Publications (21)138.6 Total impact
 [Show abstract] [Hide abstract]
ABSTRACT: Quantum information processing holds great promise for communicating and computing data efficiently. However, scaling current photonic implementation approaches to larger system size remains an outstanding challenge for realizing disruptive quantum technology. Two main ingredients of quantum information processors are quantum interference and singlephoton detectors. Here we develop a hybrid superconductingphotonic circuit system to show how these elements can be combined in a scalable fashion on a silicon chip. We demonstrate the suitability of this approach for integrated quantum optics by interfering and detecting photon pairs directly on the chip with waveguidecoupled singlephoton detectors. Using a directional coupler implemented with silicon nitride nanophotonic waveguides, we observe 97% interference visibility when measuring photon statistics with two monolithically integrated superconducting single photon detectors. The photonic circuit and detector fabrication processes are compatible with standard semiconductor thinfilm technology, making it possible to implement more complex and larger scale quantum photonic circuits on silicon chips.  [Show abstract] [Hide abstract]
ABSTRACT: The waveparticle duality dates back to Einstein's explanation of the photoelectric effect through quanta of light and de Broglie's hypothesis of matter waves. Quantum mechanics uses an abstract description for the behavior of physical systems such as photons, electrons, or atoms. Whether quantum predictions for single systems in an interferometric experiment allow an intuitive understanding in terms of the particle or wave picture, depends on the specific configuration which is being used. In principle, this leaves open the possibility that quantum systems always either behave definitely as a particle or definitely as a wave in every experimental run by a priori adapting to the specific experimental situation. This is precisely what is tried to be excluded by delayedchoice experiments, in which the observer chooses to reveal the particle or wave character  or even a continuous transformation between the two  of a quantum system at a late stage of the experiment. We review the history of delayedchoice gedanken experiments, which can be traced back to the early days of quantum mechanics. Then we discuss their experimental realizations, in particular Wheeler's delayed choice in interferometric setups as well as delayedchoice quantum erasure and entanglement swapping. The latter is particularly interesting, because it elevates the waveparticle duality of a single quantum system to an entanglementseparability duality of multiple systems.  [Show abstract] [Hide abstract]
ABSTRACT: Although interference is a classicalwave phenomenon, the superposition principle, which underlies interference of individual particles, is at the heart of quantum physics. An interactionfree measurements (IFM) harnesses the waveparticle duality of single photons to sense the presence of an object via the modification of the interference pattern, which can be accomplished even if the photon and the object haven't interacted with each other. By using the quantum Zeno effect, the efficiency of an IFM can be made arbitrarily close to unity. Here we report an onchip realization of the IFM based on silicon photonics. We exploit the inherent advantages of the lithographically written waveguides: excellent interferometric phase stability and mode matching, and obtain multipath interference with visibility above 98%. We achieved a normalized IFM efficiency up to 68.2%, which exceeds the 50% limit of the original IFM proposal.  [Show abstract] [Hide abstract]
ABSTRACT: Although interference is a classicalwave phenomenon, the superposition principle, which underlies interference of individual particles, is at the heart of quantum physics. An interactionfree measurements (IFM) harnesses the waveparticle duality of single photons to sense the presence of an object via the modification of the interference pattern, which can be accomplished even if the photon and the object haven't interacted with each other. By using the quantum Zeno effect, the efficiency of an IFM can be made arbitrarily close to unity. Here we report an onchip realization of the IFM based on silicon photonics. We exploit the inherent advantages of the lithographically written waveguides: excellent interferometric phase stability and mode matching, and obtain multipath interference with visibility above 98%. We achieved a normalized IFM efficiency up to 68.2%, which exceeds the 50% limit of the original IFM proposal.  [Show abstract] [Hide abstract]
ABSTRACT: Recently, various quantum simulators based on different quantum architectures are being successfully constructed, such as atoms, trapped ions, NMR, superconducting circuits, as well as single photons. For understanding molecular systems, quantum simulators are required to have very precise quantum control of the individual particles. Two main types of such quantum simulators are the analog and digital quantum simulators. Photons have the feature that they do not interact with anything easily, which provides the advantages and disadvantages for using them to realize quantum simulators. The advantageous side is a natural decoherencefree system that allows the implementations without complicated cryogenic experimental setups. Another advantage of photonic simulators is that photons are easily moved either in free space or waveguides and thus are not restricted to nearest–neighbor interaction. It has been proven that by using measurement induced interaction, one can circumvent the difficulty of photon–photon interaction and generate genuine manybody entangled photonic states.  [Show abstract] [Hide abstract]
