ScienceWeek

CONDENSED MATTER: ELECTRON SPIN AND QUANTUM COMPUTERS

The following points are made by Herbert A. Fertig (Science 2003 301:1335):

1) Perhaps the most prominent characteristic of the electron is that it carries an electric charge. Together with the rules of quantum mechanics, the electric forces between electrons and nuclei determine the chemical properties of atoms and molecules. Manipulation of electrons in semiconductors using electric forces is the principle behind the revolution in the electronics industry of the last few decades.

2) Another fundamental feature of electrons is their spin: The electron charge apparently spins like a top, endowing the electron with a magnetic dipole moment much like that of a bar magnet. Incorporating and exploiting this spin in microelectronic and optoelectronic applications is the central idea of "spintronics" (1). Several commercial applications of this idea already exist (most prominently in memory devices for computers). Many more have been proposed but require methods for producing and manipulating spin currents.

3) Unlike the charge, the spin of the electron is specified by a direction through its rotation axis. If one tries to measure the direction of this spin -- say, by passing the electrons through a magnetic field gradient -- one finds that the spin will point either "up" or "down"; the rules of quantum mechanics forbid any other result upon measurement. One could thus imagine using the spin as a bit in a computer, with the down-spin state representing 0 and the up-spin state representing 1.

4) However, quantum mechanics allows much richer possibilities than this. The electron spin can be in a state that is not just up or down, but that is a combination of the two. The full range of possibilities may be represented by an arrow directed toward any point on a "Bloch sphere". It is only upon measurement of the spin component along some direction that quantum mechanics allows only two possible results.

5) This richness of possible states makes the electron spin an ideal candidate for a "qubit", the basic component of the (as yet undeveloped) quantum computer. Quantum computers exploit the quantum dynamics of spins to vastly improve the speed of tasks such as Fourier transformation and factorization of large integers, which often cannot be performed by existing digital technology on reasonable time scales. Factorization in particular plays a key role in cryptographic schemes, and government security agencies around the world therefore have a keen interest in quantum computers.(2-5)

References (abridged):

1. S. A. Wolf et al., Science 294, 1488 (2001)

2. S. Murakami, N. Nagaosa, S.-C. Zhang, Science 301, 1348 (2003)

3. M. A. Nielsen, I. L. Chuang, Quantum Computation and Quantum Information (Cambridge Univ. Press, New York, 2000)

4. D. Loss, D. Vincenzo, Phys. Rev. A 57, 120 (1998)

5. I. Malajovich et al., Nature 411, 770 (2001)

Science http://www.sciencemag.org

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ON SPIN-BASED ELECTRONICS

The following points are made by S.A. Wolf et al (Science 2001 294:1488):

1) Until recently, the spin of the electron was ignored in mainstream charge-based electronics. A technology has emerged called "spintronics" (spin transport electronics or spin-based electronics), where it is not the electron charge but the electron spin that carries information, and this offers opportunities for a new generation of devices combining standard microelectronics with spin-dependent effects that arise from the interaction between spin of the carrier and the magnetic properties of the material.

2) Traditional approaches to using spin are based on the alignment of a spin (either "up" or "down") relative to a reference (an applied magnetic field or magnetization orientation of the ferromagnetic film). Device operations then proceed with some quantity (electrical current) that depends in a predictable way on the degree of alignment. Adding the spin degree of freedom to conventional semiconductor charge-based electronics or using the spin degree of freedom alone will add substantially more capability and performance to electronic products. The advantages of these new devices would be nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices.

3) Major challenges in this field of spintronics that are addressed by experiment and theory include the optimization of electron spin lifetimes, the detection of spin coherence in nanoscale structures, transport of spin-polarized carriers across relevant length scales and heterointerfaces, and the manipulation of both electron and nuclear spins on sufficiently fast time scales. In response, recent experiments suggest that the storage time of quantum information encoded in electron spins may be extended through their strong interplay with nuclear spins in the solid state. Moreover, optical methods for spin injection, detection, and manipulation have been developed that exploit the ability to precisely engineer the coupling between electron spin and optical photons.

