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ScienceWeek
2004 11 June C2 QUANTUM PHYSICS: ON ATOM-PHOTON ENTANGLEMENT
The following points are made by B.B. Blinov (Nature 2004 428:153):
1) An outstanding goal in quantum information science is the faithful mapping of quantum information between a stable quantum memory and a reliable quantum communication channel(1). This would allow, for example, quantum communication over remote distances(2), quantum teleportation(3) of matter, and distributed quantum computing over a "quantum internet".
2) Because quantum states cannot in general be copied, quantum information can only be distributed in these and other applications by entangling the quantum memory with the communication channel.
3) Atom-photon entanglement has been implicit in many previous experimental systems, from early measurements of Bell inequality violations in atomic cascade systems to fluorescence studies in trapped atomic ions. A prime example of current interest is strongly coupled cavity quantum electrodynamics, where individual atoms interact with photons in single-mode cavities(5). Another example is the continuous-variable entanglement between ensembles of atoms and light fields observed in systems containing macroscopic numbers of atoms and photons. However, atom-photon entanglement has not been directly observed in previous experiments, as the individual atoms and photons have not been under sufficient control.
4) The authors report quantum entanglement between an ideal quantum memory -- represented by a single trapped 111-Cd+ ion --and an ideal quantum communication channel provided by a single photon emitted spontaneously from the ion. Appropriate coincidence measurements between the quantum states of the photon polarization and the trapped ion memory are used to verify their entanglement directly. The direct observation of entanglement between stationary and "flying" qubits(4) is accomplished without using cavity quantum electrodynamic techniques(5) or prepared non-classical light sources(3). The authors suggest this source of entanglement may be used for a variety of quantum communication protocols(2), and for seeding large-scale entangled states of trapped ion qubits for scalable quantum computing.
References (abridged):
1. DiVincenzo, D. The physical implementation of quantum computation. Fortschr. Phys. 48, 771-783 (2000)
2. Duan, L.-M., Lukin, M., Cirac, J. I. & Zoller, P. Long-distance quantum communication with atomic ensembles and linear optics. Nature 414, 413-418 (2001)
3. Bouwmeester, D., Ekert, A. & Zeilinger, A. (eds) Quantum Cryptography, Quantum Teleportation, Quantum Computation (Springer, Springer, 2000)
4. Gheri, K., Ellinger, K., Pellizzari, T. & Zoller, P. Photon-wavepackets as flying quantum bits. Fortschr. Phys. 46, 401-415 (1998)
5. Haroche, S., Raimond, J. M. & Brune, M. in Experimental Quantum Computation and Information (eds de Martini, F. & Brune, M.) 3-36 (Proc. Int. School of Physics Enrico Fermi, course CXLVIII, IOS Press, Amsterdam, 2002)
Nature http://www.nature.com/nature
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ON QUANTUM ENTANGLEMENT
The following points are made by B.M. Terhal et al (Physics Today 2003 April):
1) Erwin Schroedinger (1887-1961) coined the word entanglement in 1935 in a three-part paper (Naturwiss. 1935 48:807; 49:823,844; Engl. trans.: Proc. Am. Philos. Soc. 1980 124:323) on the "present situation in quantum mechanics." His article was prompted by Albert Einstein, Boris Podolsky, and Nathan Rosen's now celebrated "EPR paper" that had raised fundamental questions about quantum mechanics earlier that year.
2) Einstein and his coauthors had recognized that quantum theory allows very particular correlations to exist between two physically distant parts of a quantum system; those correlations make it possible to predict the result of a measurement on one part of a system by looking at the distant part. On that basis, the EPR paper argued that the distant predicted quantity should have a definite value even before being measured if the theory were to claim completeness and respect locality. However, because quantum mechanics disallows such definite values prior to measuring, the EPR authors concluded that, from a classical perspective, quantum theory must be incomplete.
3) Schroedinger's 1935 perspective comes closer to the modern view: The wavefunction or state vector gives us all the information that we can have about a quantum system. About entangled quantum states, he wrote, "The whole is in a definite state, the parts taken individually are not," which we now understand as the essence of pure-state entanglement. In that same 1935 article, Schroedinger also introduced his famous cat as an extreme illustration of entanglement: A cat physically isolated in a box with a decaying atom and vial of cyanide represents a quantum state having macroscopic degrees of freedom. If the atom were to decay and trigger the release of cyanide, the cat would die. The quantum-mechanical description of the system is a coherent superposition of one state in which the atom is still excited and the cat alive, and another state in which the atom has decayed and the cat is dead. The isolated cat-trigger-atom-cyanide system as a whole is in a definite entangled state, even though the cat itself exists as a probabilistic mixture of being alive or dead.
