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Dr. Guilherme Tosi and Professor Andrea Morello at the University of New South Wales quantum computing labs with a dilution refrigerator, which cools silicon chips down to 0.01 ̊ above absolute zero.

Flip-Flop Qubit Could Make Silicon the King of Quantum Computing

Headline-grabbing quantum computing efforts by Google and IBM have mostly focused on building quantum bits, called qubits, out of loops of superconducting materials. But a recent breakthrough could enable a different technology based on spin-based silicon qubits to eventually dominate the rise of quantum computers.


The theoretical advantage of quantum computing rests upon qubits representing multiple states simultaneously, whereas classical computing bits can only represent information as either a 1 or 0. Toward that end, Australian and U.S. researchers have developed qubits based on either the nuclear or electron spin state of phosphorus atoms embedded in silicon. Their latest work has yielded the concept of a “flip-flop qubit” that combines both electron and nuclear spin states—an approach that enables neighboring qubits to remain coupled together despite being separated by larger physical distances. In turn, that makes it much easier to build control schemes for the large arrays of qubits necessary for full-fledged quantum computing.

“[T]he real challenge when trying to fabricate and operate 100, 1,000, or millions of qubits is how to lay out the classical components, such as interconnects and readout transistors,” says Andrea Morello, a quantum physicist at the University of New South Wales, in Australia. “So, having the qubits spaced out by 200 to 500 nanometers from each other means that we have all that space between them to intersperse all the classical control and readout infrastructure, while using fabrication technologies that are already commonplace in the semiconductor industry.”

Morello did not mince words in describing his team’s conceptual insight—spearheaded by lead author Guilherme Tosi and published in the Sept. 6 2017 online issue of the journal Nature Communicationsas a “stroke of genius.” It seems counterintuitive at first glance, because it apparently gives up the potential for two qubits—one based on an electron spin and one based on nuclear spin—in exchange for just one qubit.

But the researchers based at the University of New South Wales and Purdue University in the United States soon realized that the advantage of the flip-flop qubit comes from inducing an electric dipole—separation of positive and negative charges—by pulling the electron a little bit away from the nucleus of the phosphorus atoms (which are themselves embedded in silicon). That electric dipole enables the spin-based silicon qubits to remain entangled together over longer distances and able to influence one another through quantum physics. The wider separations between individual qubits make it easier to squeeze in the classical computing circuitry necessary for controlling qubits.

This conceptual breakthrough in building larger arrays of spin-based silicon qubits could transform the future direction of quantum computing. With the notable exception of the Canadian company D-Wave, most companies are focused on building universal gate-model quantum computers that can tackle a wide range of problems. The largest universal quantum computing machines built so far have been based on superconducting qubit arrays—an approach embraced by tech giants such as Google and IBM. By the end of 2017, Google aims to build a 49-qubit chip based on superconducting qubits that can definitively prove quantum computing’s ability to outperform classical computers for the first time.

The Australian team pushing for spin-based silicon qubits has more modest goals by comparison: developing a 10-qubit array by 2022. But the new flip-flop qubit approach could make it “much more realistic and economical” to expand beyond 10 qubits and eventually scale up to thousands or millions of qubits, Morello says. (Both the spin-based silicon qubits and superconducting qubits can be manufactured relatively easily based on modern semiconductor industry techniques.)

Morello envisions spin-based silicon qubits potentially taking over the lead in the quantum computing race from superconducting qubits, possibly within a decade. That is because larger arrays of superconducting qubits could eventually run into scaling issues because of their relatively large individual qubit sizes. By comparison, researchers could theoretically place more than one million spin-based silicon qubits on a square millimeter of space. “[B]ased on what we know now, I do imagine that silicon could become the system of choice at the thousands or millions of qubits level,” Morello says.

The researchers also discovered that the new flip-flop qubits can be controlled with electric fields rather than magnetic fields. That’s a big deal because the electron and nuclear spins of phosphorus atoms respond only very weakly to magnetic fields. Previously, researchers tried creating stronger magnetic fields that could affect the spin states—but at the cost of also creating electrical fields that could interfere with components designed to readout the information from the qubits.

Morello and his colleagues eventually realized that flip-flop qubits respond strongly to resonant electric fields that are “tuned to the exact frequency at which the electron and nucleus flip-flop with each other,” Morello explains. As an added bonus, the flip-flop qubits do not respond to any other electric fields outside that particular frequency. That meant the researchers could use a relatively weak electric field to control the flip-flop qubits while being assured that no other electrical field interference would disturb the qubit operations.

