For instance, as predicted by Ralph and coworkers [T C Ralph, G J Milburn, and T Downes, Phys. Rev. A, 79(2):22121, 2009, T C Ralph and J Pienaar, New Journal of Physics, 16(8):85008, 2014], a bipartite entangled system could decohere if each particle traversed through a different gravitational field gradient. We propose to study this effect in a ground to space uplink scenario. We extend the above theoretical predictions of Ralph and coworkers and discuss the scientific consequences of detecting/failing to detect the predicted gravitational decoherence. We present a detailed mission design of the European Space Agency's (ESA) Space QUEST (Space - Quantum Entanglement Space Test) mission, and study the feasibility of the mission schema.

Temporal shaping of single photons enabled by entanglement

Abstract: We present a method to produce pure single photons with an arbitrary designed temporal shape in a heralded way. As an indispensable resource, the method uses pairs of time-energy entangled photons. One photon of a pair undergoes temporal amplitude-phase modulation according to the desired shape. Subsequent frequency-resolved detection of the modulated photon heralds its entangled counterpart in a pure quantum state. The temporal shape of the heralded photon is indirectly affected by the modulation in the heralding arm. We derive conditions for which the shape of the heralded photon is given by the modulation function. The method can be implemented with various sources of time-energy entangled photons. In particular, using entangled photons from parametric down-conversion the method provides a simple means to generate pure shaped photons with an unprecedented broad range of temporal durations, from tenths of femtoseconds to microseconds. This shaping of single photons will push forward the implementation of scalable multidimensional quantum information protocols, efficient photon-matter coupling, and quantum control at the level of single quanta.

Polarization-Selective Out-Coupling of Whispering-Gallery Modes

Abstract: Nonlinear (and in particular quantum) optical experiments performed in whispering-gallery-mode resonators could be considerably enhanced by the ability to independently tune the coupling strength to pump and signal modes. The authors demonstrate such a scheme, exploiting birefringence in either the coupling prism or the resonator. Both the refined theory for coupling and the technique presented in this manuscript bear the potential to optimize many experiments in nonlinear and quantum optics.

Quantum communication with coherent states of light.

Abstract: Quantum communication offers long-term security especially, but not only, relevant to government and industrial users. It is worth noting that, for the first time in the history of cryptographic encoding, we are currently in the situation that secure communication can be based on the fundamental laws of physics (information theoretical security) rather than on algorithmic security relying on the complexity of algorithms, which is periodically endangered as standard computer technology advances. On a fundamental level, the security of quantum key distribution (QKD) relies on the non-orthogonality of the quantum states used. So even coherent states are well suited for this task, the quantum states that largely describe the light generated by laser systems. Depending on whether one uses detectors resolving single or multiple photon states or detectors measuring the field quadratures, one speaks of, respectively, a discrete- or a continuous-variable description. Continuous-variable QKD with coherent states uses a technology that is very similar to the one employed in classical coherent communication systems, the backbone of today's Internet connections. Here, we review recent developments in this field in two connected regimes: (i) improving QKD equipment by implementing front-end telecom devices and (ii) research into satellite QKD for bridging long distances by building upon existing optical satellite links.This article is part of the themed issue 'Quantum technology for the 21st century'.

Quantum-limited measurements of optical signals from a satellite in geostationary earth orbit

Abstract: Quantum key distribution (QKD) has raised increased attention over the past years as one of the most attractive quantum technologies for practical implementation. QKD has already been implemented in intra-city networks all around the world. But up to now, bridging global distances with quantum communication remains an outstanding challenge. A promising candidate to provide this link is via optical satellite communication. As space-to-ground communication is already well developed for classical applications, one can make use of the already existing technology for QKD, i.e. modern Laser Communication Terminals (LCTs) may be adapted for quantum communication. An important first step to achieve this goal is a precise characterization of the system and the channel with regard to their quantum noise behaviour.

Laser based bi-directional Gbit ground links with the Tesat transportable adaptive optical ground station

Abstract: Optical ground stations can be an alternative to radio frequency based transmit (forward) and receive (return) systems for data relay services and other applications including direct to earth optical communications from low earth orbit spacecrafts, deep space receivers, space based quantum key distribution systems and Tbps capacity feeder links to geostationary spacecrafts. The Tesat Transportable Adaptive Optical Ground Station is operational since September 2015 at the European Space Agency site in Tenerife, Spain.. This paper reports about the results of the 2016 experimental campaigns including the characterization of the optical channel from Tenerife for an optimized coding scheme, the performance of the T-AOGS under different atmospheric conditions and the first successful measurements of the suitability of the Alphasat LCT optical downlink performance for future continuous variable quantum key distribution systems.

Single sideband microwave to optical photon conversion – an electro-optic realization

Abstract: We present results on nonlinear electro-optical conversion of microwave radiation into the optical telecommunication band with more than 0.1% photon number conversion efficiency with MHz bandwidth, in a crystalline whispering gallery mode resonator.

Feasibility of squeezing detection over satellite links

Abstract: Quantum squeezing is the nonlinear quantum mechanical effect by which the Heisenberg uncertainty in one observable can be reduced below the shot noise level at the expense of increased uncertainty in its conjugate observable. We study the detection of optical quadrature squeezing via homodyne detection in satellite links. The propagation of the quantum states over large distances and through a varying gravitational potential offers the possibility to test the fundamental laws of physics at the interface of quantum mechanics and general relativity. We model the task as a binary hypothesis testing problem where the null hypothesis corresponds to the vacuum state and the alternative hypothesis corresponds to the squeezed state. Using the Chernoff bound, we derive the number of individual measurements required to limit the average error probabilities to a given value.

Witnessing quantum squeezing with binary homodyne detection

Abstract: Homodyne detection [1] is a typical continuous-variable measurement scheme, ubiquitously used in quantum optics and quantum information. A conventional homodyne detector performs a projective measurement onto a quadrature of the electromagnetic field mode, thus yielding a continuously distributed measurement outcome. The outcome distribution is the marginal of the Wigner function of the state. Homodyne detection also forms the basis of heterodyne, or “dual homodyne” detection, which corresponds to measuring the Q function of the state.