We observe strong violation of Bell's inequality in an Einstein, Podolsky and Rosen type experiment with independent observers. Our experiment definitely implements the ideas behind the well known work by Aspect et al. We for the first time fully enforce the condition of locality, a central assumption in the derivation of Bell's theorem. The necessary space-like separation of the observations is achieved by sufficient physical distance between the measurement stations, by ultra-fast and random setting of the analyzers, and by completely independent data registration.The stronger-than-classical correlations between entangled quantum systems, as first discovered by Einstein, Podolsky and Rosen (EPR) in 1935 [1], have ever since occupied a central position in the discussions of the foundations of quantum mechanics. After Bell's discovery [2] that EPR's implication to explain the correlations using hidden parameters would contradict the predictions of quantum physics, a number of experimental tests have been performed [3][4][5]. All recent experiments confirm the predictions of quantum mechanics. Yet, from a strictly logical point of view, they don't succeed in ruling out a local realistic explanation completely, because of two essential loopholes. The first loophole builds on the fact that all experiments so far detect only a small subset of all pairs created [6]. It is therefore necessary to assume that the pairs registered are a fair sample of all pairs emitted. In principle this could be wrong and once the apparatus is sufficiently refined the experimental observations will contradict quantum mechanics. Yet we agree with John Bell that ". . . it is hard for me to believe that quantum mechanics works so nicely for inefficient practical set-ups and is yet going to fail badly when sufficient refinements are made. Of more importance, in my opinion, is the complete absence of the vital time factor in existing experiments. The analyzers are not rotated during the flight of the particles." [7] This is the second loophole which so far has only been encountered in an experiment by Aspect et al. [4] where the directions of polarization analysis were switched after the photons left the source. Aspect et al., however, used periodic sinusoidal switching, which is predictable into the future. Thus communication slower than the speed of light, or even at the speed of light [8] could in principle explain the results obtained. Therefore this second loophole is still open.The assumption of locality in the derivation of Bell's theorem requires that the measurement processes of the two observers are space-like separated (Fig. 1). This means that it is necessary to freely choose a direction for analysis, to set the analyzer and finally to register the particle such that it is impossible for any information about these processes to travel via any (possibly unknown) channel to the other observer before he, in turn, finishes his measurement [9]. Selection of an analyzer direction has to be completely unpredictable which necessitates a...

We present a loophole-free violation of local realism using entangled photon pairs. We ensure that all relevant events in our Bell test are spacelike separated by placing the parties far enough apart and by using fast random number generators and high-speed polarization measurements. A high-quality polarization-entangled source of photons, combined with high-efficiency, low-noise, single-photon detectors, allows us to make measurements without requiring any fair-sampling assumptions. Using a hypothesis test, we compute p-values as small as 5.9 × 10−9 for our Bell violation while maintaining the spacelike separation of our events. We estimate the degree to which a local realistic system could predict our measurement choices. Accounting for this predictability, our smallest adjusted p-value is 2.3 × 10−7. We therefore reject the hypothesis that local realism governs our experiment.

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By realizing a quantum cryptography system based on polarization entangled photon pairs we establish highly secure keys, because a single photon source is approximated and the inherent randomness of quantum measurements is exploited. We implement a novel key distribution scheme using Wigner's inequality to test the security of the quantum channel, and, alternatively, realize a variant of the BB84 protocol. Our system has two completely independent users separated by 360 m, and generates raw keys at rates of 400 -800 bits/second with bit error rates arround 3%.The primary task of cryptography is to enable two parties (commonly called Alice and Bob) to mask confidential messages such, that the transmitted data are illegible to any unauthorized third party (called Eve). Usually this is done using shared secret keys. However, in principle it is always possible to intercept classical key distribution unnoticedly. The recent development of quantum key distribution 1 can cover this major loophole of classical cryptography. It allows Alice and Bob to establish two completely secure keys by transmitting single quanta (qubits) along a quantum channel. The underlying principle of quantum key distribution is that nature prohibits to gain information on the state of a quantum system without disturbing it. Therefore, in appropriately designed schemes, no tapping of the qubits is possible without showing up to Alice and Bob. These secure keys can be used in a One-Time-Pad protocol 2 , which makes the entire communication absolutely secure.Two well known concepts for quantum key distribution are the BB84 scheme and the Ekert scheme. The BB84 scheme 1 uses single photons transmitted from Alice to Bob, which are prepared at random in four partly orthogonal polarization states: 0 • , 45 • , 90 • , 135 • . If Eve tries to extract information about the polarization of the photons she will inevitably introduce errors, which Alice and Bob can detect by comparing a random subset of the generated keys.The Ekert scheme 3 is based on entangled pairs and uses Bell's inequality 4 to establish security. Both Alice and Bob receive one particle out of an entangled pair. They perform measurements along at least three different directions on each side, where measurements along parallel axes are used for key generation and oblique angles are used for testing the inequality. In 3 , Ekert pointed out that eavesdropping inevitably affects the entanglement between the two constituents of a pair and therefore reduces the degree of violation of Bell's inequality. While we are not aware of a general proof that the violation of a Bell inequality implies the security of the system, this has been shown 5 for the BB84 protocol adapted to entangled pairs and the CHSH inequality 6 .In any real cryptography system, the raw key generated by Alice and Bob contains errors, which have to be corrected by classical error correction 7 over a public channel. Furthermore it has been shown that whenever Alice and Bob share a sufficiently secure key, they can enhance its...

