Theorists predict new state of quantum matter may have big impact on electronics


Printing Graphene Chips(Nanowerk News) Constantly losing energy is something we deal with in everything we do. If you stop pedaling a bike, it gradually slows; if you let off the gas, your car also slows. As these vehicles move, they also generate heat from friction. Electronics encounter a similar effect as groups of electrons carry information from one point to another. As electrons move, they dissipate heat, reducing the distance a signal can travel. DARPA-sponsored researchers under the Mesodynamic Architectures (Meso) program, however, may have found a potential way around this fundamental problem.
Meso program researchers at Stanford University recently predicted stanene will support lossless conduction at room temperature. Stanene is the name given by the researchers to 2-D sheets of tin that are only 1-atom thick. In a paper appearing in Physical Review Letters (“Large-Gap Quantum Spin Hall Insulators in Tin Films”) the team predicts stanene would be the first topological insulator to demonstrate zero heat dissipation properties at room temperature, conducting charges around its edges without any loss. Experiments are underway to create the material in laboratory conditions. If successful, the team will use stanene to enhance devices they are building under the Meso program.
the flow of electricity along the outside edges of a new topological insulator, stanene
This image depicts the flow of electricity along the outside edges of a new topological insulator, stanene. Theorists in DARPA’s Mesodynamic Architectures (Meso) program predict stanene would have perfect energy propagation properties at room temperature. (Image: SLAC National Accelerator Laboratory)
“We recently realized there is another state of electronic matter: a topological insulator. Materials in a topologically insulating state are like paying for the gasoline to accelerate your car to highway speeds, but then cruising as far as you want on that highway without using up any more gas,” said Jeffrey Rogers, DARPA program manager. “Experiments should tell us what penalty electrons would pay for connecting to stanene in a practical application. However, the physics of stanene point to zero dissipation of heat—meaning electrons take an entropy hit once and then travel unimpeded the rest of the distance.”
Researchers at Stanford reported the first topological insulators in 2006 under a previous DARPA effort known as the Focus Center Research Program. The current Meso program developed the theory for stanene as part of research into more efficient ways to move information inside microchips. Other materials’ capabilities have come close, but only at temperatures that require extreme sub-zero temperatures created with bulky methods such as liquid helium.
“Stanene is a bold, yet compelling prediction,” said Rogers. “If the experiments underway confirm the theory, the application of a new lossless conductor becomes a very exciting prospect in the world of electronics. A host of applications—almost any time information is moved electronically from one place to another—could benefit.”
Source: DARPA

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Quantum communication controlled by resonance in ‘artificial atoms’


imagesCAMR5BLR Einstein Judging a FishResearchers at the Niels Bohr Institute, together with colleagues in the US and Australia, have developed a method to control a quantum bit for electronic quantum communication in a series of quantum dots, which behave like artificial atoms in the solid state. The results have been published in the scientific journal Physical Review Letters.

 

The experiments are carried out at ultra low temperatures close to absolute zero, which is minus 273 degrees C.

In a conventional computer, information is made up of bits, comprised of 0’s and 1’s. In a quantum computer the 0 and 1 states can simultaneously exist, allowing a kind of parallel computation in which a large number of computational states are acted upon by the machine at the same time. This can make a quantum computer exponentially faster than a conventional computer. The problem with the quantum world, however, is that you cannot allow these states to be measured, or all of the quantum magic disappears.

“We have developed a new way of controlling the electrons so that the quantum state can be controlled without measurement, using resonances familiar in atomic physics, now applied to these artificial atoms,” explains Professor Charles Marcus, director of the Center for Quantum Devices at the Niels Bohr Institute at the University of Copenhagen.

He explains that they are combining classical solid-state physics on a nanometer scale with resonance techniques of atomic physics. In a semiconducting material (GaAs) there are free electrons that move within the material structure. The information is stored in the spin of the electrons which can turn up or down. But the electrons and their spin must be controlled.

Schematic illustration of the actual ‘box’ with a triple quantum dot, where there is one single electron in each dot.

Captures electrons and controls them

“We capture the electrons in ‘boxes’. Each box consist of a quantum dot, which is an artificial atom. The quantum dots are embedded in the semiconductor and each quantum dot can capture one electron. There needs to be three quantum dots next to each other using nanometer-scale electrostatic metal gates. When we open contact between the ‘boxes’ the electrons can sense each others’ presence. The three spins must coordinate their orientations because it cost extra energy to put electrons with the same spin into the same box. To lower their energy, they not only spread out among the three boxes, but they orient their spins to further lower their energy. The three boxes together form a single entity – a qubit or quantum bit,” explains Charles Marcus.

An electrical signal is now sent from outside and by rapidly opening the boxes the system begins to swing in dynamic vibrations. The researchers can use this to change the quantum state of the electrons.

“By combining three electrons in a triple quantum dot and oscillating an applied electric field at the frequency that separates adjacent energy levels, we can thus control the spins of the electrons without measuring them,” explains Charles Marcus.

Quantum computers for extreme applications

First, the technique itself was discovered. The next step is not just a single sequence with three quantum dots, but several sequences. Each sequence forms one qubit and now a series of qubits need to talk to each other. This could be realised by a quantum computer with more bits.

“The potential of a quantum computer is that it will be able to perform multiple calculations at once. In that way it will be much faster than conventional computers and will be able to solve tasks that cannot currently be solved, because it simply takes too long,” says Charles Marcus.

Quantum computers are not expected to be something everyone will own, but rather an advanced set of tools for researchers who need to make extreme calculations.

The research is described in three articles in Physical Review Letters:

  1. Quantum-Dot-Based Resonant Exchange Qubit >>>
  2. Electrically Protected Resonant Exchange Qubits in Triple Quantum Dots >>>
  3. Two-Qubit Gates for Resonant Exchange Qubits >>>