Teleportation Perspectives

PERSPECTIVES PHYSICS

Teleporting a Quantum State to Distant Matter

A quantum state is teleported between two atoms that are 1 meter apart through their entanglement with photons.

M. S. Kim and Jaeyoon Cho

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School of Mathematics and Physics, Queen’s University Belfast, BT7 1NN, UK. E-mail: [email protected] (M.S.K); [email protected] (J.C.)

comes to embody the quantum state of the original photon (or can be converted to it via a unitary transformation). However, the quantum teleportation of states between two matter systems at macroscopic distances was not to be realized for another 10 years. Quantum teleportation of electronic-level quantum states was initially performed between pairs of ions trapped in a harmonic potential a few micrometers in size (5, 6). This physical system may be useful for quantum gate operations between nonadjacent qubits, but the experimental principle could not be extended to long-distance teleportation because of the molecular dimensions of a harmonic potential within which entanglement generation and joint measurement can be performed. Spins, atoms, and ions are well suited for logical gate operations and storage, whereas photons are advantageous for long-distance communication. Thus, the development of

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ter, they interfered and exited together in one of the two output ports. However, when the two photons were distinguishable, they did not interfere. Beugnon et al. earlier reported a similar result, but the atoms were separated by only 6 µm (9). This so-called Hong-Ou-Mandel interferometer (10) works because of the quantum nature of single photons and is frequently used to prove the indistinguishability of two single photons. Experiments (7) and (8) are important building blocks for Olmschenk et al.’s teleportation (2). Matsukevich and Kuzmich noted that atom-photon coupling becomes stronger by increasing the number of atoms in a trap, and showed quantum state transfer between photons and atomic clouds (11). Earlier, a pair of nonclassically corrected photons was generated by an atomic cloud (12, 13). Quantum teleportation between photons, with the atomic cloud used as a memory, was successfully performed (14). This achievement was

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ne of the many paradoxes introduced by quantum mechanics is entanglement. When two or more objects are entangled, knowing the quantum states of individual objects separately does not enable us to know the whole system because of their strong correlation. Quantum information processing exploits these entangled states in applications such as computation and cryptography, and one of its most useful tools is teleportation (1), which transfers a quantum state between two systems in separate locations. On page 486 of this issue, Olmschenk et al. report the teleportation of a quantum state between two ytterbium ions (Yb+) that are separated by a distance of 1 m (2). Although photon states have been teleported over much longer distances (3, 4), the teleportation of quantum states of stationary particles with mass over macroscopic distances has important practical implications such as the simultaneous transfer and storage of a quantum state at a fixed remote place. Quantum teleportation requires two communication channels, one for classical information and the other for entangled quantum states, set up between the sender (“Alice”) and the receiver (“Bob”). Alice performs a joint measurement between her particle of their entangled pair and a particle prepared in a quantum bit or qubit—which, like a classical bit, has values of 0 and 1 but can also be in a superposition of two quantum states—which she wants to send to Bob. Upon Alice’s measurement, Bob’s particle is left in a quantum state that can be recovered with simple transformations as Alice’s qubit; Alice sends the results of her measurement (1) to Bob along the classical channel, then he knows if he has the correct qubit that Alice teleported, or must recover it with what is called a unitary transformation. In 1997, Bouwmeester et al. reported the first experimental realization of quantum teleportation for photon states (3). A pair of photons, which are entangled in their polarization states, was generated from a higher-energy photon through parametric down-conversion, and by the joint measurement of one of the pair and the original photon, the other photon

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Entanglement swaps with atoms. (Left) A laser pulse excites the electronic energy state (from the lower S levels to the upper P levels). A subsequently emitted photon in either blue or red is entangled with the atom. (Right) The two emitted photons are mixed on a beam splitter (BS). Teleportation is successful if a different-color photon is detected in each of the photomultiplier tubes (PMT).

atom-photon entanglement has been an important prerequisite to the transmission of quantum states over long distances between atoms by photonic qubit states. For example, Blinov et al. (7) connected the two polarization states of a photon with the two possible states of an atom after it emitted a photon. The same group later showed quantum interference of two single photons emitted from two Yb+ ions in their respective traps, which are a distance of 1 m apart (8). When two indistinguishable photons were sent into two input ports of a beam split-

then followed by the experimental proof of entangling two atomic clouds with photons as a mediator (15). Olmschenk et al. do not follow the standard teleportation protocol because of the difficulty in establishing an entangled channel. They use entanglement swapping instead, which makes the scheme versatile and efficient as it allows expansion into a series of quantum teleportation. In their experiment, two Yb+ ions are stored in independent traps that are separated by 1 m.

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PERSPECTIVES surement causes atom B to embody the initial quantum state α|0〉 + β|1〉 of atom A (again, via a unitary transformation). The authors confirmed the success of the quantum teleportation through full tomography of the atomic state. There are several advantages to Olmschenk et al.’s experimental approach. Single atoms in free space are used in both ends of the teleportation, so the state preparation and measurement are easy. The single atoms also ensure the emission of single photons. The use of photons for a quantum channel opens the possibility for teleportation to reach long distances. This experiment is an important step toward the realization of quantum repeaters with built-in memory (16), which is a key component in long-distance quantum communication. With the recent experimental advances, the theoretically presumed quantum paradoxes are slowly revolutionizing information technology.

