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. PHYSICS O 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
... e 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 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 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-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 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. A B Fiber channel + |0͘ |1͘ Yb + Yb + Fiber channel PMT PMT BS 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).