4) For the three decades following the 1935 articles, the debate about entanglement and the "EPR dilemma" -- how to make sense of the presumably nonlocal effect one particle's measurement has on another -- was philosophical in nature, and for many physicists it was nothing more than that. The 1964 publication (J.S. Bell: Physics 1964 1:195) by John Bell changed that situation dramatically. Bell derived correlation inequalities that can be violated in quantum mechanics but have to be satisfied within every model that is local and complete -- so-called local hidden-variable models. Bell's work made it possible to test whether local hidden-variable models can account for observed physical phenomena. Early and ongoing recent experiments showing violations of such Bell inequalities have invalidated local hidden-variable models and lend support to the quantum-mechanical view of nature. In particular, an observed violation of a Bell inequality demonstrates the presence of entanglement in a quantum system.
Physics Today http://www.physicstoday.org
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Notes by ScienceWeek:
A "hidden variables theory" is one of a class of physical theories which deny that the quantum state of a physical system is a complete specification. The hidden variables are those components of the hypothetical complete state that are not contained in the quantum state.
"Bell's inequality", formulated by John Bell (1928-1990) in 1964, is one of a family of inequalities concerning the probabilities of joint occurrence of certain events in two well-separated parts of a composite system, the inequality implied by any hidden variables theory that satisfies an appropriate locality condition. In this context, in general, a locality condition is a condition such that no interaction between two entities can occur in less time than the time required for light to travel from one entity to the other. For example, any apparent instantaneous effect of one entity upon the other entity implies locality is not obeyed.
"Bell's theorem" is the theorem that no hidden variables theory satisfying an appropriate locality condition can make statistical predictions in complete agreement with those of quantum mechanics. In other words, there are situations in which quantum mechanics predicts a violation of Bell's inequality. Another formulation is that any hidden variables theory that forbids instantaneous interactions cannot make predictions in complete agreement with those of quantum mechanics.
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QUANTUM COMPUTING: IONS, PHOTONS, AND COMMUNICATION LINKS
The following points are made by Eugene Polzik (Nature 2004 428:129):
1) The initial proposal(1) for a quantum computer by Ignacio Cirac and Peter Zoller in 1995 has since been followed up by a train of theoretical and experimental breakthroughs, which last year arrived at the demonstration of elementary quantum logic gates using trapped ions(2,3). Recently, Blinov et al(4) reported the first observation of entanglement between a trapped ion and light -- a significant step towards building a quantum network.
2) Entanglement is a quantum correlation between various parts of a system and is required for processing quantum information. The quantum logic gates(2,3) involving trapped ions were built with short-range entanglement, over only a few micrometers, created by electrical interaction between the ions. Such short-range interaction is not suitable for linking distant nodes of a quantum computer, let alone a large-scale network of such computers. Quantum networks should be linked with light, which is the best long-distance carrier of information, be it classical or quantum.
3) In a classical computer, bits of information are physically implemented as charges on tiny capacitors, and can take two distinct values, usually denoted 0 and 1. Quantum mechanics, however, allows for a superposition of states. This superposition, called a quantum bit or "qubit", dramatically enhances computing and communication capabilities. More specifically, in the work of Blinov et al(4), a qubit formed by a trapped ion can exist in a superposition of two different orientations of its magnetic momentum, say "up" and "down", or +1 and -1. The trick by which Blinov et al(4) entangled an ion and a photon was to bring the ion into this superposition of states through the emission of the photon.
4) According to the conservation of angular momentum, if the ion is created in a spin-down state, the emitted photon is circularly polarized (a property of its electric-field vector) in the right-hand direction. Similarly, if the ion is created in a spin-up state, the emitted photon has left-hand circular polarization. Most importantly, if the ion ends up in an unknown superposition of spin-up and spin-down states, the emitted photon has a complementary superposition state. This is the essence of entanglement of two qubits.
5) Such an entangled state has been generated before, most often between two photons, but also for two ions(2,3), two atoms(5) or an atom and a microwave photon in a cavity(5). In fact, entanglement between atoms and light has been hinted at in a variety of experiments: for example, in the quantum correlations in light emitted by atomic ensembles; in work on spin squeezing and the entanglement of atomic ensembles; and in early experiments on Bell inequalities, in which two photons were emitted by a single atom.
6) The real breakthrough achieved by Blinov et al.4 is that, for the first time, entanglement has been observed between a stationary computational qubit (a trapped ion) and a "flying" communication qubit (an optical photon). The emitted photon can carry a unique piece of information about the state of the ion over a long distance. Another advantage of the system is that trapped ions have exceptionally long lifetimes in entangled states. Although in this experiment the lifetime of demonstrated entanglement did not exceed a microsecond, it can potentially be increased by many orders of magnitude.
References (abridged):
1. Cirac, J. I. & Zoller, P. Phys. Rev. Lett. 74, 4091-4094 (1995)
2. Liebfried, D. et al. Nature 422, 412-415 (2003)
3. Schmidt-Kaler, F. et al. Nature 422, 408-411 (2003)
4. Blinov, B. B., Moehring, D. L., Duan, L. -M. & Monroe, C. Nature 428, 153-157 (2004)
5. Rauschenbeutel, A. et al. Science 288, 2024-2028 (2000)
Nature http://www.nature.com/nature
ScienceWeek http://scienceweek.com
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