ATMOSPHERE

References and Notes 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

C. H. Bennett et al., Phys. Rev. Lett. 70, 1895 (1993). S. Olmschenk, et al., Science 323, 486 (2009). D. Bouwmeester et al., Nature 390, 575 (1997). H. de Riedmatten et al., Phys. Rev. Lett. 92, 047904 (2004). M. Riebe et al., Nature 429, 734 (2004). M. D. Barrett et al., Nature 429, 737 (2004). B. B. Blinov, D. L. Moehring, L.-M. Duan, C. Monroe, Nature 428, 153 (2004). P. Maunz et al., Nat. Phys. 3, 538 (2007). J. Beugnon et al., Nature 440, 779 (2006). C. K. Hong, Z. Y. Ou, L. Mandel, Phys. Rev. Lett. 59, 2044 (1987). D. N. Matsukevich, A. Kuzmich, Science 306, 663 (2004). A. Kuzmich et al., Nature 423, 731 (2003). C. H. van der Wal et al., Science 301, 196 (2003). Y.-A. Chen et al., Nat. Phys. 4, 103 (2008). Z.-S. Yuan et al., Nature 454, 1098 (2008). L.-M. Duan, M. D. Lukin, J. I. Cirac, P. Zoller, Nature 414, 413 (2001). We acknowledge the UK Engineering and Physical Sciences Research Council and the UK Quantum Information Processing Interdisciplinary Research Centre at Oxford for financial support.

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The quantum state α|0〉 + β|1〉, composed of electronic energy levels, to be teleported is written onto atom A with a microwave pulse. Another microwave pulse prepares atom B in a definite superposition state |0〉 + |1〉. Next, an ultrafast laser pulse puts each atom into an excited state (see the figure, left panel). The excited atoms then emit a photon and return back to their initial states (either |0〉 or |1〉); the emitted photon’s energy depends on the electronic state in which the atom initially resided. The energy of the emitted photon becomes entangled with the atomic state. The two photons from the two atoms are jointly measured with a beam splitter, which mixes the two input photons (see the figure, right panel), and the atom-photon entanglement is transferred to interatom entanglement, which is the entanglement swap. Next, atom A is measured to yield 0 or 1, and the measurement outcome is sent to atom B. This final mea-

10.1126/science.1169279

Radiocarbon analysis elucidates the sources of the pollutants responsible for the “brown clouds” over South Asia.

Sources of Asian Haze Sönke Szidat

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Department of Chemistry and Biochemistry, University of Berne, Freiestrasse 3, 3012 Berne, Switzerland. E-mail: [email protected]

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Organic carbon

Black carbon and organic carbon Black carbon and organic carbon

BIOGENIC EMISSIONS

BIOMASS BURNING AEROSOL FORMATION

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Source apportionment. Radiocarbon analysis allows fossil and nonfossil sources of black carbon and organic carbon to be identified. [Adapted from (12)]

burning, submicrometer soot particles (consisting mostly of black carbon) are mainly produced in diesel engines and during heating with hard and brown coal. Organic carbon contains lighter, nonabsorptive organic compounds, which are directly emitted or formed by atmospheric oxidation of precursor gases (5, 6); both anthropogenic and biogenic sources are known. The consequences of high carbonaceousaerosol concentrations are especially severe in the case of the tropical atmospheric brown clouds (2). First, pollution from particulate matter is responsible for cardiovascular and respiratory diseases, inducing acute symptoms, chronic diseases, or even mortality (7, 8); this is intensified in South Asia due to

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high concentrations of black carbon, which is carcinogenic in addition to the other health effects (7). Second, the haze layer causes cooling and takes up air moisture persistently so that rain events become rarer during the dry season, but are intensified when they occur (2). Efforts to reduce the extent of atmospheric brown clouds require knowledge of their sources. Several approaches have been used to apportion sources and quantify emissions, but all have drawbacks (5, 6, 9). Emission-based inventories evaluate all potential emission processes of an atmospheric species on a local scale and extrapolate them to a larger scale (4). This estimation is reasonably accurate and precise for species with few different emission

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arbonaceous aerosols—that is, the carbon-containing aerosol fraction, such as soot—can affect both climate and human health, especially in regions where the atmosphere contains high levels of such particles. Yet, knowledge of the sources of the aerosols is limited. On page 495 of this issue, Gustafsson et al. (1) use radiocarbon (14C) analysis as an atmospheric tracer to quantify biomass and fossil-fuel contributions to the atmospheric “brown clouds” (2) over South Asia, a persistent and large-scale pollution layer of haze. The results resolve a discrepancy between measurements of other atmospheric tracers and calculations of emission-based inventories for carbonaceous aerosols. Typically, urban particulate matter comprises ~40% carbonaceous aerosol (total carbon) (3), which is traditionally divided into black carbon and organic carbon. Black carbon is optically absorptive and chemically little reactive. It is emitted during oxygen-deficient combustion of biomass and fossil fuels (see the figure) (4, 5). Biomass burning is dominated on a global scale by fires due to slash-and-burn land clearance, waste burning in agriculture and forestry, and residential wood combustion for heating and cooking. As for fossil-fuel