"Quantum Electronics" section would cover more recent findings that leads to fascinating electrical systems of future. Also the quantum theoretical basis of many bulk electrical propoerties. For example :- cavity QED, ultra fast lasers, photonic crystals, spintronics, quantum optics, plasmonics, organic electronics etc.
We encourage you to post anything related to "Quantum Electronics" that you feel would be instructive, informative, fun, encouraging or anything that you feel would be technicaly interesting for electronics enthusiasts. Please feel free to post web links, journal articles, magzine links, news articles, product announcements, company links or anything on similar lines.
Please keep this forum technical in nature.
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Nano's Success Depends On A Rock Solid Foundation by Roger Allan, Electronic Design, August 4, 2005
Building a strong infrastructure and using selected silicon methods helps bridge the gap to the mass manufacture of nano devices.
Nanotechnology is at a nexus in its evolution: It needs a viable infrastructure to support the high-rate manufacture of nano-scale devices, particularly devices grown atom by atom and molecule by molecule, as envisioned by the technology's original proponents. Read entire article ...
Posted by: Nitin Mohan on September 13, 2005 08:34 AMA great benefit of studying at a university, as opposed to an institute with limited focus, is that a student can acquire a well-rounded education rather than simply "training".
With that idea as a backdrop, permit me to suggest budding nanotechnologists at BHUIT to also consider societal aspects of future developments in nanotechnology. Please look at
http://www.esm.psu.edu/~axl4/lakhtakia/ALNanoTechReport2004.pdf
for a brief report.
I suggest also that BHUIT-EcE students should invite professors from physics (perhaps, Prof. ON Srivastava), philosophy, and history, in addition to BHUIT, for a Saturday Symposium on Nanotechnologies.
Posted by: Akhlesh Lakhtakia on January 3, 2005 03:05 AMCarbon Nanotubes Research Collaboration between Rensselaer Polytechnic Institute and Banaras Hindu University
Source:American Chemical Society
Onkar N. Srivastava, a physics professor at Banaras Hindu University, in India, and Pulickel M. Ajayan, a materials science professor at Rensselaer Polytechnic Institute, Troy, N.Y., collaborated to create the nanotube-based structures.
Read More
R&D eyes novel, nonvolatile memories at nanoscale by Chappell Brown, EE Times
Hancock, N.H. - Nanotechnology has the ability to yield a memory chip technology that could replace everything from magnetic disks to DRAMs-at least in theory. R&D projects based on such techniques as carbon nanotubes, molecular electronics and atomic force microscope probe arrays appear to be converging on a universal memory technology and, while still highly speculative, have attracted substantial funding. Read entire article ...
Nanocell used for molecular logic, memory
IBM stores terabits of memory on a single chip
Mark Lundstrom's Summer School on Nano-MOS
Nano-MOS Resources
Nano-Hub
Online Resources for Nanotechnology - live simulations, seminars, courses, etc.
Posted by: Mukul Agrawal on August 8, 2004 12:40 AMExperimental demonstration of five-photon entanglement and open-destination teleportation
Source: Nature 430, 54 - 58 (01 July 2004)
Quantum-mechanical entanglement of three or four particles has been achieved experimentally, and has been used to demonstrate the extreme contradiction between quantum mechanics and local realism. However, the realization of five-particle entanglement remains an experimental challenge. The ability to manipulate the entanglement of five or more particles is required for universal quantum error correction. Another key process in distributed quantum information processing, similar to encoding and decoding, is a teleportation protocol that we term 'open-destination' teleportation. An unknown quantum state of a single particle is teleported onto a superposition of N particles; at a later stage, this teleported state can be read out (for further applications) at any of the N particles, by a projection measurement on the remaining particles. Here we report a proof-of-principle demonstration of five-photon entanglement and open-destination teleportation (for N = 3). In the experiment, we use two entangled photon pairs to generate a four-photon entangled state, which is then combined with a single-photon state. Our experimental methods can be used for investigations of measurement-based quantum computation and multi-party quantum communication.
Quantum Cryptography Experiment Using a Single-Photon Source
Source NTT, Japan
Reference E. Waks et al., Nature 420 (2002) 762.
Quantum cryptography, which guarantees unconditional security based on the law of quantum mechanics, is extensively studied. It is a system that provides a secret key for ciphering messages to two legitimate parties, utilizing the fact that non-orthogonal quantum states cannot be fully identified by an eavesdropper. Several experiments have been demonstrated, using attenuated laser light with an average photon number of, for example, 0.1 per pulse. However, the photon number statistics of laser light follows the Poisson distribution, and there is a finite probability of two photons in one pulse, from which an eavesdropper can steal a part of information. It is known that the amount of leaked information is larger for larger transmission loss. Thus, experiments using a single-photon source, that emits no more than two photons per pulse, are desired for long-distance systems. We successfully demonstrated a quantum cryptography experiment using a single-photon source for the first time.
