# Nanoclast

### MRAM-like Device Could Make Logic Run Backwards

Engineers at Purdue University and the University of California at Berkeley have discovered that a simple combination of commonly available devices creates a computing element that could lead to some truly strange circuits: logic that can perform its inverse operation. Some important aspects of modern computing—notably encryption—depend on there being a significant difference in difficulty between multiplication and its inverse, factoring. But the engineers are cautious about the technology’s potential for code breaking. They describe the device in this month’s issue of IEEE Electron Device Letters.

Purdue electrical engineering and computer science professor Supriyo Datta and his colleagues had been exploring new ways of computing using simulated networks of devices that have a “tunable” randomness. That is, the devices output a string of bits that are random, but can be tuned to produce more of one bit than the other. Such a device, which they called a p-bit,  could then represent a probability—40 percent say—as a string of bits of whatever length as long as for every 100 bits or so 40 of them were 1.

Many computing problems can be streamlined by computing with probabilities; Datta’s group showed their use in solving the travelling salesman problem and other important problems of optimization and inference. But Datta and post-doctoral researcher Kerem Yunus Camsari and some colleagues discovered something new about using p-bits: They could be combined in such a way that they form logic and arithmetic circuits that can perform their inverse operations—adders can subtract, multipliers can factor, and potentially stranger things can happen.

The inputs to such a logic gate would be random streams of bits, but they still obey the logic of that gate. So whenever a 2-input AND gate saw a 1 on both inputs, it would output a 1; everything else would generate a 0. But what p-bit-based gates can do that others can’t is run in reverse. Put a stream of random bits through what was the AND gate’s output and its input will produce two random streams that satisfy the AND gate’s logic:  Whenever you put a 1 on the output, both inputs would generate a 1. But if you put a 0 on the output the inputs generate a random sequence of 00, 01, and 10.

The problem was this: there was no such thing as a p-bit. “Six-months ago I’d have said we’re not close to building even one, because we’d have to build a whole new device,” says Datta.

Sayeef Salahuddin at UC Berkeley suggested to Datta and Camsari that a magnetic RAM cell combined with a transistor might be the solution. Datta and Camsari have now shown it to be true. Just those combination of things recently became available on processors and systems-on-a-chip from GlobalFoundries and TSMC.

An MRAM cell is basically a two-terminal nanoscale device called a magnetic tunnel junction. It’s made up of two ferromagnetic layers sandwiching a non-ferromagnetic layer. If the two magnetic layers have magnetic fields oriented in the same direction, current can tunnel across them with little resistance. If they’re pointing in the opposite directions from each other, the resistance spikes. MRAM stores a bit by making one of the layers changeable by a particular type of current. They’ve now been engineered so well that the flippable magnetic layer will stay stable for a decade or more.

To make a p-bit, however, you need to basically make a bad MRAM cell. Instead of needing a high current to flip the magnetic layer, you need to make it so unstable that random thermal fluctuations will knock its magnetic field back and forth at gigahertz rates. The transistor component of the p-bit is there to pull the output towards either the one or zero states as needed—tuning the randomness to encode a probability.

Datta’s group hasn’t built p-bits yet, but they’ve simulated them and are seeking access to a foundry that can do the job.

### New Carbon Nanotube Sheets Claim World’s Top Heat-Sink Performance

Fujitsu Laboratories announced last week they have developed a process to manufacture sheets of pure carbon, multi-walled perpendicular nanotubes (CNTs). The uniformly arrayed tubes are aligned in the direction of heat removal so that they can be used as heat sinks for a number of electronic applications including silicon carbide devices employed in electric vehicles and in high-performance computing.

Fujitsu says its CNT sheets can dissipate heat at a rate of 80 watts-per-meter Kelvin even with contact resistance included. That is roughly three times the thermal conductance of indium sheets, a material known for its high thermal conductivity, which makes Fujitsu’s technology the world’s top heat-dissipation performer, the company claims.

CNTs’ inherent usefulness as heat-dissipation material has long been known, but exploiting this characteristic in anything other than the simplest of applications has been a headache for material scientists.