ABSTRACT: Photonic quantum simulators are promising candidates for providing insight into other small to mediumsized quantum systems. Recent experiments have shown that photonic quantum systems have the advantage to exploit quantum interference for the quantum simulation of the ground state of Heisenberg spin systems. Here we experimentally characterize this quantum interference at a tuneable beam splitter and further investigate the measurementinduced interactions of a simulated fourspin system by comparing the entanglement dynamics using pairwise concurrence. We also study theoretically a foursite square lattice with nextnearest neighbor interactions and a sixsite checkerboard lattice, which might be in reach of current technology.  [Show abstract] [Hide abstract]
ABSTRACT: The counterintuitive features of quantum physics challenge many commonsense assumptions. In an interferometric quantum eraser experiment, one can actively choose whether or not to erase whichpath 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 delayedchoice 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 spacelike separated from the interference. Our setup employs hybrid pathpolarization entangled photon pairs, which are distributed over an optical fiber link of 55 m in one experiment, or over a freespace 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 wavelike behavior would depend on a causally disconnected choice. It is therefore suggestive to abandon such pictures altogether.  [Show abstract] [Hide abstract]
ABSTRACT: We present a highfidelity quantum teleportation experiment over a highloss freespace 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 freespace channel attenuation of 31 dB corresponds to the estimated attenuation regime for a downlink from a lowearthorbit satellite to a ground station. We also discussed various important technical issues for future experiments, including the dark counts of singlephoton detectors, coincidencewindow width etc. Our experiment tested the limit of performing quantum teleportation with stateoftheart resources. It is an important step towards future satellitebased quantum teleportation and paves the way for establishing a worldwide quantum communication network.  [Show abstract] [Hide abstract]
ABSTRACT: The quantum internet is predicted to be the nextgeneration information processing platform, promising secure communication and an exponential speedup 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 feedforward of the classical Bellstate 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 freespace link whose attenuation corresponds to the path between a satellite and a ground station. Here we report such an experiment, using active feedforward in real time. The experiment uses two freespace 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 frequencyuncorrelated polarizationentangled photon pair source, ultralownoise singlephoton detectors and entanglementassisted clock synchronization. The average teleported state fidelity is well beyond the classical limit of twothirds. Furthermore, we confirm the quality of the quantum teleportation procedure without feedforward by complete quantum process tomography. Our experiment verifies the maturity and applicability of such technologies in realworld scenarios, in particular for future satellitebased quantum teleportation.  [Show abstract] [Hide abstract]
ABSTRACT: Photonic quantum simulators are promising candidates for providing insight into other small to mediumsized 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 Heisenbergtype 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 fourphoton quantum system. We further show that photonic quantum simulations of generalized Heisenberg interactions on a foursite square lattice and a sixsite checkerboard lattice are in reach of current technology.  [Show abstract] [Hide abstract]
ABSTRACT: Quantum teleportation [1] is a quintessential prerequisite of many quantum information processing protocols [24]. By using quantum teleportation, one can circumvent the nocloning theorem [5] 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 [6] and of squeezed states [7], researchers have progressively extended the communication distance in teleportation, usually without active feedforward of the classical Bellstate measurement result which is an essential ingredient in future applications such as communication between quantum computers. Here we report the first longdistance quantum teleportation experiment with active feedforward 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 frequencyuncorrelated polarizationentangled photon pair source, ultralownoise singlephoton detectors, and entanglementassisted 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 feedforward) by complete quantum process tomography. Our experiment confirms the maturity and applicability of the involved technologies in realworld scenarios, and is a milestone towards future satellitebased quantum teleportation.  [Show abstract] [Hide abstract]
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 delayedchoice 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 measurementimplemented via a highspeed tunable bipartite state analyzer and a quantum random number generatoron two of the photons into the timelike 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". 