The photon source was an InAs semiconductor quantum dot fabricated at Stanford University. In order to extract photons efficiently, the dot was embedded in a post-shaped microcavity (Fig. 1). When illuminated by pump pulses, it emitted one photon per pulse via spontaneous emission between particular energy levels. The emission efficiency was 6 %, and the probability that it emitted two photons per pulse was one tenth of conventional laser light. Using this light source, we carried out an quantum cryptography experiment based on the uncertainty among four polarization states (called BB84), and obtained desired correlation between the transmitter and receiver with an error rate of 2.5 %. Then, applying error correction and privacy amplification to these data, we successfully created an unconditionally secured secret key for ciphering messages. The final key creation rate was 25 kbit/s. We also carried out the same experiment for various channel losses between the transmitter and receiver both for our single-photon source and a laser source (Fig. 2). The results showed that, though the key creation rate was lower in the low loss region because of the low emission efficiency, the single-photon source could achieve longer transmission distance than laser light.
Carbon Nanotube Transistor (Nano-FET) is 'Better Than Silicon'
Source: NanotechWeb.org
Researchers at Stanford University, Cornell University and Purdue University in the US have produced a carbon nanotube transistor that they claim has better properties than silicon transistors of an equivalent size. The device uses zirconium oxide rather than silicon dioxide, which has a lower dielectric constant, as the gate insulator.
Nanotube transistor specialists
Transistor team
High-dielectric-constant materials are useful as gate insulators as they can provide efficient charge injection into transistor channels and reduce direct-tunnelling leakage currents.
"Using high-K dielectrics as gate insulators for molecular electronics is an obvious step to take to realize the full potential of molecule-based devices," Stanford scientist Hongjie Dai told nanotechweb.org. "We achieved the highest performance carbon nanotube field-effect transistors made to date by integrating zirconia gate insulators. For instance, 70 mV/decade subthreshold swings are obtained - approaching the theoretical limit for transistors."
The scientists used semiconducting single-walled nanotubes (SWNTs) to make p-type field-effect-transistors (FETs). They formed the zirconia gate insulators by atomic layer deposition, creating zirconia films of about 8 nm thick. Fortunately, the process did not significantly degrade key transistor performance parameters of the nanotubes, such as mobility.
The team converted p-type ZrO2/SWNT-FETs to n-type transistors by heating them in molecular hydrogen at 400°C for one hour. The properties of the n-type transistors, although good, were not as ideal as the p-type FETs. The researchers also made a NOT logic gate, i.e. an inverter, by connecting a p- and n-type ZrO2/SWNT-FET. The device had a high voltage gain.
Now Dai says that the researchers, who reported their work in Nature Materials, are working to improve device performance and exploring the fundamental limit.
About the author
Liz Kalaugher is editor of nanotechweb.org.
Spintronics Goes Organic
Author :- Belle Dumé
Source :- Physics Web
Physicists have moved a step closer to creating a new generation of
"spintronic" devices that exploit the spin of electrons as well as their charge.
Jing Shi and colleagues at the University of Utah in the US have made the
first organic "spin valve" - a device that changes resistance depending on
the applied magnetic field. Previous spin valves were made from metals or
insulators (Z Xiong et al. 2004 Nature 427 821).
A spin valve consists of a thin layer of metal or insulator sandwiched
between two ferromagnetic electrodes. The spin of the electrons passing through
the device can be flipped by an external magnetic field, which changes the
resistance of the two ferromagnetic layers. This effect, known as magnetoresistance,
has already been used to make highly sensitive magnetic-recording devices
and memory chips.
Extending these spin-dependent effects to semiconductor materials has,
however, proved difficult. Shi and co-workers have now built a spin valve
with a 100 nanometre thick organic semiconductor made from aluminium and
hydroxyquinoline. The semiconductor was sandwiched between a layer of cobalt
and an alloy of lanthanum, strontium and magnesium (see figure).
To test their device, the Utah team first calculated the current that
flowed through the semiconductor when the two electrodes were magnetized in
the same direction - or parallel - and then in opposite directions - or anti-parallel.
Shi and colleagues found that that the current increased by as much as 40%
when the magnetization of the electrodes was switched from anti-parallel
to parallel. This constitutes giant magnetoresistance.
At present, the device only works at low temperatures - between about
-260°C to about -40°C - but Shi's team says that the experiment is
"a proof of concept that sets the stage for more practical applications".
The long-term aim is to make the device work at room temperature. The group
believes that organic semiconductors have many advantages over conventional
semiconductors, such as those made from silicon. They are simpler to make,
are flexible and their resistance can be tuned by doping.
Posted by: Mukul Agrawal on June 30, 2004 03:44 PM
Spintronics: Microelectronic devices that function by using the spin of the electron are a nascent multibillion-dollar industry--and may lead to quantum microchips
Authors:- David D. Awschalom, Michael E. Flatté and Nitin Samarth
Source:- Sientific American
Following is pretty long but very intersting article for any new comer by one of the pioneers of Spintronics :- David Awschalom of Santa Barbra University.
