ABSTRACT: The counterintuitive features of quantum physics challenge many common-sense assumptions. In an interferometric quantum eraser experiment, one can actively choose whether or not to erase which-path information (a particle feature) of one quantum system and thus observe its wave feature via interference or not by performing a suitable measurement on a distant quantum system entangled with it. In all experiments performed to date, this choice took place either in the past or, in some delayed-choice arrangements, in the future of the interference. Thus, in principle, physical communications between choice and interference were not excluded. Here, we report a quantum eraser experiment in which, by enforcing Einstein locality, no such communication is possible. This is achieved by independent active choices, which are space-like separated from the interference. Our setup employs hybrid path-polarization entangled photon pairs, which are distributed over an optical fiber link of 55 m in one experiment, or over a free-space link of 144 km in another. No naive realistic picture is compatible with our results because whether a quantum could be seen as showing particle- or wave-like behavior would depend on a causally disconnected choice. It is therefore suggestive to abandon such pictures altogether.
Proceedings of the National Academy of Sciences 01/2013; · 9.68 Impact Factor
ABSTRACT: We present a high-fidelity quantum teleportation experiment over a high-loss free-space channel between two laboratories. We teleported six states of three mutually unbiased bases and obtained an average state fidelity of 0.82(1), well beyond the classical limit of 2/3. With the obtained data, we tomographically reconstructed the process matrices of quantum teleportation. The free-space channel attenuation of 31 dB corresponds to the estimated attenuation regime for a down-link from a low-earth-orbit satellite to a ground station. We also discussed various important technical issues for future experiments, including the dark counts of single-photon detectors, coincidence-window width etc. Our experiment tested the limit of performing quantum teleportation with state-of-the-art resources. It is an important step towards future satellite-based quantum teleportation and paves the way for establishing a worldwide quantum communication network.
Optics Express 10/2012; 20(21):23126-37. · 3.59 Impact Factor
ABSTRACT: 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.
Nature 09/2012; 489(7415):269-73. · 36.28 Impact Factor
ABSTRACT: Quantum teleportation  is a quintessential prerequisite of many quantum
information processing protocols [2-4]. By using quantum teleportation, one can
circumvent the no-cloning theorem  and faithfully transfer unknown quantum
states to a party whose location is even unknown over arbitrary distances. Ever
since the first experimental demonstrations of quantum teleportation of
independent qubits  and of squeezed states , researchers have
progressively extended the communication distance in teleportation, usually
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. Here we report the first long-distance quantum
teleportation experiment with active feed-forward in real time. The experiment
employed two optical links, quantum and classical, over 143 km free space
between the two Canary Islands of La Palma and Tenerife. To achieve this, the
experiment had to employ novel techniques such as a frequency-uncorrelated
polarization-entangled photon pair source, ultra-low-noise single-photon
detectors, and entanglement-assisted clock synchronization. The average
teleported state fidelity was well beyond the classical limit of 2/3.
Furthermore, we confirmed the quality of the quantum teleportation procedure
(without feed-forward) by complete quantum process tomography. Our experiment
confirms the maturity and applicability of the involved technologies in
real-world scenarios, and is a milestone towards future satellite-based quantum
ABSTRACT: Photonic quantum simulators are promising candidates for providing insight
into other small- to medium-sized quantum systems. The available photonic
quantum technology is reaching the state where significant advantages arise for
the quantum simulation of interesting questions in Heisenberg spin systems.
Here we experimentally simulate such spin systems with single photons and
linear optics. The effective Heisenberg-type interactions among individual
single photons are realized by quantum interference at the tunable direction
coupler followed by the measurement process. The effective interactions are
characterized by comparing the entanglement dynamics using pairwise concurrence
of a four-photon quantum system. We further show that photonic quantum
simulations of generalized Heisenberg interactions on a four-site square
lattice and a six-site checkerboard lattice are in reach of current technology.
