Novel Devices ♦ news digest
Such perfect reproducibility is important as it opens the door to quantum dot architectures free of uncontrolled variations, which is necessary for technologies ranging from nanophotonics to quantum information processing.
Now scientists from Paul-Drude-Institute for Solid- State Physics in Berlin, NTT Basic Research Laboratories, Japan; and the Naval Research Laboratory (NRL), USA have managed to create quantum dots with identical, deterministic sizes, according to a recent report in Nature Nanotechnology.
Quantum dots are often called artificial atoms because, like real atoms, they confine electrons to quantised states with discrete energies. But real atoms are identical, whereas most quantum dots comprise hundreds or thousands of atoms, with variations in size and shape and, consequently, unavoidable variability in their wavefunctions and energies.
Creating atomically precise quantum dots requires every atom of the quantum dot to be placed in a precisely specified location without error, and multiple dots to be arranged in exactly defined configurations without variation. The researchers achieved this goal using a scanning tunnelling microscope (STM) to manipulate the atoms and an atomically precise surface template to define a lattice of allowed atomic positions.
The template was the surface of an InAs crystal, which has a regular pattern of indium vacancies and a low concentration of native indium adatoms adsorbed above the vacancy sites. The adatoms are ionized +1 donors and can be moved with the STM tip by vertical atom manipulation. The team assembled quantum dots consisting of linear chains of N = 6 to 25 indium atoms.
Because the indium atoms are strictly confined to the regular lattice of vacancy sites, every quantum dot with N atoms is essentially identical, with no intrinsic variation in size, shape, or position. This means that quantum dot “molecules” consisting of several coupled chains will reflect the same invariance.
Steve Erwin, a physicist at NRL and the team’s theorist, pointed out that “this greatly simplifies the task of creating, protecting, and controlling degenerate states in quantum dot molecules, which
is an important prerequisite for many technologies.” In quantum computing, for example, qubits with doubly degenerate ground states offer protection against environmental decoherence.
By combining the invariance of quantum dot molecules with the intrinsic symmetry of the InAs vacancy lattice, the team say they have created degenerate states that are surprisingly resistant to environmental perturbations by defects.
The reproducibility and high fidelity offered by these quantum dots makes them excellent candidates for studying fundamental physics. Looking forward, the team also anticipates that the elimination of uncontrolled variations in quantum dot architectures will offer many benefits to a broad range of future quantum dot technologies from nanophotonics to quantum information processing.
Figures a,b, c and d above show the quantized states of a digital quantum dot in which electrons are confined by a chain of ionized indium adatoms. Picture a, is a topographic STM image (0.1 nA, _0.3 V) of a chain of indium adatoms assembled on InAs(111)A. Twenty-two indium atoms were placed on adjacent indium-vacancy sites of the (2 x 2)-reconstructed surface. b, shows the atomic structure of the image section indicated in a. The surface consists of indium (green) and arsenic (orange) atoms, and the chain is formed by In adatoms (black circles) adsorbed above vacancy sites. c, shows the differential conductance (dI/dV)
Issue VI 2014
www.compoundsemiconductor.net 155
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160