As rapid progress in the miniaturization of semiconductor electronic devices leads toward chip features smaller than 100 nanometers in size, device engineers and physicists are inevitably faced with the looming presence of quantum mechanics--that counterintuitive and sometimes mysterious realm of physics wherein wavelike properties dominate the behavior of electrons. Pragmatists in the semiconductor device world are busy conjuring up ingenious ways to avoid the quantum world by redesigning the semiconductor chip within the context of "classical" electronics [see "A Vertical Leap for Microchips," by Thomas H. Lee; Scientific American, January]. Yet some of us believe that we are being offered an unprecedented opportunity to define a radically new class of device that would exploit the idiosyncrasies of the quantum world to provide unique advantages over existing information technologies.
One such idiosyncrasy is a quantum property of the electron known as spin, which is closely related to magnetism. Devices that rely on an electron's spin to perform their functions form the foundation of spintronics (short for spin-based electronics), also known as magnetoelectronics.
Information-processing technology has thus far relied on purely charge-based devices--ranging from the now quaint vacuum tube to today's million-transistor microchips. Those conventional electronic devices move electric charges around, ignoring the spin that tags along for the ride on each electron.
Possible Solutions
Magnetism (and hence electron spin) has nonetheless always been important for information storage. For instance, even the earliest computer hard drives used magnetoresistance--a change in electrical resistance caused by a magnetic field--to read data stored in magnetic domains. It is no surprise that the information storage industry has provided the initial successes in spintronics technology. Most laptop computers now come fitted with high-capacity hard drives that pack an unprecedented amount of data into each square millimeter. The drives rely on a spintronic effect, giant magnetoresistance (GMR), to read such dense data.
More sophisticated storage technologies based on spintronics are already at an advanced stage: in the next few years, MRAM (magnetic random-access memory), a new type of computer memory, will go on the market. MRAMs would retain their state even when the power was turned off, but unlike present forms of nonvolatile memory, they would have switching rates and rewritability challenging those of conventional RAM.
In today's read heads and MRAMs, key features are made of ferromagnetic metallic alloys. Such metal-based devices make up the first--and most mature--of three categories of spintronics. In the second category, the spin-polarized currents flow in semiconductors instead of metals. Achieving practical spintronics in semiconductors would allow a wealth of existing microelectronics techniques to be co-opted and would also unleash many more types of devices made possible by semiconductors' high-quality optical properties and their ability to amplify both optical and electrical signals. Examples include ultrafast switches and fully programmable all-spintronics microprocessors. This avenue of research may lead to a new class of multifunctional electronics that combine logic, storage and communications on a single chip.
Researchers must answer several major questions before the second category of devices can take off as a viable industry: Can we devise economic ways to combine ferromagnetic metals and semiconductors in integrated circuits? Can we make semiconductors that are ferromagnetic at room temperature? What is an efficient way to inject spin-polarized currents, or spin currents, into a semiconductor? What happens to spin currents at boundaries between different semiconductors? How long can a spin current retain its polarization in a semiconductor?
Our own research groups are working on these questions but are keeping one eye also on the more distant and speculative quarry that is the third category of devices: ones that manipulate the quantum spin states of individual electrons. This category includes spintronic quantum logic gates that would enable construction of large-scale quantum computers, which would extravagantly surpass standard computers for certain tasks. A diverse assortment of exotic technologies is aimed toward that goal: ions in magnetic traps, "frozen" light, ultracold quantum gases called Bose-Einstein condensates and nuclear magnetic resonance of molecules in liquids--there are many ways to skin a quantum cat.
We believe it makes sense instead to build on the extensive foundations of conventional electronic semiconductor technology. Indeed, a recent series of unexpected discoveries appears to support our hunch that semiconductor spintronics provides a feasible path for developing quantum computers and other quantum information machines. Whether one looks at the near term for tomorrow's consumer electronics or at the more distant prospect of quantum computing, spintronics promises to be revolutionary.
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Exploiting Spin Currents
An intuitive notion of how an electron's spin works is suggested by the name itself. Imagine a small electrically charged sphere that is spinning rapidly. The circulating charges on the sphere amount to tiny loops of electric current, which create a magnetic field similar to the earth's magnetic field. Scientists traditionally depict the rotation by a vector, or arrow, that points along the sphere's axis of rotation. Immersing the spinning sphere in an external magnetic field changes its total energy according to how its spin vector is aligned with the field.
In some ways, an electron is just like such a spinning sphere of charge--an electron has a quantity of angular momentum (its "spin") and an associated magnetism, and in an ambient magnetic field its energy depends on how its spin vector is oriented. But there the similarities end and the quantum peculiarities begin. Electrons seem to be ideal dimensionless points, not little spheres, so the simple picture of their spin arising from actual rotation doesn't work. In addition, every electron has exactly the same amount of spin, equal to one half the fundamental quantum unit of angular momentum. That property is hardwired into the mathematics that describes all the elementary particles of matter, a result whose significance and meaning are another story entirely. The bottom line is that the spin, along with a mass and a charge, is a defining characteristic of an electron.
