A HIERARCHICAL PRIORITY ENCODER

A normal priority encoder encodes only the highest-order data line. But in many situations, not only the highest but the second-highest priority information is also needed. The circuit presented here encodes both the highest-priority information as well as the second-highest priority information of an 8-line incoming data. The circuit uses the standard octal priority encoder 74148 that is an 8-line-to-3-line (4-2-1) binary encoder with active-‘low’ data inputs and outputs. The first encoder (IC1) generates the highest-priority value, say, F. The active- ‘low’ output (A0, A1, A2) of IC1 is inverted by gates N9 through N11 and fed to a 3-line-to-8-line decoder (74138) that requires active-‘high’ inputs. The decoded outputs are active-‘low’. The decoder identifies the highest-priority data line and that data value is cancelled using XNOR gates (N1 through N8) to retain the second- highest priority value that is generated by the second encoder. To understand the logic, let the incoming data lines be denoted as L0 to L7. Lp is the highest-priority line (active-‘low’) and Lq the second highest priority line (active-‘low’). Thus Lp=0 and Lq=0. All lines above Lp and also between Lp and Lq (denoted as Lj) are at logic 1. All lines below Lq logic state are irrelevant, i.e. ‘don’t care’. Here p is the highest-priority value and q the second-highest-priority value. (Obviously, q has to be lower than p, and the minimum possible value for p is taken as ‘1’.) Priority encoder IC1 generates binary output F2, F1, F0, which represents the value of p in active-‘low’ format. The complemented F2, F1, and F0 are applied to 3-line-to-8-line (one out of eight outputs is active-‘low’) decoder 74138. Let the output lines of 74138 be denoted as M0 through M7. Now only one line is active-‘low’ among M0 through M7, and that is Mp (where the value of p is explained as above). Therefore the logic level of line Mp is ‘0’ and that of all other M lines ‘1’. The highest-priority line is cancelled using eight XNOR gates as shown in the figure. Let the output lines from XNOR gates be N0 through N7. Consider inputs Lp and Mp of the corresponding XNOR gate. Since Mp = 0 and also Lp = 0, the output of this XNOR gate is Np = complement of Lp = 1. All other L’s are not changed because the corresponding M’s are all 1’s. Thus data lines N0 through N7 are same as L0 through L7, except that the highest-priority level in L0 through L7 is cancelled in N0 through N7. The highest-priority level in N0 through N7 is the second-highest priority Left over from L0 through L7, i.e. Nq=0 and Nj=1 for q

For example, let L0 through L7 = X X X 0 1 1 0 1. Here the highest ‘0’ line is L6 and the next highest is L3 (X denotes ‘don’t care’). Thus p=6 and q=3. Now the active-‘low’ output of the first priority encoder will be F2 F1 F0 = 0 0 1. The input to 74138 is 1 1 0 and it outputs M0 through M7 = 1 1 1 1 1 1 0 1. Since M6=0, only L6 is complemented by XNOR gates. Thus the outputs of XNORs are N0 through N7 = X X X 0 1 1 1 1. Now N3=0 and the highest priority for ‘N’ is 3. This value is recovered by priority encoder 2 (IC3) as S2 S1 S0 = 1 0 0.

Quantum Computing Taps Nucleus of Single Atom

A team of Australian engineers at the University of New South Wales (UNSW) has demonstrated a quantum bit based on the nucleus of a single atom in silicon, promising dramatic improvements for data processing in ultra-powerful quantum computers of the future. Quantum bits, or qubits, are the building blocks of quantum computers, which will offer enormous advantages for searching expansive databases, cracking modern encryption, and modelling atomic-scale systems such as biological molecules and drugs.
The world-first result, to be published in Nature on April 18, brings these machines one-step closer, describing how information was stored and retrieved using the magnetic spin of a nucleus.
"We have adapted magnetic resonance technology, commonly known for its application in chemical analysis and MRI scans, to control and read-out the nuclear spin of a single atom in real time," says Associate Professor Andrea Morello from the School of Electrical Engineering and Telecommunications at UNSW.
The nucleus of a phosphorus atom is an extremely weak magnet, which can point along two natural directions, either "up" or "down." In the strange quantum world, the magnet can exist in both states simultaneously -- a feature known as quantum superposition.
The natural positions are equivalent to the "zero" and "one" of a binary code, as used in existing classical computers. In this experiment, the researchers controlled the direction of the nucleus, in effect "writing" a value onto its spin, and then "reading" the value out -- turning the nucleus into a functioning qubit.
"We achieved a read-out fidelity of 99.8 per cent, which sets a new benchmark for qubit accuracy in solid-state devices," says UNSW Scientia Professor Andrew Dzurak, who is also Director of the Australian National Fabrication Facility at UNSW, where the devices were made.
The accuracy of the UNSW team's nuclear spin qubit rivals what many consider to be today's best quantum bit -- a single atom in an electromagnetic trap inside a vacuum chamber. The development of this "Ion Trap" technology was awarded the 2012 Nobel Prize in physics.
"Our nuclear spin qubit operates at a similar level of accuracy, but it's not in a vacuum chamber -- it's in a silicon chip that can be wired up and operated electrically like normal integrated circuits," says Morello. "Silicon is the dominant material in the microelectronics industry, which means our qubit is more compatible with existing industry technology and is more easily scaleable."
Morello's PhD student Jarryd Pla is the lead experimental author of the work, which was conducted in collaboration with the groups led by Dzurak and Professor David Jamieson at the University of Melbourne. Morello, Dzurak and Jamieson are all Program Managers in the ARC Centre of Excellence for Quantum Computation and Communication Technology.
In September 2012, the same UNSW team reported in Nature the first functional quantum bit based on an electron bound to a phosphorus atom embedded in silicon, "writing" information onto its spin and then "reading" the spin state back out.
With their latest result, the team has dug even deeper into the atomic structure to manipulate and measure the spin of its nucleus. This is the core of an atom, containing most of its mass, but its diameter is only about one-millionth that of the atom's diameter.
"This means it's more challenging to measure, but it's almost completely immune to disturbances from the outside world, which makes it an exceptional quantum bit," says UNSW engineering PhD student Jarryd Pla. "Our nuclear spin qubit can store information for longer times and with greater accuracy. This will greatly enhance our ability to carry out complex quantum calculations once we put many of these qubits together."
Electron spin qubits will likely act as the main "processor" bits for quantum computers of the future, coupled with other electrons to perform calculations. But nuclear spin qubits could also be integrated and could provide a useful memory function or help implement two-bit logic gates between the electronic qubits, the researchers say.
Demonstrating quantum memories and two-qubit logic gates is the main focus of the UNSW team for the near future. They are also exploring ways of improving the accuracy of their nuclear and electron spin qubits even further, by moving to a purer form of silicon.