ABSTRACT: Motivated by the question, which kind of physical interactions and processes
are needed for the production of quantum entanglement, Peres has put forward
the radical idea of delayed-choice entanglement swapping. There, entanglement
can be "produced a posteriori, after the entangled particles have been measured
and may no longer exist". In this work we report the first realization of
Peres' gedanken experiment. Using four photons, we can actively delay the
choice of measurement-implemented via a high-speed tunable bipartite state
analyzer and a quantum random number generator-on two of the photons into the
time-like future of the registration of the other two photons. This effectively
projects the two already registered photons onto one definite of two mutually
exclusive quantum states in which either the photons are entangled (quantum
correlations) or separable (classical correlations). This can also be viewed as
"quantum steering into the past".
ABSTRACT: We realize quantum gates for path qubits with a high-speed, polarization-independent and tunable beam splitter. Two electro-optical modulators act in a Mach-Zehnder interferometer as high-speed phase shifters and rapidly tune its splitting ratio. We test its performance with heralded single photons, observing a polarization-independent interference contrast above 95%. The switching time is about 5.6 ns, and a maximal repetition rate is 2.5 MHz. We demonstrate tunable feed-forward operations of a single-qubit gate of path-encoded qubits and a two-qubit gate via measurement-induced interaction between two photons.
Optics Express 11/2011; 19(23):22723-30. · 3.59 Impact Factor
ABSTRACT: Optical implementations of qubits play an important role for quantum information science. Photons are ideal carriers of quantum information due to low decoherence rates and are easily controllable by standard off‐the‐shelf components. Over the last decade the degree of control over photonic single‐ and two‐qubit operations has improved substantially, which recently enables the complex state engineering of various multi‐photon states. The control of current four‐ and six‐photon entangled states provides access for the investigation of interesting properties of multi‐particle entangled states and for overcoming the random nature of spontaneous emission sources.
AIP Conference Proceedings. 10/2011; 1363(1):13-16.
ABSTRACT: Bell's theorem shows that local realistic theories place strong restrictions on observable correlations between different systems, giving rise to Bell's inequality which can be violated in experiments using entangled quantum states. Bell's theorem is based on the assumptions of realism, locality, and the freedom to choose between measurement settings. In experimental tests, "loopholes" arise which allow observed violations to still be explained by local realistic theories. Violating Bell's inequality while simultaneously closing all such loopholes is one of the most significant still open challenges in fundamental physics today. In this paper, we present an experiment that violates Bell's inequality while simultaneously closing the locality loophole and addressing the freedom-of-choice loophole, also closing the latter within a reasonable set of assumptions. We also explain that the locality and freedom-of-choice loopholes can be closed only within nondeterminism, i.e., in the context of stochastic local realism.
Proceedings of the National Academy of Sciences 11/2010; 107(46):19708-13. · 9.68 Impact Factor
ABSTRACT: Quantum simulators are controllable quantum systems that can reproduce the
dynamics of the system of interest, which are unfeasible for classical
computers. Recent developments in quantum technology enable the precise control
of individual quantum particles as required for studying complex quantum
systems. Particularly, quantum simulators capable of simulating frustrated
Heisenberg spin systems provide platforms for understanding exotic matter such
as high-temperature superconductors. Here we report the analog quantum
simulation of the ground-state wavefunction to probe arbitrary Heisenberg-type
interactions among four spin-1/2 particles . Depending on the interaction
strength, frustration within the system emerges such that the ground state
evolves from a localized to a resonating valence-bond state. This spin-1/2
tetramer is created using the polarization states of four photons. The
single-particle addressability and tunable measurement-induced interactions
provide us insights into entanglement dynamics among individual particles. We
directly extract ground-state energies and pair-wise quantum correlations to
observe the monogamy of entanglement.