There does not need to be only one winner between spin-based silicon qubits and superconducting qubits in the long-term race for quantum computing. Some superconducting qubit architectures, called transmon qubits, could naturally interface with flip-flop qubits based in silicon, Morello says. And besides, superconducting qubits are often manufactured as a layer on top of a silicon chip. It’s very possible that researchers may want to eventually “mix and match flip-flop qubits in silicon with superconducting qubits” in some future quantum computing applications—a way to leverage the best of both worlds.

Conductance map in bias energy versus chemical potential

Majorana Particles Grab the Limelight and Remain Center Stage

Last week, the long-anticipated confirmation that the Majorana quasiparticle actually exists finally came to pass. With that confirmation also came a method for using it in a new kind of quantum computing that is far more stable than what’s currently available.

That research appears to have been a watershed moment. Two new papers released this week share some of the same origins as last week’s research, but with some significant new twists.

In the first, published in Nature Communications, researchers at the University of Sydney in Australia and Microsoft's Station Q, also in Sydney, offer further proof of the existence of Majorana fermions in semiconducting nanowires. This confirmation takes the form of proof that an electron inside these one-dimensional semiconducting nanowires will have a quantum spin opposite to its momentum in a finite magnetic field.

The other paper, which appears in the journal Science Advances, details the work of an international team of scientists led by a group at the University of Pittsburgh. That team has provided a new approach to generating Majorana fermions in nanowires that involves the use of quantum dots.

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Scanning electron microscope image showing the nano-scale optical quantum memory fabricated in yttrium orthovanadate (YVO). The schematic shows that this device is an optical cavity that contains Nd atoms.

Quantum Optical Memory Device One Thousand Times Smaller Than Previous Options

The prospects for optical quantum networks—in which information is transmitted by encoding data using the quantum state of photons—has been on the upswing lately, leading to some commercial quantum photonics products currently on the market.

However, optical quantum memory, which is one of the key technologies for the realization of widespread, optical, quantum networks, remains a sticking point. Optical quantum memory is a device that takes a photon and encodes it with information. Unfortunately, the devices developed to date have been too large and inefficient to operate in a chip-scale quantum device.

Now an international team of researchers led by a group from the California Institute of Technology (Caltech) has managed to create an optical quantum memory device that is over 1000 times smaller than anything previously available. Not only is the device significantly smaller than anything that has come before it—ensuring will fit into on-chip devices—it is capable of on-demand retrieval of the stored data.

In research described in the journal Science, researchers from Caltech, the National Institute of Standards and Technology (NIST), and the University of Verona, Italy have collaborated on the development of a nano-sized cavity containing neodymium. That cavity in turn creates a crystal cavity that enhances the interaction between light and the cavity’s neodymium at the single photon level.

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Scanning electron microscope image of growing InP nanowires thereby forming multiple junctions

Elusive Majorana Particle Takes Major Step Towards Quantum Computing

An international team of researchers has fashioned a device from nanowires that may finally prove the existence of long-theorized quasiparticles known as Majorana particles. Once these Majorana particles are identified and isolated, they could form the basis of a quantum bit—or qubit—that would process information in a new kind of quantum computer with improved stability.

Ever since 1937, when the Italian physicist Ettore Majorana first theorized the existence of the quasiparticle that takes his name, there has been much effort to prove that it really exists, with little to show for it. But this changed back in 2012, when researchers at Delft University of Technology (TU Delft) in the Netherlands saw strong hints of Majoranas when they sent electrons into a semiconducting nanowire placed alongside superconducting material.

Since that 2012 Delft research, there have been a number of experiments that have reported evidence of Majoranas in a similar system. However, all of those experiments, including the original one at Delft, left open the possibility of alternative explanations for the results. So, unil now, there has been no “smoking-gun” evidence of Majoranas, said Hao Zhang, a post-doc at TU Delft, in an e-mail interview with IEEE Spectrum.

There remained one definitive way to prove the existence of these Majorana particles and that was for them to exchange places along the nanowire, a phenomenon referred to as exchanging statistics of the particles. These statistics describe how the quantum mechanics of the system change when two indistinguishable particles switch places.

This exchange of places along the nanowire is also called “braiding”. These braids form the logic gates of topological quantum computers. However, no one could see how this braiding was possible because the act of getting the particles to pass each other in this nanowire would annihilate them.

If this braiding of these quasiparticles could somehow be artificially induced, researchers theorized, it would result in a far more stable method for quantum computing than employing trapped quantum particles. That’s because the system wouldn’t be susceptible to outside influences like thermal fluctuations.

