ScienceWeek

QUANTUM INFORMATION: ON STORING SINGLE PHOTONS

The following points are made by Philippe Grangier (Nature 2005 438:749):

1) The basic unit of quantum information, the quantum bit or qubit, can be encoded in various physical quantities, such as the polarization states of photons, or the spin states of atomic nuclei. To make qubits practically useful, random coupling of them with the external world -- an effect known as decoherence --must at all costs be avoided or corrected. This makes photons (the quanta of light) particularly suitable for qubit transmission, as they can travel over very long distances with very little decoherence. For qubit storage, encoders such as atoms come into their own: they can be kept in "traps" for long periods, again avoiding deleterious decoherence effects from outside.

2) New work[1,2] contrives to combine the two crucial aspects of transport and storage: the experiments generate a single photon on demand, catch it and store it in a remote atomic memory, and release it some time later. The advance is potentially highly significant for the field of quantum cryptography, also known as quantum key distribution (QKD). This emerging technology promises absolutely secure transmission of the key codes that are essential to decipher any encrypted message.

3) Previous advances in quantum key distribution have owed much to the fact that photons that are used to encode the keys are very good qubit carriers: apart from maintaining a robust quantum state throughout transmission, they can be detected efficiently and with low levels of noise. But light signals cannot -- whether viewed classically or quantum-mechanically -- propagate over infinite distances in optical fibers. They are in fact dampened exponentially with distance: by a factor of two over 15 kilometers, and by a factor of a hundred over 100 kilometers. In classical optical telecommunications, this problem is solved by using simple, readily available devices known as repeaters, which can amplify and reshape the transmitted signal.

4) But a good classical repeater is no use in the quantum regime: it is much too noisy, and creates so many errors that any quantum key being transmitted would not survive. To put the problem in more quantum-mechanical terms, a classical repeater breaks down quantum entanglement. This delicate phenomenon is associated with very strong, non-classical correlations between the states of two widely separated qubits, and is a crucial element of all quantum communication schemes: in effect, it allows any useful qubit to be "teleported" directly to its destination, avoiding transmission losses[3].

5) So quantum communication must re-invent the repeater concept, using quantum hardware that preserves coherence. This is feasible in principle[4]: a quantum repeater would be nothing more than a small quantum processor. The exact number of qubits that would have to be stored and processed in such a repeater to ensure high-fidelity quantum communication over thousands of kilometers is an open issue. But it is likely to be in the range of tens or hundreds much lower than the number required for a fully fledged quantum computer.[5]

References (abridged):

1. Chanelière, T. et al. Nature 438, 833 836 (2005)

2. Eisaman, M. D. et al. Nature 438, 837 841 (2005)

3. Bennett, C. H. et al. Phys. Rev. Lett. 70, 1895 (1993)

4. Briegel, H.-J. , Dür, W. , Cirac, J. I. & Zoller, P. Phys. Rev. Lett. 81, 5932 5935 (1998)

5. Duan, L.-M. , Lukin, M. , Cirac, J. I. & Zoller, P. Nature 414, 413 418 (2001)

Nature http://www.nature.com/nature

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QUANTUM COMPUTING: ON QUBITS

The following points are made by L.I. Glazman and R.C. Ashoori (Science 2004 304:524):

1) Exploiting quantum mechanics as a tool for computation may allow entirely new ways of performing a number of difficult computing tasks, such as drastically cutting the time necessary for factoring a number by its primes, the cornerstone of modern cryptography. At the heart of quantum computing is the concept of a "qubit" (quantum bit), which requires a whole new set of physical devices to replace the "classical" devices implementing the conventional bit operations. A conventional bit may reside in one of two states at a time. In contrast with binary logic, the qubit must be capable of residing in a superposition of two states. The element of a quantum computation consists of a quantum-mechanical evolution of such a superposition. As with the bits in the conventional computer, the qubits must act together, requiring controllable interactions between them.(1)

2) Building devices to store and process qubits is a challenging problem. In a typical field-effect transistor in a computer chip, 10,000 to 100,000 electrons participate in a single switching event. It is impossible to isolate, out of such a complex system, two quantum mechanical states that would evolve coherently to play the role of a qubit. Just over a decade ago, physicists first learned to measure the movements of single electrons in semiconductor structures that isolate electrons into small "quantum dots". In such "single-electron transistors", the electrical conductance depends strongly on the position of a single electron. However, the same factors that make single-electron detection simple also complicate construction of a quantum computer based on sensing an electron's position. Charged electrons are easily jostled by stray electric fields, and electrons placed in delicate entangled quantum states rapidly lose quantum coherence.

