System Bits: Aug. 9

Using trapped ions as quantum bits
MIT researchers reminded that quantum computers are largely hypothetical devices that could perform some calculations much more rapidly than conventional computers can, and instead of the bits of classical computation — which can represent 0 or 1 — quantum computers consist of quantum bits, or qubits, which can, in some sense, represent 0 and 1 simultaneously.

And while quantum systems with as many as 12 qubits have been demonstrated in the lab, building quantum computers complex enough to perform useful computations will require miniaturizing qubit technology, much the way the miniaturization of transistors enabled modern computers, with trapped ions the most widely studied qubit technology. However, they’ve historically required a large and complex hardware apparatus — until now.

Researchers from MIT and MIT Lincoln Laboratory report an important step toward practical quantum computers, with a paper describing a prototype chip that can trap ions in an electric field and, with built-in optics, direct laser light toward each of them. (Source: MIT)

MIT and MIT Lincoln Laboratory researchers have reported an important step toward practical quantum computers, describing a prototype chip that can trap ions in an electric field and, with built-in optics, direct laser light toward each of them.

The Quantum Information and Integrated Nanosystems group at Lincoln Laboratory was one of several research groups already working to develop simpler, smaller ion traps known as surface traps. The researchers explained that a standard ion trap looks like a tiny cage, whose bars are electrodes that produce an electric field. Ions line up in the center of the cage, parallel to the bars. A surface trap, by contrast, is a chip with electrodes embedded in its surface. The ions hover 50 micrometers above the electrodes.

Although cage traps are intrinsically limited in size, surface traps could, in principle, be extended indefinitely. With current technology, they would still have to be held in a vacuum chamber, but they would allow many more qubits to be crammed inside.

For this reason, they believe that surface traps are a key technology to enable these systems to scale to the very large number of ions that will be required for large-scale quantum computing.

Quantum computer module combines proven hw/sw techniques
While the promise of quantum computers is speedy solutions for difficult problems, building large-scale, general-purpose quantum devices is a problem fraught with technical challenges, reminded University of Maryland researchers.

To date, many research groups have created small but functional quantum computers by combining a handful of atoms, electrons or superconducting junctions in order to demonstrate quantum effects and run simple quantum algorithms; i.e., small programs dedicated to solving particular problems.

Close-up photo of an ion trap. (Source: University of Maryland)

However, these laboratory devices are often hard-wired to run one program or limited to fixed patterns of interactions between their quantum constituents. Making a quantum computer that can run arbitrary algorithms requires the right kind of physical system and a suite of programming tools. Atomic ions, confined by fields from nearby electrodes, are among the most promising platforms for meeting these needs, the team asserted.

Now, the researchers have introduced what they say is the first fully programmable and reconfigurable quantum computer module.

The new device — dubbed a module because of its potential to connect with copies of itself — takes advantage of the unique properties offered by trapped ions to run any algorithm on five quantum bits, or qubits, which is the fundamental unit of information in a quantum computer.

The team explained that for any computer to be useful, the user should not be required to know what’s inside, and very few people care what their iPhone is actually doing at the physical level. They noted that this experiment brings high-quality quantum bits up to a higher level of functionality by allowing them to be programmed and reconfigured in software.

The new module builds on decades of research into trapping and controlling ions. It uses standard techniques but also introduces novel methods for control and measurement. This includes manipulating many ions at once using an array of tightly-focused laser beams, as well as dedicated detection channels that watch for the glow of each ion.

Neural dust opens door to electroceuticals
The first dust-sized, wireless sensors have been built by University of California, Berkeley engineers that can be implanted in the body, bringing closer the day when a Fitbit-like device could monitor internal nerves, muscles or organs in real time.

These batteryless sensors could also be used to stimulate nerves and muscles, opening the door to “electroceuticals” to treat disorders such as epilepsy or to stimulate the immune system or tamp down inflammation, they said.

The neural dust implanted in the muscles and peripheral nerves of rats is unique in that ultrasound is used both to power and read out the measurements. Ultrasound technology is already well-developed for hospital use, and ultrasound vibrations can penetrate nearly anywhere in the body, unlike radio waves, the researchers said.

Michel Maharbiz, an associate professor of electrical engineering and computer sciences at UC Berkeley, and one of the study’s two main authors believes the long-term prospects for neural dust are not only within nerves and the brain, but much broader, and having access to in-body telemetry has never been possible because there has been no way to put something super tiny in super deep. But now a speck of nothing can be place next to a nerve or organ, GI tract or a muscle, and data be read out it.

The sensor mote contains a piezoelectric crystal (silver cube) plus a simple electronic circuit that responds to the voltage across two electrodes to alter the backscatter from ultrasound pulses produced by a transducer outside the body. The voltage across the electrodes can be determined by analyzing the ultrasound backscatter.
(Source: UC Berkeley)

The sensors, which the researchers have already shrunk to a 1 millimeter cube – about the size of a large grain of sand – contain a piezoelectric crystal that converts ultrasound vibrations from outside the body into electricity to power a tiny, on-board transistor that is in contact with a nerve or muscle fiber. A voltage spike in the fiber alters the circuit and the vibration of the crystal, which changes the echo detected by the ultrasound receiver, typically the same device that generates the vibrations. The slight change, called backscatter, allows them to determine the voltage.

They believe that while the experiments so far have involved the peripheral nervous system and muscles, the neural dust motes could work equally well in the central nervous system and brain to control prosthetics especially because today’s implantable electrodes degrade within 1 to 2 years, and all connect to wires that pass through holes in the skull. Whereas wireless sensors – dozens to a hundred – could be sealed in, avoiding infection and unwanted movement of the electrodes.

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Week In Review: Design (Aug 5)
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