In an ordinary electric current, the spins point at random and play no role in determining the resistance of a wire or the amplification of a transistor circuit. Spintronic devices, in contrast, rely on differences in the transport of "spin up" and "spin down" electrons. In a ferromagnet, such as iron or cobalt, the spins of certain electrons on neighboring atoms tend to line up. In a strongly magnetized piece of iron, this alignment extends throughout much of the metal. When a current passes through the ferromagnet, electrons of one spin direction tend to be obstructed. The result is a spin-polarized current in which all the electron spins point in the other direction.
A ferromagnet can even affect the flow of a current in a nearby nonmagnetic metal. For example, present-day read heads in computer hard drives use a device dubbed a spin valve, wherein a layer of a nonmagnetic metal is sandwiched between two ferromagnetic metallic layers. The magnetization of the first layer is fixed, or pinned, but the second ferromagnetic layer is not. As the read head travels along a track of data on a computer disk, the small magnetic fields of the recorded 1's and 0's change the second layer's magnetization back and forth, parallel or antiparallel to the magnetization of the pinned layer. In the parallel case, only electrons that are oriented in the favored direction flow through the conductor easily. In the antiparallel case, all electrons are impeded. The resulting changes in the current allow GMR read heads to detect weaker fields than their predecessors, so that data can be stored using more tightly packed magnetized spots on a disk, increasing storage densities by a factor of three.
Our own research groups are working on these questions but are keeping one eye also on the more distant and speculative quarry that is the third category of devices: ones that manipulate the quantum spin states of individual electrons. This category includes spintronic quantum logic gates that would enable construction of large-scale quantum computers, which would extravagantly surpass standard computers for certain tasks. A diverse assortment of exotic technologies is aimed toward that goal: ions in magnetic traps, "frozen" light, ultracold quantum gases called Bose-Einstein condensates and nuclear magnetic resonance of molecules in liquids--there are many ways to skin a quantum cat.
We believe it makes sense instead to build on the extensive foundations of conventional electronic semiconductor technology. Indeed, a recent series of unexpected discoveries appears to support our hunch that semiconductor spintronics provides a feasible path for developing quantum computers and other quantum information machines. Whether one looks at the near term for tomorrow's consumer electronics or at the more distant prospect of quantum computing, spintronics promises to be revolutionary.
Exploiting Spin Currents
An intuitive notion of how an electron's spin works is suggested by the name itself. Imagine a small electrically charged sphere that is spinning rapidly. The circulating charges on the sphere amount to tiny loops of electric current, which create a magnetic field similar to the earth's magnetic field. Scientists traditionally depict the rotation by a vector, or arrow, that points along the sphere's axis of rotation. Immersing the spinning sphere in an external magnetic field changes its total energy according to how its spin vector is aligned with the field.
In some ways, an electron is just like such a spinning sphere of charge--an electron has a quantity of angular momentum (its "spin") and an associated magnetism, and in an ambient magnetic field its energy depends on how its spin vector is oriented. But there the similarities end and the quantum peculiarities begin. Electrons seem to be ideal dimensionless points, not little spheres, so the simple picture of their spin arising from actual rotation doesn't work. In addition, every electron has exactly the same amount of spin, equal to one half the fundamental quantum unit of angular momentum. That property is hardwired into the mathematics that describes all the elementary particles of matter, a result whose significance and meaning are another story entirely. The bottom line is that the spin, along with a mass and a charge, is a defining characteristic of an electron.
In an ordinary electric current, the spins point at random and play no role in determining the resistance of a wire or the amplification of a transistor circuit. Spintronic devices, in contrast, rely on differences in the transport of "spin up" and "spin down" electrons. In a ferromagnet, such as iron or cobalt, the spins of certain electrons on neighboring atoms tend to line up. In a strongly magnetized piece of iron, this alignment extends throughout much of the metal. When a current passes through the ferromagnet, electrons of one spin direction tend to be obstructed. The result is a spin-polarized current in which all the electron spins point in the other direction.
A ferromagnet can even affect the flow of a current in a nearby nonmagnetic metal. For example, present-day read heads in computer hard drives use a device dubbed a spin valve, wherein a layer of a nonmagnetic metal is sandwiched between two ferromagnetic metallic layers. The magnetization of the first layer is fixed, or pinned, but the second ferromagnetic layer is not. As the read head travels along a track of data on a computer disk, the small magnetic fields of the recorded 1's and 0's change the second layer's magnetization back and forth, parallel or antiparallel to the magnetization of the pinned layer. In the parallel case, only electrons that are oriented in the favored direction flow through the conductor easily. In the antiparallel case, all electrons are impeded. The resulting changes in the current allow GMR read heads to detect weaker fields than their predecessors, so that data can be stored using more tightly packed magnetized spots on a disk, increasing storage densities by a factor of three.
Another three-layered device, the magnetic tunnel junction, has a thin insulating layer between two metallic ferromagnets. Current flows through the device by the process of quantum tunneling: a small number of electrons manage to jump through the barrier even though they are forbidden to be in the insulator. The tunneling current is obstructed when the two ferromagnetic layers have opposite orientations and is allowed when their orientations are the same.