In research described in the journal Nature, the researchers, from TU Delft, Eindhoven University of Technology in the Netherlands, and the University of California, Santa Barbara, created a hashtag-like device made from nanowires. It provided a four-way intersection in which two Majorana particles could exchange places in the nanowire-based structure without coming in contact with each other and annihilating each other.

“These braiding experiments can give experimental results, which are unique to Majoranas, and cannot be mimicked by other alternative scenarios,” says Zhang. “Thus it can be treated as the smoking-gun evidence.”

Braiding not only provides definitive evidence of Majoranas, but perhaps more importantly, it also proves the feasibility of topological quantum computing in which the fundamental assumption of their operation is based on the braiding phenomenon. In other words, the braiding not only proves the existence of Majoranas, but also provides the mechanism by which they could serve as the basis of a qubit for a topological quantum computer.

This means that the quantum information (qubit) can be stored and manipulated simply by braiding (swapping) of Majoranas. “This process of braiding is supposed to be robust against error since the outcome only depends on the order of braiding operations,” adds Zhang.

In the video below, you can see a description of how the Majoranas are formed from the combining of semiconductor nanowires with a superconductor material, and how once formed can be manipulated into serving as qubits in a topological quantum computer.

This robustness against error depends on Majorna’s ability to maintain superposition.  In previous quantum computing proposals, the unpaired electrons of certain ions can assume either of two spin states, up or down—or in terms of digital logic, 0 or 1. When these ions are hit with a microwave pulse, the unpaired electron can take on both the 0 and 1 state simultaneously. These two states constitute what is termed superposition.

Until now, it has only been possible to maintain a superposition state for very short periods of time because the spin states of neighboring atoms quickly destroy the coherent state. This makes the life of the qubit too short for it to perform the desired number of quantum computations.

“This is the biggest advantage of Majorana qubit compared to other qubits,” says Zhang. “The Majorana qubit should have longer coherence time (robust against error) due to its topological protection.”

Zhang says that they are already working on the engineering of a qubit based on these Majoranas that will involve the fabrication of a microwave pulse circuit.

Image: Drexel University

Nanodiamonds May Help Make Lithium-Ion Batteries Better and Safer

Microscopic diamonds added to lithium-based batteries could help prevent the fires and explosions that can bedevil the energy storage devices, a new study finds. This advance could also help lay the foundation for lithium-based batteries with pure lithium electrodes that can store up to 10 times more energy than today’s lithium-ion batteries, researchers say.

Lithium-based batteries have become notorious for safety incidents where they can burst into flames or even explode. A key reason such hazards can occur is the formation of dendrites—tendril-like deposits of lithium that can grow long enough to pierce the barrier separating a lithium-ion battery's positive and negative halves and cause it a short circuit.

Dendrites form when a battery electrode degrades and metal ions deposit onto the electrode's surface. To avoid dendrite formation, today’s lithium-based batteries don’t use a pure lithium metal electrode. Instead they often use an electrode made of graphite that’s filled with lithium. Although the graphite helps suppress dendrite growth, such electrodes also store only about 1/10th the energy pure lithium could.

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Illustration of a torsionally tethered coiled harvester electrode and counter and reference electrodes in an electrochemical bath, showing the coiled yarn before and after stretch.

Nanotube-Based Yarns Harvest Energy From Twisting and Stretching

An international team of researchers led by researchers at the University of Texas (UT) at Dallas—where they have been working on making carbon nanotube-based yarns for well over a decade—has devised a way to make these carbon nanotube yarns into devices that can harvest energy from stretching or twisting them.

In research described in the journal Science, the initial results show promise for immediate use in powering small sensor nodes in Internet of Things (IoT) applications. The team says its nanotube yarns could produce larger amounts of energy by flexing and twisting in response to the movement of ocean waves.

While it appears as though these nanotube yarns are exploiting a piezoelectric effect, in which a material can generate an electric charge in response to applied mechanical stress, the yarn’s behavior makes it more closely tied to so-called electroactive polymers (EAPs), which are a kind of artificial muscle.

“Basically what's happening is when we stretch the yarn, we're getting a change in capacitance of the yarn. It’s that change that allows us to get energy out,” explains Carter Haines, associate research professor at UT Dallas and co-lead author of the paper describing the research, in an interview with IEEE Spectrum.

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Artist's rendering of bioreactor (left) loaded with bacteria decorated with cadmium sulfide, light-absorbing nanocrystals (middle) to convert light, water and carbon dioxide into useful chemicals (right).

