In quantum computing, the query as to what physical system, and which degrees of freedom inside that system, could also be used to encode quantum bits of data — qubits, briefly — is at the guts of many research projects carried out in physics and engineering laboratories. Superconducting qubits, spin qubits, and qubits encoded within the motion of trapped ions are already recognised widely as prime candidates for future practical applications of quantum computers; other systems should be higher understood and thus offer a stimulating ground for fundamental investigation.
Rebekka Garreis, Chuyao Tong, Wister Huang and their colleagues within the group of Professors Klaus Ensslin and Thomas Ihn from the Department of Physics at ETH Zurich have been looking into bilayer graphene (BLG) quantum dots, often known as a possible platform for spin qubits, to seek out out if one other degree of freedom of BLG could be used to encode quantum information. Their latest findings, just published in Nature Physics with collaborators from the National Institute for Materials Science in Japan, show that the so-called valley degree of freedom in BLG is related to quantum states which might be extremely long-lived and are thus price considering further as a further resource for solid-state quantum computing.
It’s all within the lattice structure
Graphene is a two-dimensional material given by a single layer of carbon atoms certain in a hexagonal lattice structure. Its sheet-like appearance is deceitful, as graphene is among the many strongest materials on Earth; its mechanical and electronic properties are of great interest to many industry sectors. In bilayer graphene, the system utilized by the researchers, two sheets of carbon atoms lie on top of one another. Each graphene and BLG are semimetals, as they lack the characteristic energy band gap present in semiconductors and, most notably, insulators. Nevertheless, a tunable band gap could be engineered in BLG by applying an electrical field perpendicularly to the plane of the sheets.
Opening a band gap is essential to make use of BLG as a bunch material for quantum dots, that are nanometer-scale ‘boxes’ able to confining single or few electrons. Often fabricated in semiconductor host materials, quantum dots offer excellent control over individual electrons. Because of this, they’re a vital platform for spin qubits, systems where quantum information is encoded within the electron spin degree of freedom.
Because quantum information is rather more vulnerable to being corrupted — and due to this fact turn into unsuitable for computational tasks — by the encircling environment than its classical counterpart, researchers who study different qubit candidates must characterise their coherence properties: these tell them how well and for a way long quantum information can survive of their qubit system. In most traditional quantum dots, electron spin decoherence could be brought on by the spin-orbit interaction, which introduces an unwanted coupling between the electron spin and the vibrations of the host lattice, and the hyperfine interaction between the electron spin and the encircling nuclear spins. In graphene in addition to in other carbon-based materials, spin-orbit coupling and hyperfine interaction are each weak: this makes graphene quantum dots especially appealing for spin qubits. The outcomes reported by Garreis, Tong and co-authors add yet another promising facet to the image.
The hexagonal lattice of BLG could be imaged with specific microscopy techniques. The hexagonal symmetry observed on this so-called real space can also be present in momentum space, where the vertices of the lattice don’t correspond to the spatial locations of carbon atoms but to values of momentum related to the free electrons on the lattice. In momentum space, free electrons are present in the local minima and maxima of the energy landscape, namely at points where the conduction and valence bands meet. These energy extrema are called valleys. In BLG, the hexagonal symmetry dictates the existence of two degenerate energy valleys (that’s, characterised by the identical electron energy) corresponding to opposite electron momentum values. This valley degree of freedom could be treated in much the identical way as electron spin in BLG — the truth is, valleys in graphene are commonly called pseudo-spins. While valley states in bilayer graphene were known before, their suitability as practical qubits remained unclear until now.
There’s much promise within the valley
Garreis, Tong and colleagues considered a double quantum dot — that’s, two dots with tunable coupling — in BLG and measured the relief time for valley and spin states. The relief time sets the temporal scale over which the system makes a transition from one valley or spin state to a different and, in consequence of the relief process, loses its energy and becomes unsuitable for further qubit operations. The research team finds that valley states have rest times exceeding half a second, a result that points to promising coherence properties for future valley qubits. The spin rest time measurement within the BLG double quantum dot gives a price below 25 ms, which is way shorter than the relief time for valley states but is in good agreement with spin rest times measured in semiconductor quantum dots. Importantly, each values are acceptable for high-quality qubit manipulation and readout.
Within the paper, the researchers also highlight points that decision for further experimental and theoretical investigation. They present data showing the dependence of the relief times for spin and valley states on two parameters which might be expected to play a task within the states’ rest dynamics. One parameter is the energy detuning: that is the energy difference between the bottom states of two distinct configurations for the double quantum dot. Various the detuning means acting on the energy difference between the states involved in the relief process. The opposite parameter is often known as inter-dot coupling and determines how easily an electron in a single quantum dot can ‘trespass’ into the territory of the opposite dot. The authors report behaviours that can’t be explained through the mechanisms which might be often at play in quantum-dot spin qubits. The relief time is shown to extend with higher energy detuning, which does not match observations in other systems. Remarkably, various the inter-dot coupling leaves the valley rest time unaffected.
It’s clear that a more complete understanding of the mechanisms affecting valley and spin rest times is required to discover which variables may go best for manipulating future valley qubits. Meanwhile, the findings presented by Garreis, Tong and collaborators make the case for adding valley states in BLG quantum dots to the landscape of solid-state quantum computing.
