arXiv subjects

Mario Krenn

Publications and source records attributed to Mario Krenn.

85 records · Page 5Linked to original sources

arXiv2016

Twisted Light Transmission over 143 kilometers

Spatial modes of light can potentially carry a vast amount of information, making them promising candidates for both classical and quantum communication. However, the distribution of such modes over large distances remains difficult. Intermodal coupling complicates their use with common fibers, while free-space transmission is thought to be strongly influenced by atmospheric turbulence. Here we show the transmission of orbital angular momentum modes of light over a distance of 143 kilometers between two Canary Islands, which is 50 times greater than the maximum distance achieved previously. As a demonstration of the transmission quality, we use superpositions of these modes to encode a short message. At the receiver, an artificial neural network is used for distinguishing between the different twisted light superpositions. The algorithm is able to identify different mode superpositions with an accuracy of more than 80% up to the third mode order, and decode the transmitted message with an error rate of 8.33%. Using our data, we estimate that the distribution of orbital angular momentum entanglement over more than 100 kilometers of free space is feasible. Moreover, the quality of our free-space link can be further improved by the use of state-of-the-art adaptive optics systems.

physics.optics↗

arXiv2016

Automated Search for new Quantum Experiments

Quantum mechanics predicts a number of at first sight counterintuitive phenomena. It is therefore a question whether our intuition is the best way to find new experiments. Here we report the development of the computer algorithm Melvin which is able to find new experimental implementations for the creation and manipulation of complex quantum states. And indeed, the discovered experiments extensively use unfamiliar and asymmetric techniques which are challenging to understand intuitively. The results range from the first implementation of a high-dimensional Greenberger-Horne-Zeilinger (GHZ) state, to a vast variety of experiments for asymmetrically entangled quantum states -- a feature that can only exist when both the number of involved parties and dimensions is larger than 2. Additionally, new types of high-dimensional transformations are found that perform cyclic operations. Melvin autonomously learns from solutions for simpler systems, which significantly speeds up the discovery rate of more complex experiments. The ability to automate the design of a quantum experiment can be applied to many quantum systems and allows the physical realization of quantum states previously thought of only on paper.

quant-ph↗

arXiv2015

Cyclic transformation of orbital angular momentum modes

The spatial modes of photons are one realization of a QuDit, a quantum system that is described in a D-dimensional Hilbert space. In order to perform quantum information tasks with QuDits, a general class of D-dimensional unitary transformations is needed. Among these, cyclic transformations are an important special case required in many high-dimensional quantum communication protocols. In this paper, we experimentally demonstrate a cyclic transformation in the high-dimensional space of photonic orbital angular momentum (OAM). Using simple linear optical components, we show a successful four-fold cyclic transformation of OAM modes. Interestingly, our experimental setup was found by a computer algorithm. In addition to the four-cyclic transformation, the algorithm also found extensions to higher-dimensional cycles in a hybrid space of OAM and polarization. Besides being useful for quantum cryptography with QuDits, cyclic transformations are key for the experimental production of high-dimensional maximally entangled Bell-states.

quant-ph↗

arXiv2015

On Small Beams with Large Topological Charge

Light beams can carry a discrete, in principle unbounded amount of angular momentum. Examples of such beams, the Laguerre-Gauss modes, are frequently expressed as solutions of the paraxial wave equation. There, they are eigenstates of the orbital angular momentum (OAM) operator. The paraxial solutions predict that beams with large OAM could be used to resolve arbitrarily small distances - a dubious situation. Here we show how to solve that situation by calculating the properties of beams free from the paraxial approximation. We find the surprising result that indeed one can resolve smaller distances with larger OAM, although with decreased visibility. If the visibility is kept constant (for instance at the Rayleigh criterion, the limit where two points are reasonably distinguishable), larger OAM does not provide an advantage. The drop in visibility is due to a field in the direction of propagation, which is neglected within the paraxial limit.

