Metamorphic materials for quantum computing
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troduction Inspired by the potential to solve problems that cannot be efficiently solved by any classical computer, researchers have exerted enormous effort in the last two decades to develop a quantum computer.1 Data processing in a quantum computer is performed through the manipulation of quantum information, the fundamental unit of which is called (by analogy with classical computation) a qubit. Conceptually, a qubit is a quantum mechanical system that can be in any linear combination α|0> + β|1> (where α and β are complex numbers representing probability amplitudes) of two fundamental basis states, denoted as |0> and |1>. Given the restriction that |α|2 + |β|2 = 1, the state of the qubit is often represented schematically by a point on a unit sphere (the “Bloch sphere”). The state of the qubit (i.e., its position on the Bloch sphere) can be manipulated through appropriate physical operations, and by allowing qubits to interact with one another, the states of multiple qubits can become entangled. A detailed discussion of these manipulations and interactions is beyond the scope of the present article, but two points warrant emphasis: In order for the qubit to function well, the basis states should be unique and energetically distinct from other states accessible to the system, and unintentional perturbations of the system by its environment should be minimized. Physical embodiments of qubits have included the
polarization states of photons; the energy levels of trapped ions; the nuclear spins in molecules; the spin states of confined electrons in solids; and the charge, flux, or phase of pairs of electrons bound together at low temperature (Cooper pairs) in superconducting materials.2 In semiconductors, qubits have been realized by electrons confined to point defects, such as nitrogen-vacancy centers in diamond and phosphorus donors in silicon, and to selfassembled nanostructures.3 An alternative approach is to first confine electrons within a two-dimensional (2D) quantum well, and then to apply biases to gates above the well to enclose the electrons within an electrostatically defined quantum dot. In this approach, quantum information can be encoded in the spin state of one or more electrons confined to the quantum dot. This approach has been implemented in very high-quality GaAs/AlGaAs heterostructures,4 GaInAs quantum wells,5 silicon metal oxide semiconductor nanostructures,6 and Si/SiGe heterostructures.7 All but the last of these implementations are able to make use of high-quality, single-crystal substrates on which the active device layers can be grown coherently and pseudomorphically, without the complication of plastic deformation. That is not the case with the Si/SiGe system, although it has seen remarkable progress in recent years.8 Quantum devices implemented in the Si/SiGe system rely on metamorphic substrates—strain-relaxed, compositionally
Peter W. Deelman, HRL Laboratories, LLC, USA; [email protected] Lisa F. Edge, HRL Laboratories, LLC, USA; [email protected] Clayton A. Jackson, HRL Laboratories, LLC, USA;
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