ABSTRACT: A significant limitation of practical quantum key distribution (QKD) setups is currently their limited operational range. It has recently been emphasized (X. Ma, C.-H. F. Fung, and H.-K. Lo., Phys. Rev. A, 76:012307, 2007) that entanglement-based QKD systems can tolerate higher channel losses than systems based on weak coherent laser pulses (WCP), in particular when the source is located symmetrically between the two communicating parties, Alice and Bob. In the work presented here, we experimentally study this important advantage by implementing different entanglement-based QKD setups on a 144~km free-space link between the two Canary Islands of La Palma and Tenerife. We established three different configurations where the entangled photon source was placed at Alice's location, asymmetrically between Alice and Bob and symmetrically in the middle between Alice and Bob, respectively. The resulting quantum channel attenuations of 35~dB, 58~dB and 71~dB, respectively, significantly exceed the limit for WCP systems. This confirms that QKD over distances of 300~km and even more is feasible with entangled state sources placed in the middle between Alice and Bob. Comment: 14 pages, 5 figures, 1 table
ABSTRACT: An on-demand single-photon source is a fundamental building block in quantum
science and technology. We experimentally demonstrate the proof of concept for
a scheme to generate on-demand single photons via actively multiplexing several
heralded photons probabilistically produced from pulsed spontaneous parametric
down-conversions (SPDCs). By utilizing a four-photon-pair source, an active
feed-forward technique, and an ultrafast single-photon router, we show a
fourfold enhancement of the output photon rate. Simultaneously, we maintain the
quality of the output single-photon states, confirmed by correlation
measurements. We also experimentally verify, via Hong-Ou-Mandel interference,
that the router does not affect the indistinguishability of the single photons.
Furthermore, we give numerical simulations, which indicate that photons based
on multiplexing of four SPDC sources can outperform the heralding based on
highly advanced photon-number-resolving detectors. Our results show a route for
on-demand single-photon generation and the practical realization of scalable
linear optical quantum information processing.
ABSTRACT: We demonstrate hybrid entanglement of photon pairs via the experimental violation of a Bell inequality with two different degrees of freedom (DOF), namely the path (linear momentum) of one photon and the polarization of the other photon. Hybrid entangled photon pairs are created by Spontaneous Parametric Down Conversion and coherent polarization to path conversion for one photon. For that photon, path superposition is analyzed, and polarization superposition for its twin photon. The correlations between these two measurements give an S-parameter of S=2.653+/-0.027 in a CHSH inequality and thus violate local realism for two different DOF by more than 24 standard deviations. This experimentally supports the idea that entanglement is a fundamental concept which is indifferent to the specific physical realization of Hilbert space.
ABSTRACT: The predictions of quantum mechanics can be in striking contradiction with local realism if entanglement exists between distant systems. Bell's theorem shows that local realistic theories, such as classical physics, place a strong restriction on observable correlations between different systems in experiments, giving rise to Bell's inequality . This allows an experimental test of whether nature itself agrees with local realism or quantum mechanics. To derive his inequality, Bell made three assumptions: realism (objects possess definite properties prior to and independent of observation), locality (space-like separated events cannot causally influence each other), and freedom of choice (the choice of measurement settings is free or random). In experimental tests of Bell's inequality, there may be "loopholes" which allow observed violations to still be explained by local realistic theories. Many Bell tests have been performed which violate Bell's inequality [2-13], some which have closed individual loopholes, specifically the locality loophole  and the fair-sampling loophole . Another crucial loophole, which has been discussed theoretically in Ref.  but not yet addressed experimentally, is related to Bell's freedom-of-choice assumption. Here we report an experiment using entangled photons, which for the first time closes this loophole by randomly switching measurement settings and space-like separating the setting choice from the photon pair emission. There has previously been much experimental and theoretical progress towards a complete loophole-free Bell test (e.g., Refs [14-18]). However, our experiment, which simultaneously closes the locality and the freedom-of-choice loopholes, is the first to close more than one of the three crucial loopholes at the same time.