physics.optics↗

arXiv2015

Physical meaning of the radial index of Laguerre-Gauss beams

The Laguerre-Gauss modes are a class of fundamental and well-studied optical fields. These stable, shape-invariant photons - exhibiting circular-cylindrical symmetry - are familiar from laser optics, micro-mechanical manipulation, quantum optics, communication, and foundational studies in both classical optics and quantum physics. They are characterized, chiefly, by two modes numbers: the azimuthal index indicating the orbital angular momentum of the beam - which itself has spawned a burgeoning and vibrant sub-field - and the radial index, which up until recently, has largely been ignored. In this manuscript we develop a differential operator formalism for dealing with the radial modes in both the position and momentum representations, and - more importantly - give for the first time the meaning of this quantum number in terms of a well-defined physical parameter: the "intrinsic hyperbolic momentum charge".

quant-ph↗

arXiv2015

Multi-photon entanglement in high dimensions

Entanglement lies at the heart of quantum mechanics $-$ as a fundamental tool for testing its deep rift with classical physics, while also providing a key resource for quantum technologies such as quantum computation and cryptography. In 1987 Greenberger, Horne, and Zeilinger realized that the entanglement of more than two particles implies a non-statistical conflict between local realism and quantum mechanics. The resulting predictions were experimentally confirmed by entangling three photons in their polarization. Experimental efforts since have singularly focused on increasing the number of particles entangled, while remaining in a two-dimensional space for each particle. Here we show the experimental generation of the first multi-photon entangled state where both $-$ the number of particles and the number of dimensions $-$ are greater than two. Interestingly, our state exhibits an asymmetric entanglement structure that is only possible when one considers multi-particle entangled states in high dimensions. Two photons in our state reside in a three-dimensional space, while the third lives in two dimensions. Our method relies on combining two pairs of photons, high-dimensionally entangled in their orbital angular momentum, in such a way that information about their origin is erased. Additionally, we show how this state enables a new type of "layered" quantum cryptographic protocol where two parties share an additional layer of secure information over that already shared by all three parties. In addition to their application in novel quantum communication protocols, such asymmetric entangled states serve as a manifestation of the complex dance of correlations that can exist within quantum mechanics.

quant-ph↗

arXiv2015

Twisted photon entanglement through turbulent air across Vienna

Photons with a twisted phase front can carry a discrete, in principle unbounded amount of orbital angular momentum (OAM). The large state space allows for complex types of entanglement, interesting both for quantum communication and for fundamental tests of quantum theory. However, the distribution of such entangled states over large distances was thought to be infeasible due to influence of atmospheric turbulence, indicating a serious limitation on their usefulness. Here we show that it is possible to distribute quantum entanglement encoded in OAM over a turbulent intra-city link of 3 kilometers. We confirm quantum entanglement of the first two higher-order levels (with OAM=$\pm 1 \hbar$ and $\pm 2 \hbar$). They correspond to four new quantum channels orthogonal to all that have been used in long-distance quantum experiments so far. Therefore a promising application would be quantum communication with a large alphabet. We also demonstrate that our link allows access to up to 11 quantum channels of OAM. The restrictive factors towards higher numbers are technical limitations that can be circumvented with readily available technologies.

quant-ph↗

arXiv2014

Communication with spatially modulated Light through turbulent Air across Vienna

The transverse spatial modes of light offer a large state-space with interesting physical properties. For exploiting it in future long-distance experiments, spatial modes will have to be transmitted over turbulent free-space links. Numerous recent lab-scale experiments have found significant degradation in the mode quality after transmission through simulated turbulence and consecutive coherent detection. Here we experimentally analyze the transmission of one prominent class of spatial modes, the orbital-angular momentum (OAM) modes, through 3 km of strong turbulence over the city of Vienna. Instead of performing a coherent phase-dependent measurement, we employ an incoherent detection scheme which relies on the unambiguous intensity patterns of the different spatial modes. We use a pattern recognition algorithm (an artificial neural network) to identify the characteristic mode pattern displayed on a screen at the receiver. We were able to distinguish between 16 different OAM mode superpositions with only ~1.7% error, and use them to encode and transmit small grey-scale images. Moreover, we found that the relative phase of the superposition modes is not affected by the atmosphere, establishing the feasibility for performing long-distance quantum experiments with the OAM of photons. Our detection method works for other classes of spatial modes with unambiguous intensity patterns as well, and can further be improved by modern techniques of pattern recognition.