### 2D Materials Push Paper Electronics Towards the Internet of Things

Paper-based electronics have primarily been limited to use in printed organic electronics. While this is promising for commodity applications such as packaging tags and toys, the speed of organic semiconductors is not suitable for most radio-frequency applications. Among the uses for which paper-based electronic devices have been heretofore unsuitable is connecting to the cloud over Bluetooth frequencies for the Internet of Things (IoT), smart sensors, and other smart applications.

Researchers at the University of Texas at Austin (UT-Austin) are reporting this week at the International Electron Devices Meeting that graphene and molybdenum disulfide (MoS2), with their extraordinary conductivity, can enable paper-based electronics to achieve the frequency required to make them fit for IoT and smart sensor applications. The researchers claim that this work represents the first time that high-performance two-dimensional (2D) transistors have been demonstrated on a paper substrate.

### Perovskites Challenge Quantum Dots as Champion Emitters of Light

An international research team from the University of Maryland and ETH Zurich in Switzerland has successfully demonstrated that a nanocrystal of perovskite can serve as a quantum emitter of light, and, when coupled with a nanophotonic cavity, can dramatically improve the efficiency of the light emission.

The resulting device and method, described in the journal Applied Physics Letters, could be used to build nanolasers and optical devices that exhibit much faster response times than currently possible.

### 4 Strange New Ways to Compute

With Moore’s Law slowing, engineers have been taking a cold hard look at what will keep computing going when it’s gone. Certainly artificial intelligence will play a role. So might quantum computing. But there are stranger things in the computing universe, and some of them got an airing at the IEEE International Conference on Rebooting Computing in November.

There were also some cool variations on classics such as reversible computing and neuromorphic chips. But some less-familiar ones got their time in the sun too, such as photonics chips that accelerate AI, nano-mechanical comb-shaped logic, and a “hyperdimensional” speech recognition system. What follows includes a taste of both the strange and the potentially impactful.

### Nanoscale Magnetic Circuits Expand Into Three Dimensions

Researchers at the University of Cambridge in the UK have broken the paradigm of two-dimensional circuits used to store and transmit data and created a nanoscale magnetic circuit that can send along bits of information in three dimensions.

These new magnetic-based circuits have the potential to form architectures where logic and memory circuits are merged together, according to the researchers. This merger could reduce the energy required for data transfer associated with any logic operation and could prove to be a boon for battery-powered applications, such as mobile phones or the Internet of Things.

### Nanosys Wants Printing Quantum Dot Displays to be as Cheap as Printing a T-Shirt

Quantum dots have established themselves as a go-to material for photoluminescence, in which light is emitted when stimulated by a light source. Based on this capability, companies such as Nanosys have been able to help display companies like Samsung capture a growing segment of the display market from competing technologies such as LED-backlit LCD and organic light-emitting diode (OLED) displays.

Nanosys currently has more than 60 quantum dot-enabled products on the market, and the company now wants to make a big push to expand the capabilities of quantum dots beyond just photoluminescence into the area of electroluminescence, where photons are emitted in the presence of an electric field or current. Nanosys expects this development to lead to a new era of what Nanosys is terming: Electro Luminescent Quantum Dot (ELQD) displays.

Executives at Nanosys believe that ELQD displays have the potential to disrupt the display industry over the next decade. The displays don’t need a backlight and, because each subpixel is addressable, the display wastes no energy while the light travels from the backlight to the pixel. This should translate into lower power consumption, along with wider viewing angles, purer colors, and perfect black levels, according to Jeff Yurek, Director of Marketing and Investor Relations at Nanosys.

“We expect to see these displays in the three to five year timeframe,” said Yurek. “We think that quantum dots have the potential to deliver on the promise of OLED.”

### A Mix of Nanomaterials Leads to a New Quantum Photonic Circuit Architecture

Scientists at the National Institute of Standards and Technology (NIST) have developed an underlying architecture for a new class of quantum photonic circuits—chip-based devices that leverage the quantum properties of light to process and communicate information.

In research described in the journal Nature Communications, the NIST researchers and their collaborators in China and the UK developed a class of devices that are composed of networks of low-loss waveguides and single-photon sources, all on a single chip. The resulting quantum circuit architecture could have an impact on photonic quantum computing and simulation, as well as metrology and communications.