3) Aside from its charge, an electron carries with it an elementary magnetic moment characterized by its spin. There are two quantized states for the spin, and an electron may exist in a superposition of the two. The electron's spin is much less perturbed than its position, allowing for long coherence times (2) and raising hopes of building an electron-spin-based quantum computer. Although it is much harder to measure the state of a single electron's spin, progress in this direction has been reported recently (2).

4) Qubit connections pose another central problem in moving toward spin-based quantum computing. Computation requires the means to control the interaction between the spins of distant electrons, each working as qubits. Craig et al (1) have demonstrated a controllable interaction between spins localized in two quantum dots that need not be adjacent to each other. Understanding their measurement involves several key notions, in particular the use of quantum dots as "artificial atoms" and how these "atoms" interact.(3-5)

References (abridged):

1. N. J. Craig et al., Science 304, 565 (2004)

2. R. Hanson et al., Phys. Rev. Lett. 91, 196802 (2003)

3. L. P. Kouwenhoven, C. M. Marcus, Phys. World 11, 35 (June 1998)

4. J. M. Ziman, Principles of the Theory of Solids (Cambridge University Press, New York, 1995)

5. L. P. Kouwenhoven, L. I. Glazman, Phys. World 14, 33 (January 2001)

Science http://www.sciencemag.org

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QUANTUM INFORMATION: ON QUANTUM MEMORY

The following points are made by Jean-Michel Raimond (Nature 2004 432:453):

1) Most of the information we get through the World Wide Web travels encoded on inch-long laser pulses rushing at light-speed down hair-thin glass fibers many thousands of leagues under the seas. One day, that information might be coded onto the quantum properties of these pulses, the weird rules of quantum logic opening a wealth of new possibilities(1). Processing this information would require that it be copied from the light onto motionless objects, to be stored. But quantum states are fragile, and copying them is not easy. In a step towards the realization of a quantum-information network, Julsgaard et al(2) have demonstrated such a quantum memory, in which the state of a faint laser pulse is faithfully copied onto an ensemble of atoms.

2) The power and strangeness of the quantum arise from a few striking properties. Quantum systems can be in a superposition of states, allowing quantum bits (qubits) to take two logical values at once. Quantum states cannot be cloned -- a copy operation inevitably destroys the original. This is a key point for quantum cryptography(3), as an eavesdropper cannot access quantum information without revealing his presence. Quantum systems can be entangled, forming a single entity whatever the physical distance between them, and these weird correlations are used for teleportation(4) -- a quantum "fax machine" that transmits a quantum state independently of the particle that carries it.

3) Light quanta (photons) or faint laser pulses are excellent carriers of quantum information. They travel unaffected over long distances, are easily read out in detectors, and hence have been thoroughly exploited for quantum tests(5), cryptography(3), and teleportation(4). For many purposes, however, their main quality -- that they travel at the speed of light -- is an inconvenience, as they cannot be stored for any extended time; the best optical resonators available so far can store photons for a few tens of microseconds only. It is thus rather difficult to process the quantum information that light carries in a quantum way. We need quantum memories.

4) One natural approach, when it comes to single photons, is to map them onto the state of a single atom, the interface being provided by a resonant cavity, for instance. Atom-photon information-exchange experiments have already been realized, but at the expense of using quite complex experimental techniques. Moving to the many-photon case, mesoscopic light pulses can be "stopped" in their tracks: the light velocity is reduced to mere meters per second, even to zero, in an atomic medium that has an extraordinarily large index of refraction. The light field can then be mapped onto an atomic excitation and can be retrieved later (milliseconds later in real situations). But so far these experiments, although spectacular, have involved rather intense pulses of light, whose quantum properties are not apparent.

References (abridged):

1. Bennett, C. H. & Di Vincenzo, D. P. Nature 404, 247-255 (2000)

2. Julsgaard, B., Sherson, J., Cirac, J. I., Fiurcek, J. & Polzik, E. S. Nature 432, 482-486 (2004)

3. Gisin, H., Ribordy, G., Tittel, W. & Zbinden, H. Rev. Mod. Phys. 74, 145-195 (2002)

4. Bouwmeester, D. et al. Nature 390, 575-579 (1997)

5. Zeilinger, A. Rev. Mod. Phys. 71, S288-S297 (1999)

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