Magnetic tunnel junctions form the basis of the MRAM chips mentioned earlier. Each junction can store one bit of data in the orientation of its unpinned ferromagnetic layer. That layer retains its magnetic state whether the power is on or off, at least until it is deliberately rewritten again.
Whereas the metallic spintronic devices just described provide new ways to store information, semiconductor spintronics may offer even more interesting possibilities. Because conventional semiconductors are not ferromagnetic, one might wonder how semiconductor spintronic devices can work at all. One solution employs a ferromagnetic metal to inject a spin-polarized current into a semiconductor.
In 1990 Supriyo Datta and Biswajit A. Das, then at Purdue University, proposed a design for a spin-polarized field-effect transistor, or spin FET. In a conventional FET, a narrow semiconductor channel runs between two electrodes named the source and the drain. When voltage is applied to the gate electrode, which is above the channel, the resulting electric field drives electrons out of the channel (for instance), turning the channel into an insulator. The Datta-Das spin FET has a ferromagnetic source and drain so that the current flowing into the channel is spin-polarized. When a voltage is applied to the gate, the spins rotate as they pass through the channel and the drain rejects these antialigned electrons.
A spin FET would have several advantages over a conventional FET. Flipping an electron's spin takes much less energy and can be done much faster than pushing an electron out of the channel. One can also imagine changing the orientation of the source or drain with a magnetic field, introducing an additional type of control that is not possible with a conventional FET: logic gates whose functions can be changed on the fly.
As yet, however, no one has succeeded in making a working prototype of the Datta-Das spin FET because of difficulties in efficiently injecting spin currents from a ferromagnetic metal into a semiconductor. Although this remains a controversial subject, recent optical experiments carried out at various laboratories around the world indicate that efficient spin injection into semiconductors can indeed be achieved by using unconventional materials, called magnetic semiconductors, that incorporate magnetism by doping the semiconductor crystals with atoms such as manganese.
Some magnetic semiconductors have been engineered to show ferromagnetism, providing a spintronic component called a gateable ferromagnet, which may one day play an important role in spin transistors. In this device, a small voltage would switch the semiconductor between nonmagnetic and ferromagnetic states. A gateable ferromagnet could in turn be used as a spin filter--a device that, when switched on, passes one spin state but impedes the other.
The filtering effect could be amplified by placing the ferromagnet in a resonant tunnel diode. Conventional resonant tunnel diodes allow currents to flow at a specific voltage, one at which the electrons have an energy that is resonant with the tunneling barrier. The version incorporating a ferromagnet would have a barrier with different resonant voltages for up and down spins.
The most exciting developments in semiconductor spintronics will probably be devices we have not imagined yet. A key research question for this second category of spintronics is how well electrons can maintain a specific spin state when traveling through a semiconductor or crossing from one material to another. For instance, a spin FET will not work unless the electrons remain polarized on entering the channel and after traveling to its far end.
Another three-layered device, the magnetic tunnel junction, has a thin insulating layer between two metallic ferromagnets. Current flows through the device by the process of quantum tunneling: a small number of electrons manage to jump through the barrier even though they are forbidden to be in the insulator. The tunneling current is obstructed when the two ferromagnetic layers have opposite orientations and is allowed when their orientations are the same.
Magnetic tunnel junctions form the basis of the MRAM chips mentioned earlier. Each junction can store one bit of data in the orientation of its unpinned ferromagnetic layer. That layer retains its magnetic state whether the power is on or off, at least until it is deliberately rewritten again.
Whereas the metallic spintronic devices just described provide new ways to store information, semiconductor spintronics may offer even more interesting possibilities. Because conventional semiconductors are not ferromagnetic, one might wonder how semiconductor spintronic devices can work at all. One solution employs a ferromagnetic metal to inject a spin-polarized current into a semiconductor.
In 1990 Supriyo Datta and Biswajit A. Das, then at Purdue University, proposed a design for a spin-polarized field-effect transistor, or spin FET. In a conventional FET, a narrow semiconductor channel runs between two electrodes named the source and the drain. When voltage is applied to the gate electrode, which is above the channel, the resulting electric field drives electrons out of the channel (for instance), turning the channel into an insulator. The Datta-Das spin FET has a ferromagnetic source and drain so that the current flowing into the channel is spin-polarized. When a voltage is applied to the gate, the spins rotate as they pass through the channel and the drain rejects these antialigned electrons.
A spin FET would have several advantages over a conventional FET. Flipping an electron's spin takes much less energy and can be done much faster than pushing an electron out of the channel. One can also imagine changing the orientation of the source or drain with a magnetic field, introducing an additional type of control that is not possible with a conventional FET: logic gates whose functions can be changed on the fly.
As yet, however, no one has succeeded in making a working prototype of the Datta-Das spin FET because of difficulties in efficiently injecting spin currents from a ferromagnetic metal into a semiconductor. Although this remains a controversial subject, recent optical experiments carried out at various laboratories around the world indicate that efficient spin injection into semiconductors can indeed be achieved by using unconventional materials, called magnetic semiconductors, that incorporate magnetism by doping the semiconductor crystals with atoms such as manganese.