Operating the device consists of producing streams of single photons, launching them into a network of waveguides and beamsplitters in which they are allowed to interfere with each other, and then detecting them at the network’s outputs.

### Nanodevice Maps the Angle of Light to Color and Vice Versa

An international team of researchers has added a small twist to typical light filters. They’ve fabricated a nanoscale device that divides incident light into its different component colors based on the direction the light is coming from. Their device that could drastically alter how color displays are designed and even improve the efficiency of solar cells.

To accomplish this trick, the researchers have moved beyond the periodic architectures of a generation of tunable optical devices. In other words, they’ve switched from making filters with equally spaced grooves to an aperiodic architecture wherein the grooves are not equally spaced.

Researchers at the U.S. National Institute of Standards and Technology (NIST), in collaboration with Syracuse University and Nanjing University in China, have discovered that when incident light strikes these aperiodic grooves (nanoscale trenches on an otherwise opaque metal film) it triggers the creation of surface waves, referred to as plasmons.

In research described in the journal Nature Communications, the scientists were able to engineer the structure so that plasmons generated by individual grooves meet at the device’s central through slit with the same phase (constructive interference), but at multiple user-defined angles and wavelengths. The result: an angle-dependent color filtering response.

The angular response of an equivalent device with uniformly spaced grooves is limited by the grooves’ uniformity. In diffraction gratings, for example, the groove dimensions are restricted by the diffraction limit. But because the wavelength of plasmons is much smaller than the incident light, the researchers are able to sidestep the diffraction limit and shrink the groove dimensions so that they are much smaller than the wavelength of incident light.

“Plasmons, because of their shorter wavelengths, help us shrink the device footprint to the micron scale while at the same time allowing us to engineer light-matter interaction at the nanoscale,” explained NIST physicist and co-author of the paper Amit Agrawal.  “By incorporating engineered aperiodicity into the device, we are able to get multi-functional response: angle-to-color, or color-to-angle mapping.”

Currently, displays and consumer cameras typically use what is referred to as Bayer color filters. One super-pixel in a display is in fact a combination of three sub-pixels, one for each RGB color. Any combination thereof will provide the desired color for that super-pixel. However, this comes at a loss of resolution by a factor of three.

Because the footprint of the device developed by Agrawal and his colleagues (Matthew Davis and Wenqi Zhu of NIST and the University of Maryland, along with NIST physicist Henri Lezecmatches) matches the size of individual pixels (10 micrometers or so), Agrawal envisions placing these color filtering devices on top of individual CCD pixels, and performing color filtering by changing the angle of incidence using a micromirror-array based device.

Another alternative Agrawal suggests is using the device in reverse. In this way, one can direct the RGB colors coming at different angles to three pixels forming a super-pixel (same as an LCD).

While this all sounds very appealing, Agrawal is quick to caution that the efficiency of the devices at this point is still pretty low because of scattering and absorption losses in the metal film. So, although there is some work to be done to improve it, Agrawal believes that the incorporation of such devices on top of an array of CCD pixels or on top of multi-junction solar cells, for example, should be pretty straightforward.

“There is already a precedent for it,” said Agrawal  “Since these devices are angle sensitive (in the case of forward illumination), there will be some engineering required to illuminate them at various angles with micromirror arrays. However, at this point it is a matter of making an effort to increase the efficiency.”

The key to this efficiency-raising effort will be working with higher quality metal films than can be created using traditional physical vapor deposition techniques. It will also call for further optimization of the design process, according to Agrawal.

While the researchers grapple with those issues, Agrawal notes that the approach can also be easily extended to other regions of the electromagnetic spectrum, such as mid-infrared and the terahertz, where there are more choices for metal, and the losses are significantly smaller.

Agrawal adds: “In terms of efficiency, these devices should work remarkably well at those wavelengths and are especially suitable for applications such as sensing. Some biological and chemical molecules have spectral signatures at those wavelengths, so it makes logical sense for us to extend the operation of these devices to longer wavelengths.”