The aim of the QIPS project (financed by ESA) is to explore quantum phenomena and to demonstrate quantum communication over long distances. Based on the current state-of-the-art a first study investigating the feasibility of space based quantum communication has to establish goals for mid-term and long-term missions, but also has to test the feasibility of key issues in a long distance ground-to-ground experiment. We have therefore designed a proof-of-concept demonstration for establishing single photon links over a distance of 144 km between the Canary Islands of La Palma and Tenerife to evaluate main limitations for future space experiments. Here we report on the progress of this project and present first measurements of crucial parameters of the optical free space link.
John Bell’s theorem of 1964 states that local elements of physical reality, existing independent of measurement, are inconsistent with the predictions of quantum mechanics (Bell, J. S. (1964), Physics (College. Park. Md). Specifically, correlations between measurement results from distant entangled systems would be smaller than predicted by quantum physics. This is expressed in Bell’s inequalities. Employing modifications of Bell’s inequalities, many experiments have been performed that convincingly support the quantum predictions. Yet, all experiments rely on assumptions, which provide loopholes for a local realist explanation of the measurement. Here we report an experiment with polarization-entangled photons that simultaneously closes the most significant of these loopholes. We use a highly efficient source of entangled photons, distributed these over a distance of 58.5 meters, and implemented rapid random setting generation and high-efficiency detection to observe a violation of a Bell inequality with high statistical significance. The merely statistical probability of our results to occur under local realism is less than 3.74×10-31, corresponding to an 11.5 standard deviation effect.
In the field of quantum communication the teleportation1 of single quanta plays a fundamental role in numerous quantum information-processing protocols. Quantum teleportation allows to faithfully transfer unknown quantum states over arbitrary distances and constitutes a method to circumvent the no-cloning theorem2. Even formally completely independent particles can become entangled via the process of entanglement swapping3. In a future quantum communication network4 this will be of utmost importance, enabling quantum computers to become globally interconnected. In order to prove the feasibility of quantum teleportation under optical link attenuations that will arise in a future space-application scenario, we extended the communication distance to 143 km, employing an optical free-space link between the two Canary Islands of La Palma and Tenerife. This work proofs the feasibility of ground-based freespace quantum teleportation. With our setup we were able to achieve coincidence production rates and fidelities to cope with the optical link attenuation, resulting from various experimental and technical challenges, which will arise in a quantum transmission between a ground-based transmitter and a low-earth-orbiting satellite receiver5. In our experiment we gained an average state fidelity for the teleported quantum states of more than 6 standard deviations beyond the classical limit of 2/3 and a process fidelity of 0.710(42). We expect that many of the features implemented in this experiment will be key blocks for future investigations.
We design and built a novel optical terminal specifically designed for free-space communication operating at light levels
at the quantum limit, such as in quantum communication. Our system is particularly well suited for this task, as it is
based on an all-spheric catadioptric design, which allows for large and un-obstructed apertures. This design offers an
easier and cheaper approach to building high-quality optical terminals with large apertures than schemes based on offaxis
parabolic mirrors. We utilized an off-axis version of the original Schwarzschild concentric design, and correct the
spherical aberration by substituting the original on-axis secondary spherical mirror with an off-axis catadioptric
secondary mirror. A prototype of the optical terminal was realized and tested and it meet the expected performance.
Fundamental quantum optics test as well as quantum cryptography and quantum teleportation are based on the
distribution of single quantum states and quantum entanglement respectively. We will discuss recent experimental
achievements in the field of long-distance quantum communication via optical fiber as well as in free-space over
a record breaking distance of 144 km. The European Space Agency (ESA) has supported a range of studies
in the field of quantum physics and quantuminformation science in space for several years, and consequently a
mission proposal Space-QUEST Quantum Entanglement for Space Experiments was submitted to the European
Life and Physical Sciences in Space Program. This proposal envisions to perform space-to-ground quantum
communication tests from the International Space Station (ISS) and will presented in this article.