4) The author describes a new paradigm of electronics based on the spin degree of freedom of the electron. Either adding the spin degree of freedom to conventional charge-based electronic devices or using the spin alone has the potential advantages of nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices. To successfully incorporate spins into existing semiconductor technology, one has to resolve technical issues such as efficient injection, transport, control and manipulation, and detection of spin polarization as well as spin-polarized currents. Recent advances in new materials engineering hold the promise of realizing spintronic devices in the near future.(1-5)

References (abridged):

1. M. Baibich, et al., Phys. Rev. Lett. 61, 2472 (1988)

2. J. Barnas, A. Fuss, R. Camley. P. Grunberg and W. Zinn, Phys. Rev. B 42, 8110 (1990)

3. G. Prinz, Science 282, 1660 (1998)

4. B. Dieny, et al., J. Appl. Phys. 69, 4774 (1991)

5. S. Parkin, D. Mauri, Phys. Rev. B 44 7131 (1991)

Science http://www.sciencemag.org

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ON SPINTRONICS AND QUANTUM COMPUTING

The following points are made by J.H. Smet et al (Nature 2002 415:281):

1) Discussions of future information-processing technologies often assign a prominent role to the spin degree of freedom in addition to (or instead of) the charge degree of freedom(1), which is exploited in today's mainstream electronics. In the short term, this "spintronics" may deliver products with enhanced functionality or improved performance, such as high-speed, high-density non-volatile random access memories: whereas on a much longer timescale, contributions to the very challenging realm of quantum computation(2,3) have been anticipated.

2) Quantum computation attempts to benefit from correlations and dissipationless transformations of coupled quantum-mechanical systems. The main incentive is a certain degree of parallelism that computational schemes based on such principles bring with them. For example, such schemes offer algorithms for prime factorization(4) and for exhaustive search(5); unlike any apparatus based on classical physics, a quantum computer should be able to solve these problems in polynomial time -- provided that it can be implemented in a real machine, as energy dissipation is a fundamental source of concern.

3) There has been a wide variety of proposals for practical implementations of rudimentary logic gates in which quantum memory registers -- based on any of the abundant two-level systems in physics, like spin-1/2 electrons and nuclei -- can be externally manipulated. These proposals range from trap configurations in atom or ion physics, to techniques of nuclear magnetic resonance spectroscopy as used in organic chemistry, to a very bold all-electronic approach for nuclear spin solid-state devices9 that would marry the merits of electronics fabrication technology with the virtues of quantum computation.

4) Many of the ideas produced by workers in the spintronics and quantum computing communities may be deemed far out of reach. But they have sparked efforts to develop new ways to accomplish the more fundamental task of controlling and measuring the nuclear spin polarization in solid-state devices, in view of the dearth of existing techniques for locally manipulating nuclear spins. Particularly appealing is the use of mobile objects, like conduction electrons in semiconductors, as mediators to both probe and modify nuclear spins. Gating and optical techniques are able to tailor precisely the population and energy distribution of such electrons, especially when they are constrained to move in two dimensions -- as in quantum wells or field-effect transistors -- or even fewer dimensions. The creation of non-equilibrium populations of spin-polarized electrons using coherent polarized light pulses or gating techniques have, for example, already enabled dynamical control of nuclear spins or the electronic generation of net nuclear spin polarization. Progress in this area will rely on experiments specifically geared towards expanding limited knowledge of controlled spin interactions and the microscopic interaction processes that take place between spin systems in such low-dimensional structures.

5) In summary: The authors report procedures that carry out the controlled transfer of spin angular momentum between electrons --confined to two dimensions and subjected to a perpendicular magnetic field -- and the nuclei of the host semiconductor, using gate voltages only. The authors demonstrate that the spin transfer rate can be enhanced near a ferromagnetic ground state of the electron system, and that the induced nuclear spin polarization can be subsequently stored and "read out". These techniques can also be combined into a spectroscopic tool to detect the low-energy collective excitations in the electron system that promote the spin transfer. The existence of such excitations is contingent on appropriate electron–electron correlations, and these can be tuned by changing, for example, the electron density via a gate voltage.

References (abridged):

1. Prinz, G. A. Magnetoelectronics. Science 282, 1660-1663 (1998)

2. Bennett, C. H. & DiVincenzo, D. P. Quantum information and computation. Nature 404, 247-255 (2000)

3. Steane, A. Quantum computing. Rep. Prog. Phys. 61, 117-173 (1998)

4. Ekert, A. & Jozsa, R. Quantum computation and Shor's factoring algorithm. Rev. Mod. Phys. 68, 733-753 (1996)

5. Grover, L. K. Quantum mechanics helps in searching for a needle in a haystack. Phys. Rev. Lett. 79, 325-328 (1997)

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