Article: A highspeed tunable beam splitter for feedforward photonic quantum information processing
[Show abstract] [Hide abstract]
ABSTRACT: We realize quantum gates for path qubits with a highspeed, polarizationindependent and tunable beam splitter. Two electrooptical modulators act in a MachZehnder interferometer as highspeed phase shifters and rapidly tune its splitting ratio. We test its performance with heralded single photons, observing a polarizationindependent interference contrast above 95%. The switching time is about 5.6 ns, and a maximal repetition rate is 2.5 MHz. We demonstrate tunable feedforward operations of a singlequbit gate of pathencoded qubits and a twoqubit gate via measurementinduced interaction between two photons.  [Show abstract] [Hide abstract]
ABSTRACT: Quantum simulators are controllable quantum systems that can reproduce the dynamics of the system of interest, which are typically unfeasible for classical computers. The recent developments of quantum technology enable the precise control of individual quantum particles as required for studying complex quantum systems. In particular, quantum simulators capable of simulating frustrated Heisenberg spin systems provide a platform for understanding exotic matter such as hightemperature superconductors. Here we report the analog quantum simulation of arbitrary Heisenbergtype interactions among four spin1/2 particles. This spin1/2 tetramer is the twodimensional archetype system whose ground state belongs to the class of valencebond states. Depending on the interaction strength, frustration within the system emerges such that the ground state evolves from a localized to a resonating valencebond state. This spin1/2 tetramer is created using the polarization states of four photons. The precise singleparticle addressability and a tunable measurementinduced interaction allows us to obtain fundamental insights into entanglement dynamics among individual particles by observing the frustration of entanglement, governed by quantum monogamy.  [Show abstract] [Hide abstract]
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 freedomofchoice loophole, also closing the latter within a reasonable set of assumptions. We also explain that the locality and freedomofchoice loopholes can be closed only within nondeterminism, i.e., in the context of stochastic local realism.  [Show abstract] [Hide abstract]
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 hightemperature superconductors. Here we report the analog quantum simulation of the groundstate wavefunction to probe arbitrary Heisenbergtype interactions among four spin1/2 particles . Depending on the interaction strength, frustration within the system emerges such that the ground state evolves from a localized to a resonating valencebond state. This spin1/2 tetramer is created using the polarization states of four photons. The singleparticle addressability and tunable measurementinduced interactions provide us insights into entanglement dynamics among individual particles. We directly extract groundstate energies and pairwise quantum correlations to observe the monogamy of entanglement.  [Show abstract] [Hide abstract]
ABSTRACT: An ondemand singlephoton source is a fundamental building block in quantum science and technology. We experimentally demonstrate the proof of concept for a scheme to generate ondemand single photons via actively multiplexing several heralded photons probabilistically produced from pulsed spontaneous parametric downconversions (SPDCs). By utilizing a fourphotonpair source, an active feedforward technique, and an ultrafast singlephoton router, we show a fourfold enhancement of the output photon rate. Simultaneously, we maintain the quality of the output singlephoton states, confirmed by correlation measurements. We also experimentally verify, via HongOuMandel 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 photonnumberresolving detectors. Our results show a route for ondemand singlephoton generation and the practical realization of scalable linear optical quantum information processing.  [Show abstract] [Hide abstract]
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 entanglementbased 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 entanglementbased QKD setups on a 144~km freespace 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 
Conference Paper: Hybrid Entanglement and a Nonlocal Quantum Eraser
[Show abstract] [Hide abstract]
ABSTRACT: We demonstrate for the first time hybrid entanglement of photon pairs generated 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. Polarization entangled photon pairs are created by spontaneous parametric down conversion. Then, a modified MachZehnder interferometer is used to convert the polarization DOF of one photon to a path DOF. For that photon, path superposition is analyzed and polarization superposition for its twin photon.  [Show abstract] [Hide abstract]
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 Sparameter 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.
Publication Stats
455  Citations  
138.60  Total Impact Points  
Top Journals
Institutions

2014

Yale University
New Haven, Connecticut, United States


20092014

Austrian Academy of Sciences
 Institute for Quantum Optics and Quantum Information  IQOQI Innsbruck
Wien, Vienna, Austria