Super-Nanotubes: 'Remarkable' Spray-On Coating Combines Carbon Nanotubes With Ceramic

Researchers from the National Institute of Standards and Technology (NIST) and Kansas State University have demonstrated a spray-on mixture of carbon nanotubes and ceramic that has unprecedented ability to resist damage while absorbing laser light. Coatings that absorb as much of the energy of high-powered lasers as possible without breaking down are essential for optical power detectors that measure the output of such lasers, which are used, for example, in military equipment for defusing unexploded mines. The new material improves on NIST's earlier version of a spray-on nanotube coating for optical power detectors and has already attracted industry interest.

"It really is remarkable material," NIST co-author John Lehman says. "It's a way to make super-nanotubes. It has the optical, thermal and electrical properties of nanotubes with the robustness of the high-temperature ceramic."

The composite was developed by Kansas State. NIST researchers suggested using toluene to uniformly coat individual nanotubes with a ceramic shell. They also performed damage studies showing how well the composite tolerates exposure to laser light. NIST has developed and maintained optical power standards for decades. In recent years, NIST researchers have coated optical detectors with nanotubes because of their unusual combination of desirable properties, including intense black color for maximum light absorption.

The new composite consists of multiwall carbon nanotubes and a ceramic made of silicon, boron, carbon and nitrogen. Boron boosts the temperature at which the material breaks down. The nanotubes were dispersed in toluene, to which a clear liquid polymer containing boron was added drop by drop, and the mixture was heated to 1,100 degrees C. The resulting composite was then crushed into a fine powder, dispersed in toluene, and sprayed in a thin coat on copper surfaces. Researchers baked the test specimens and then exposed them to a far-infrared laser beam of the type used to cut hard materials.

Analysis revealed that the coating absorbed 97.5 percent of the light and tolerated 15 kilowatts of laser power per square centimeter for 10 seconds. This is about 50 percent higher damage tolerance than other research groups have reported for similar coatings -- such as nanotubes alone and carbon paint -- tested with the same wavelength of light, according to the paper. The nanotubes and graphene-like carbon absorb light uniformly and transmit heat well, while the oxidation-resistant ceramic boosts damage resistance. The spray-on material also adheres well to the copper surface. As an added bonus, the composite can be produced easily in large quantities.

After light exposure, the coatings were analyzed using several different techniques. Electron microscopy revealed no major destruction such as burning or deformation. Other tests showed the coating to be adaptable, with the ceramic shell partially oxidizing into a stable layer of silicon dioxide (quartz).

40-Metre Direct Conversion Receiver

Using the circuit of direct-conversion receiver described here, one can listen to amateur radio QSO signals in CW as well as in SSB mode in the 40-metre band. The circuit makes use of three n-channel FETs (BFW10). The first FET (T1) performs the function of ant./RF amplifier-cum-product detector, while the second and third FETs (T2 and T3) together form a VFO (variable frequency oscillator) whose output is injected into the gate of first FET (T1) through 10pF capacitor C16. The VFO is tuned to a frequency which differs from the incoming CW signal frequency by about 1 kHz to produce a beat frequency note in the audio range at the output of transformer X1, which is an audio driver transformer of the type used in transistor radios. The audio output from transformer X1 is connected to the input of audio amplifier built around IC1 (TBA820M) via volume control VR1. An audio output from the AF amplifier is connected to an 8-ohm, 1-watt speaker. The receiver can be powered by a 12-volt power-supply, capable of sourcing around 250mA current. Audio output stage can be substituted with a readymade L-plate audio output circuit used in transistor amplifiers, if desired. The necessary data regarding the coils used in the circuit is given in the circuit diagram itself.