Cyborg Bacteria Change the Game in Carbon Dioxide Reduction

Researchers at the University of California Berkeley may have introduced a new era in artificial photosynthesis. They’ve devised a way to cover bacteria with cadmium sulfide semiconductor nanocrystals to breakdown carbon dioxide (CO2) into acetic acid, a precursor to a host of other products.

In research that is being presented at the 254th National Meeting & Exposition of the American Chemical Society (ACS), Kelsey K. Sakimoto, a researcher at the lab of Peidong Yang at Berkeley, has demonstrated that the non-photosynthetic bacterium known as moorella thermoacetica can self-assemble into nanocrystal-clad cyborgs that essentially supercharge the bacteria into CO2-reduction powerhouses.

“It's actually a natural, overlooked feature of their biology,” explains Sakimoto in an e-mail interview with IEEE Spectrum. “This bacterium has a detoxification pathway, meaning if it encounters a toxic metal, like cadmium, it will try to precipitate it out, thereby detoxifying it. So when we introduce cadmium ions into the growth medium in which M. thermoacetica is hanging out, it will convert the amino acid cysteine into sulfide, which precipitates out cadmium as cadmium sulfide. The crystals then assemble and stick onto the bacterium through normal electrostatic interactions.”

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Schematic representation of the C60-based molecular spin-photovoltaic (MSP) device

Fullerene Device Acts as Both Solar Cell and a Current Inverter

An international team of researchers has developed a photovoltaic cell based on a combination of magnetic electrodes and C60 fullerenes— sometimes referred to as Buckyballs—that increases the photovoltaic efficiency of their device by 14 percent over photovoltaics using ordinary materials and architecture.

In research described in the journal Science, scientists from China, Germany, and Spain have taken spin valves—devices based on giant magnetoresistance and used in magnetic memory and sensors—and combined them with photovoltaic materials. The result offers a new way for solar cells to convert light into electricity.

“The device is simply a photovoltaic cell,” says Luis Hueso, research professor and leader of the Nanodevices Group at CIC nanoGUNE in Spain, in an e-mail interview with IEEE Spectrum. “However, we are using magnetic electrodes (cobalt and nickel-iron) rather than standard indium tin oxide (ITO) and aluminum as commonly used in organic photovoltaics.” The magnetic electrodes provide electrons with a certain orientation of their spin, creating what’s called a spin polarized current. Using these electrodes increased the photovoltaic efficiency by 14 percent compared to using ordinary electrodes, he says.

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Columbia researchers wired a single molecular cluster to gold electrodes to show that it exhibits a quantized and controllable flow of charge at room temperature.

Single-Molecule Transistors Get Reproducibility and Room-Temperature Operation

Molecular electronics promise a time when the basic building blocks of electronics are individual molecules. Of course, the transistor is today’s fundamental building block for computing. And along these lines, five years ago, researchers showed that it was possible to make a transistor from a single atom.

That work and much that has followed since is not going to lead to practical devices any time soon. But all of this basic research may someday result in practical devices.

In the latest step in this long journey, researchers at Columbia University have fabricated a small cluster of atoms into a two-terminal transistor capable of switching from insulator to conductor when charge is added or removed, one electron at a time.

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Crystal structure of HfSe 2 and ZrSe 2

Two-Dimensional Versions of High-K Materials Offer New Future for Chips

Silicon has been the mainstay of chips for much of their history (a history you can explore in IEEE Spectrum’s Chip Hall of Fame). This is in large part because silicon possesses a “Goldilocks” band gap of 1.1 electron Volts (eV), which makes it possible to operate integrated circuits at a low voltage, leading to reduced leakage of current.

Another key feature of silicon is that it can be used to make a convenient “native” insulator, in the form of silicon oxide. Silicon oxide managed to serve as an insulator for silicon circuits for many generations of chips, isolating components and reducing gate leakage currents, until high-K dielectrics took over the job a decade ago.

Now researchers at Stanford University and SLAC National Accelerator Laboratory have found that some of the most sought after high-K materials—namely hafnium selenide (HfSe2) and zirconium selenide (ZrSe2)—possess the same perfect band gap seen in silicon when they are thinned down to two-dimensional (2D) materials. As a result, the Stanford researchers have discovered a 2D material version of the handy silicon/silicon dioxide combination that enabled generations of chip designs. But in this case the combination can be shrunk down ten times smaller.

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IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

Dexter Johnson
Madrid, Spain
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