Some magnetic semiconductors have been engineered to show ferromagnetism, providing a spintronic component called a gateable ferromagnet, which may one day play an important role in spin transistors. In this device, a small voltage would switch the semiconductor between nonmagnetic and ferromagnetic states. A gateable ferromagnet could in turn be used as a spin filter--a device that, when switched on, passes one spin state but impedes the other.
The filtering effect could be amplified by placing the ferromagnet in a resonant tunnel diode. Conventional resonant tunnel diodes allow currents to flow at a specific voltage, one at which the electrons have an energy that is resonant with the tunneling barrier. The version incorporating a ferromagnet would have a barrier with different resonant voltages for up and down spins.
The most exciting developments in semiconductor spintronics will probably be devices we have not imagined yet. A key research question for this second category of spintronics is how well electrons can maintain a specific spin state when traveling through a semiconductor or crossing from one material to another. For instance, a spin FET will not work unless the electrons remain polarized on entering the channel and after traveling to its far end.
The question of how fast spin polarization decays becomes all the more acute if one is to build a quantum computer based on electron spins. That application requires control over a property known as quantum coherence, in essence the pure quantum nature of all the computer's data-carrying components. Quantum data in semiconductors based on the charges of electrons tend to lose coherence, or dissipate, in mere picoseconds, even at cryogenic temperatures. Quantum data based on spin should be intrinsically more sturdy. Curiously enough, our research groups stumbled on significant basic results regarding coherent electron spins while doing experiments aimed at developing practical magnetic semiconductors.
A Pleasant Surprise
In 1997 at the University of California at Santa Barbara, we were experimenting on zinc selenide (ZnSe), a long-studied conventional semiconductor. The ZnSe was meant to be a control for an ongoing project studying magnetic semiconductors. In our experiment we used pulses of circularly polarized light to excite pools of electrons in the ZnSe into identical spin states. In a circularly polarized light wave, instead of oscillating in intensity, the electric and magnetic fields rotate in a circle, transverse to the direction of the light.
We sent the ultrashort (100-femtosecond) pulses horizontally through the semiconductor, exciting electrons into horizontal spin states, initially aligned with the light beam. In a vertical ambient magnetic field the electron spins precess--the direction of each electron's spin vector rotates in the horizontal plane, similar to how a tilted gyroscope precesses in the earth's gravitational field. The precession enables us to monitor how long these states remain coherent, but the horizontal spin state has another, more important property.
For a baseball, say, horizontal spinning is nothing special and is quite distinct from the two vertical modes of spinning. For electrons, however, the horizontal quantum spin states are actually coherent superpositions of the spin-up and spin-down states. In effect, such electrons are in both the up and the down state at the same time. This is precisely the kind of coherent superposition of states employed by quantum computers.
Each electron spin can represent a bit; for instance, a 1 for spin up and a 0 for spin down. With conventional computers, engineers go to great lengths to ensure that bits remain in stable, well-defined states. A quantum computer, in contrast, relies on encoding information within quantum bits, or qubits, each of which can exist in a superposition of 0 and 1. By having a large number of qubits in superpositions of alternative states, a quantum computer intrinsically contains a massive parallelism so that quantum algorithms can operate on many different numbers simultaneously. Unfortunately, in most physical systems, interactions with the surrounding environment rapidly disrupt these superposition states. A typical disruption would effectively change a superposition of 0 and 1 randomly into either a 0 or a 1, a process called decoherence. State-of-the-art qubits based on the charge of electrons in a semiconductor remain coherent for a few picoseconds at best--and only at temperatures too low for practical applications. The rapid decoherence occurs because the electric force between charges is strong and long range. In traditional semiconductor devices, this strong interaction is beneficial, permitting delicate control of current flow with small electric fields. To quantum coherent devices, however, it is anathema.
Electron-spin qubits interact only weakly with the environment surrounding them, principally through magnetic fields that are nonuniform in space or changing in time. Such fields can be effectively shielded. The goal of our experiment was to create some of these coherent spin states in a semiconductor to see how long they could survive. The results are also useful for understanding how to design devices such as spin transistors that do not depend on maintaining and detecting the quantum coherence of an individual electron's spin.
Our experiment measured the decoherence rate by monitoring the precession of the spins. Each electron would continue precessing as long as its superposition remained coherent. We used weak pulses of light to monitor the precession, in effect obtaining stroboscopic images of the spin dynamics. As the electrons precessed, the measured signal oscillated, in magnitude; as coherence was lost, the amplitude of the oscillations fell to zero.
Much to our surprise, the optically excited spin states in ZnSe remained coherent for several nanoseconds at low temperatures--1,000 times as long as charge-based qubits. The states even survived for a few nanoseconds at room temperature. Subsequent studies of electrons in gallium arsenide (GaAs, a high-quality semiconductor commonly used in everyday applications such as cellular phones and CD players) have shown that, under optimal conditions, spin coherence in a semiconductor could last hundreds of nanoseconds at low temperatures.