Quantum computation promises to solve fundamental, yet otherwise intractable, problems across a range of active fields of research. Recently, universal quantum logic-gate sets-the elemental building blocks for a quantum computer-have been demonstrated in several physical architectures. A serious obstacle to a full-scale implementation is the large number of these gates required to build even small quantum circuits. Here, we present and demonstrate a general technique that harnesses multi-level information carriers to significantly reduce this number, enabling the construction of key quantum circuits with existing technology. We present implementations of two key quantum circuits: the three-qubit Toffoli gate and the general two-qubit controlled-unitary gate. Although our experiment is carried out in a photonic architecture, the technique is independent of the particular physical encoding of quantum information, and has the potential for wider application.T he realization of a full-scale quantum computer presents one of the most challenging problems facing modern science. Even implementing small-scale quantum algorithms requires a high level of control over multiple quantum systems. Recently, much progress has been made with demonstrations of universal quantum gate sets in a number of physical architectures including ion traps 1,2 , linear optics 3-6 , superconductors 7,8 and atoms 9,10 . In theory, these gates can now be put together to implement any quantum circuit and build a scalable quantum computer. In practice, there are many significant obstacles that will require both theoretical and technological developments to overcome. One is the sheer number of elemental gates required to build quantum logic circuits.Most approaches to quantum computing use qubits-the quantum version of bits. A qubit is a two-level quantum system that can be represented mathematically by a vector in a two-dimensional Hilbert space. Realizing qubits typically requires enforcing a twolevel structure on systems that are naturally far more complex and which have many readily accessible degrees of freedom, such as atoms, ions or photons. Here, we show how harnessing these extra levels during computation significantly reduces the number of elemental gates required to build key quantum circuits. Because the technique is independent of the physical encoding of quantum information and the way in which the elemental gates are themselves constructed, it has the potential to be used in conjunction with existing gate technology in a wide variety of architectures. Our technique extends a recent proposal 11 , and we use it to demonstrate two key quantum logic circuits: the Toffoli and controlled-unitary 12 gates. We first outline the technique in a general context, then present an experimental realization in a linear optic architecture: without our resource-saving technique, linear optic implementations of these gates are infeasible with current technology. Simplifying the Toffoli gateOne of the most important quantum logic gates is the Toffoli 1...

The quantum internet is predicted to be the next-generation information processing platform, promising secure communication and an exponential speed-up in distributed computation. The distribution of single qubits over large distances via quantum teleportation is a key ingredient for realizing such a global platform. By using quantum teleportation, unknown quantum states can be transferred over arbitrary distances to a party whose location is unknown. Since the first experimental demonstrations of quantum teleportation of independent external qubits, an internal qubit and squeezed states, researchers have progressively extended the communication distance. Usually this occurs without active feed-forward of the classical Bell-state measurement result, which is an essential ingredient in future applications such as communication between quantum computers. The benchmark for a global quantum internet is quantum teleportation of independent qubits over a free-space link whose attenuation corresponds to the path between a satellite and a ground station. Here we report such an experiment, using active feed-forward in real time. The experiment uses two free-space optical links, quantum and classical, over 143 kilometres between the two Canary Islands of La Palma and Tenerife. To achieve this, we combine advanced techniques involving a frequency-uncorrelated polarization-entangled photon pair source, ultra-low-noise single-photon detectors and entanglement-assisted clock synchronization. The average teleported state fidelity is well beyond the classical limit of two-thirds. Furthermore, we confirm the quality of the quantum teleportation procedure without feed-forward by complete quantum process tomography. Our experiment verifies the maturity and applicability of such technologies in real-world scenarios, in particular for future satellite-based quantum teleportation.

We demonstrate a wavelength-tunable, fiber-coupled source of polarization- entangled photons with extremely high spectral brightness and quality of entanglement. Using a 25 mm PPKTP crystal inside a polarization Sagnac interferometer we detect a spectral brightness of 273000 pairs (s mW nm)(-1), a factor of 28 better than comparable previous sources while state tomography showed the two-photon state to have a tangle of T = 0.987. This improvement was achieved by use of a long crystal, careful selection of focusing parameters and single-mode fiber coupling. We demonstrate that, due to the particular geometry of the setup, the signal and idler wavelengths can be tuned over a wide range without loss of entanglement.

We produce two identical keys using, for the first time, entangled trinary quantum systems (qutrits) for quantum key distribution. The advantage of qutrits over the normally used binary quantum systems is an increased coding density and a higher security margin. The qutrits are encoded into the orbital angular momentum of photons, namely Laguerre-Gaussian modes with azimuthal index l +1, 0 and −1, respectively. The orbital angular momentum is controlled with phase holograms. In an Ekert-type protocol the violation of a three-dimensional Bell inequality verifies the security of the generated keys. A key is obtained with a qutrit error rate of approximately 10%.