Entanglement-based quantum cryptography has the appealing advantage of intuitively more evident security. While
originally, weak laser pulse schemes were implemented earlier as technologically simpler, it is now possible to build
entanglement-based quantum key distribution systems on a technically equally advanced level. The existing polarization-based
systems as developed in Vienna now cover distances of the order of 100 km in fiber and of 144 km in free space.
In a recent fiber experiment, an asymmetric source is used such that one photon at the 1.550 nm telecom wavelength is
transmitted to Bob, while the other photon at 810 nm is locally measured by Alice. It turns out that polarization
entanglement is rather robust, certainly over distances of 100 km in fibers. In a recent long-distance free-space
experiment, one photon was sent over 144 km from the Canary Island of La Palma to the island of Tenerife, while again
the other photon was measured locally. The receiving station uses the OGS telescope operated by the European Space
Agency ESA. This experiment opens up the possibility for future quantum key distribution using satellites.
In the emerging field of quantum information technology the two basic subfields are quantum communication
and quantum computation. Photonic qubits are considered as most promising information carriers for this
new technology due to the immense advantage of suffering negligible decoherence. Additionally, the very small
photon-photon interactions can be replaced by inducing effective nonlinearities via measurements which allow for
the implementation of crucial two-qubit gate operations. Although the spontaneous parametric down-conversion
gives access to the generation of highly entangled few-photon states, such as four-qubit cluster states which
allow to demonstrate the new concept of the one-way quantum computer, its applicability is highly limited
due to the poor scaling of the simultaneous emission of more than one-entangled photon pair. Therefore of
particular interest is the reversible mapping of qubits from photon states to atomic states. This might allow
the implementation of photonic quantum repeaters for long-distance quantum communication or the generation
of arbitrary multi-photon states as required for linear-optics quantum computing. Thus for the realization of
such a quantum network several approaches for achieving the required quantum control between matter and
photons have been studied during the past few years. Recent experiments demonstrating the generation of
narrow-bandwidth single photons using a room-temperature ensemble of 87Rb atoms and electromagnetically
induced transparency should emphasize the progress towards such a quantum network.
We report on the experimental implementation of a BB84-type quantum key distribution protocol over a 144 km free-space link using weak coherent laser pulses. The security was assured by employing decoy state analysis, and optimization of the link transmission was achieved with bi-directional active telescope tracking. This enabled us to distribute a secure key at a rate of 11 bits/s at an attenuation of about 35dB. Utilizing a simple transmitter setup and an optical ground station capable of tracking spacecraft in low earth orbit, this outdoor experiment demonstrates the feasibility of global key distribution via satellites.
Quantum metrology utilizes nonclassical states (of light) to
outperform the accuracy limits of its classical counterpart. We
demonstrate the relevance of photon number Fock states and
polarization entanglement for the experimental realization of
interferometric quantum metrology applications.
We have tested the experimental prerequisites for a Space-to-Ground quantum communication link between satellites and an optical ground station. The feasibility of our ideas is being tested using the facilities of the ASI Matera Laser Ranging Observatory (MLRO) and existing geodetic satellites such as Lageos 1 and 2. Specific emphasis is put on the necessary technological modifications of the existing infrastructure to achieve single photon reception from an orbiting satellite.
Quantum physics experiments in space using entangled photons and
satellites are within reach of current technology. We propose a series of fundamental quantum physics experiments that make advantageous use of the space infrastructure with specific emphasis on the satellite-based distribution of entangled photon pairs. The experiments are feasible already today and will eventually lead to a Bell-experiment over thousands of kilometers, thus demonstrating quantum correlations over distances which cannot be achieved by purely earth-bound experiments.
Atoms interacting with standing light waves are a model system for the propagation of waves in static and time varying periodic media. We present here experiments studying the coherent motion of atomic deBroglie waves in periodic potentials made from on and off resonant light. We observe anomalous transmission of atoms through resonant standing light waves and experimentally confirm that atoms fulfilling the Bragg condition form a standing matter wave pattern. We furthermore demonstrate how Bragg diffraction of atomic matter waves at a time-modulated thick standing light wave can be used to coherently shift the deBroglie frequency of the diffracted atoms. Our frequency shifter for atomic matter waves is similar to an acousto-optic frequency shifter for photons.