Hazards of Holes
These experiments also revealed characteristics that are crucial for attaining long spin coherence times. Of primary importance is the nature of the carriers of spin and charge. A semiconductor has two key bands of states that can be occupied by electrons: a valence band, which is usually full, and (at a slightly higher energy) a conduction band, which is usually empty. Charge carriers in semiconductors come in two flavors: conduction electrons, which are electrons in the conduction band, and valence holes, which are electrons missing from the valence band. The holes carry a spin because in a filled valence band all the spins cancel out: the removal of one electron leaves a net spin imbalance in the same way that it leaves behind a net positive charge.
Holes have dramatically shorter spin coherence times than electrons, and spin exchange between electrons and holes is very efficient, accelerating the decoherence of both. For these reasons, it pays to have no hole carriers, a condition that is achieved by using n-doped semiconductor crystals, which are doped to have some excess electrons in the conduction band without any corresponding valence holes.
When holes have been eliminated, the dominant remaining source of decoherence comes from a relativistic effect: a body moving at high speed through an electric field sees the field partially transformed into a magnetic field. For an electron moving in a semiconductor, the crystal structure of the material provides the electric field. The spin of a fast-moving electron precesses around the resulting local magnetic field that it sees. In each of our ensembles, the 10 billion or so excited electrons have a range of velocities and therefore precess in a variety of ways. Two electron spins that start off parallel can soon evolve to point in opposite directions. As this misalignment among the electrons grows, the average spin polarization of the population diminishes, which our experiment measures as loss of coherence. This population-based origin of decoherence holds forth the hope that the spin coherence times of individual electrons may turn out to greatly exceed even the remarkably long times seen in ensembles.
Spinning into the Future
In conjunction with the carrier's lifetime, two other properties are crucial for semiconductor applications: how far excitations can be transported and how fast the state of a device can be manipulated. Macroscopic spin transport was first demonstrated in n-doped gallium arsenide. A laser pulse excited a "puddle" of coherently precessing electrons, much as in the lifetime experiments, but then a lateral electric field dragged the electrons through the crystal. Spin packets traveled more than 100 microns (a distance far exceeding the feature sizes in contemporary microelectronics) with only moderate loss of spin polarization. Recent experiments have successfully driven coherent spins across complex interfaces between semiconductor crystals of different composition (for instance, from GaAs into ZnSe). A wealth of semiconductor applications, from lasers to transistors, are based on heterostructures, which combine disparate materials. The same design techniques can be brought to bear on spintronics.
Further advances toward quantum information processing have also taken place. For example, 150-femtosecond laser pulses have been used to tilt coherent electron spins, demonstrating that such spins can, in principle, be manipulated thousands of times before their coherence is lost. Meanwhile researchers with nearer-term goals have taken strides in making new magnetic semiconductors, which may at last open the door to practical spin transistors. On every front, the spintronics revolution is racing ahead and will continue to generate technologies that would be inconceivable in a nonquantum world.
Single Photon Emission Prepares Way for Quantum Cryptography. Experiment Also Enable Alternative Approach to Quantum Computation Using Photons as Qubits
Researchers at the University of California at Santa Barbara (UCSB) report in the Dec. 22 issue of Science that they have built a device from which the emission of a single photon (particle of light) can be repeatedly detected. The ability to produce a single photon prepares the way for a whole new approach to communicating information secretly such that the information is unconditionally secure.
In addition to cryptography, the results also pertain more generally to the use of semiconductor quantum dots for quantum computation.
Quantum cryptography differs from other code schemes in that the attempt by a third party to intercept a code's key itself alters the key. It is as if the very act of listening in on a conversation makes the eavesdropper known.
Let us say that Alice wants to send a secret message to Bob. In order for the message to be secret, Alice has to employ some scheme to encode the message. And Bob needs a key to the scheme to decode the message. The crucial communication for the sake of preserving secrecy is not the message, but the key.
So Alice sends a string of single photons whose polarization successively contain the key. If a third party, Eve, tries to detect the singly transmitted photons, the act of detection causes an irreversible change in the wave function of the system. ("Wave function" denotes the quantum mechanical state of a physical system.) If Eve then tries to send the key on to Bob, the key will, in effect, bear the imprint of Eve's intermediate detection.
"Because measurements unavoidably modify the state of a single quantum system, an eavesdropper cannot gather information about the secret key without being noticed, provided that the pulses used in transmission do not contain two or more photons," state the researchers in the Science article.
Why is the single photon so important?
If the pulse that Alice sends contains two photons identically polarized (i.e., rotating the same way), Eve can in principle use a beam splitter to channel one of the photons into her detector while Bob receives the other photon. But if there is one and only one photon, it will have to choose in the presence of a beam splitter between Eve's and Bob's detectors. It is the singleness of the photon that guarantees security.