physics.optics↗

arXiv2014

Quantum Entanglement of High Angular Momenta

Single photons with helical phase structures may carry a quantized amount of orbital angular momentum (OAM) and their entanglement is important for quantum information science and fundamental tests of quantum theory. Because there is no theoretical upper limit on how many quanta of OAM a single photon can carry, it is possible to create entanglement between two particles with an arbitrary high difference in the quantum number. By transferring polarization entanglement to OAM with an interferometric scheme, we generate and verify entanglement up to 600 quanta difference in the orbital angular momentum. The only restrictive factors towards higher numbers are current technical limitations. We also experimentally demonstrate that the entanglement of very high OAM can improve the sensitivity of the angular resolution in remote sensing.

quant-ph↗

arXiv2014

Generation and Confirmation of a (100x100)-dimensional entangled Quantum System

Entangled quantum systems have properties that have fundamentally overthrown the classical worldview. Increasing the complexity of entangled states by expanding their dimensionality allows the implementation of novel fundamental tests of nature, and moreover also enables genuinely new protocols for quantum information processing. Here we present the creation of a (100x100)-dimensional entangled quantum system, using spatial modes of photons. For its verification we develop a novel nonlinear criterion which infers entanglement dimensionality of a global state by using only information about its subspace correlations. This allows very practical experimental implementation as well as highly efficient extraction of entanglement dimensionality information. Applications in quantum cryptography and other protocols are very promising.

quant-ph↗

arXiv2013

Real-Time Imaging of Quantum Entanglement

Quantum Entanglement is widely regarded as one of the most prominent features of quantum mechanics and quantum information science. Although, photonic entanglement is routinely studied in many experiments nowadays, its signature has been out of the grasp for real-time imaging. Here we show that modern technology, namely triggered intensified charge coupled device (ICCD) cameras are fast and sensitive enough to image in real-time the effect of the measurement of one photon on its entangled partner. To quantitatively verify the non-classicality of the measurements we determine the detected photon number and error margin from the registered intensity image within a certain region. Additionally, the use of the ICCD camera allows us to demonstrate the high flexibility of the setup in creating any desired spatial-mode entanglement, which suggests as well that visual imaging in quantum optics not only provides a better intuitive understanding of entanglement but will improve applications of quantum science.

quant-ph↗

arXiv2013

Entangled Singularity patterns of Photons in Ince-Gauss modes

Photons with complex spatial mode structures open up possibilities for new fundamental high-dimensional quantum experiments and for novel quantum information tasks. Here we show for the first time entanglement of photons with complex vortex and singularity patterns called Ince-Gauss modes. In these modes, the position and number of singularities vary depending on the mode parameters. We verify 2-dimensional and 3-dimensional entanglement of Ince-Gauss modes. By measuring one photon and thereby defining its singularity pattern, we non-locally steer the singularity structure of its entangled partner, while the initial singularity structure of the photons is undefined. In addition we measure an Ince-Gauss specific quantum-correlation function with possible use in future quantum communication protocols.

quant-ph↗

arXiv2012

Quantum orbital angular momentum of elliptically-symmetric light

We present a quantum mechanical analysis of the orbital angular momentum of a class of recently discovered elliptically-symmetric stable light fields --- the so-called Ince-Gauss modes. We study, in a fully quantum formalism, how the orbital angular momentum of these beams varies with their ellipticity and discover several compelling features, including: non-monotonic behavior, stable beams with real continuous (non-integer) orbital angular momenta, and orthogonal modes with the same orbital angular momenta. We explore, and explain in detail, the reasons for this behavior. These features may have application to quantum key distribution, atom trapping, and quantum informatics in general --- as the ellipticity opens up a new way of navigating the photonic Hilbert space.

quant-ph↗