Though such a simplified example of quantum encryption may not seem convincingly foolproof, establishing a key with a single photon emission has been shown by Peter Shor of AT&T and John Preskill of Caltech to be secure from the most advanced attacks.
The eight UCSB researchers have built a device that produces that one photon emission upon which quantum encryption depends. Three of the authors of the Science article, "A Quantum Dot Single-Photon Turnstile Device," are professors: Atac Imamoglu of Electrical and Computer Engineering (ECE) and of Physics, and Pierre Petroff and Evelyn Hu, both of ECE and Materials. The other authors have been either postdoctoral fellows (including first author Peter Michler) or graduate students associated with one of the faculty members' laboratories.
The device itself is mushroom-shaped and made out of semiconducting materials.
The first step was to grow a block of semiconducting materials layer by layer. Petroff's postdoc Winston Schoenfeld made the layered material using the technique molecular beam epitaxy or MBE. The base is a substrate of Gallium Arsenide. The post is made out of Aluminum Gallium Arsenide, and the ultra-thin mushroom cap or microdisk (200 nanometers thick) contains quantum dots of Indium Arsenide embedded in Gallium Arsenide.
A semiconductor quantum dot is a nanoscale box in which charge carriers are confined. These carriers can be electrons, holes (missing electrons), or excitons (bound electron-hole pairs). The carriers are confined because the band gap of the quantum dot material is lower than the band gap of the semiconducting material surrounding the quantum dot.
"Band gap" is the energy required to raise an electron from the valence band of the semiconductor crystal to its conduction band. With quantum dots, the higher band gap material of the surrounding material presents a potential barrier to the motion of the carriers out of the box. (Petroff developed the techniques to self-assemble quantum dots in 1993. He holds the patent.)
Hu and her graduate student Lidong Zhang took the block of layered material made by Petroff and Schoenfeld and shaped it into the device. "When we pattern and etch," said Hu, "we make use of the properties of the materials such that one etchant will work on one material and not another, and that ’s how we can form the post without attacking the rest of the material."
The structure of the device--ultra-thin microdisk atop post--is so important because that shape enables the removal of most of the material surrounding the quantum dots. Less material surrounding the quantum dots means less material to contribute contaminating background radiation. The researchers did not knowingly choose the microdisk design in order deliberately to limit background radiation, but discovered that the disk design did so.
"People have made microdisks before," said Hu. "What's new here is combining that device with the quantum dot and with a knowledge of what to look for."
Imamoglu, his postdocs Michler and Christopher Becher, and his graduate student Alper Kiraz knew what to look for. They took the device, tailor-made by Petroff and Hu to make this measurement of a single photon emission, and actually made the measurements. Specifically, what they found when they used laser pulses to load the quantum dots with energy is a pattern of subsequent emission without peaking. The lack of peaking indicates the singleness of photon emission.
"It's like God saying," quipped Petroff, "Let there be a photon, and there is a photon."
Imamoglu clarifies the nature of the measurement. It is not that only one photon is emitted from a quantum dot, but that among several photons emitted is one particular kind of photon. "The strong confinement enabled by the quantum dot structure ensures that the photons emitted are different, and we only look at the photon that is emitted at the lowest transition energy when the last electron-hole pair recombines."
One disadvantage of the microdisk for quantum cryptography is its lack of preferential output. But Imamoglu figures that some other shape, perhaps elliptical instead of circular, will cure that problem.
What really intrigues Imamoglu is the relevance of single photon emission to quantum computing. The idea behind quantum computing is to use particle spin as the quantum-bits or qubits analogous to the zero and one binary code of electronic computing.
Much of the theorizing about quantum computing has focused on the use of the spin states of electrons. But in a soon to be published paper, computer scientist Many Knill and physicist Raymond LaFlamme of Los Alamos National Laboratory and Gerald Milburn of Queensland University in Australia show that quantum computing can be done with linear optical elements. The one missing piece, say the authors, is the availability of a single photon emission source. And that, of course, is what the UC Santa Barbara researchers provide in this Science article.
Now, Imamoglu points out, instead of thinking about quantum computing strictly in terms of electron spin states as the qubits, there is an alternate approach which envisions the polarization of light as the qubits.
What is the advantage of using the nano structure in quantum dots as a means of generating photon qubits? "Speed and ease," said Imamoglu. "This all optical approach allows for a much faster way of doing real quantum operations. And photons are easy to deal with. We have a better chance of implementing a two-bit operation with this alternative scheme."
Reference:- Science; 2000 Dec 22;290(5500):2282-5.
Authors:-Michler P, Kiraz A, Becher C, Schoenfeld WV, Petroff PM, Zhang L, Hu E, Imamoglu A
Contacts: Professor Imamoglu can be reached in Istanbul, Turkey, by phone at 011-90-216-308-3482 or by e-mail at atac@ece.ucsb.edu. Professor Petroff can be reached at (805) 893-8256. Professor Hu can be reached at 805-893-2368.
Posted by: Mukul Agrawal on May 28, 2004 01:55 PMInstitute of Technology, Banaras Hindu University
Varanasi 221005 INDIA