Silicon-based spin and charge quantum computation

Abstract

Silicon-based quantum-computer architectures have attracted attention because of their promise for scalability and their potential for synergetically utilizing the available resources associated with the existing Si technology infrastructure. Electronic and nuclear spins of shallow donors (e.g. phosphorus) in Si are ideal candidates for qubits in such proposals due to the relatively long spin coherence times. For these spin qubits, donor electron charge manipulation by external gates is a key ingredient for control and read-out of single-qubit operations, while shallow donor exchange gates are frequently invoked to perform two-qubit operations. More recently, charge qubits based on tunnel coupling in P + 2 substitutional molecular ions in Si have also been proposed. We discuss the feasibility of the building blocks involved in shallow donor quantum computation in silicon, taking into account the peculiarities of silicon electronic structure, in particular the six degenerate states at the conduction band edge. We show that quantum interference among these states does not significantly affect operations involving a single donor, but leads to fast oscillations in electron exchange coupling and on tunnel-coupling strength when the donor pair relative position is changed on a lattice-parameter scale. These studies illustrate the considerable potential as well as the tremendous challenges posed by donor spin and charge as candidates for qubits in silicon.Key words: semiconductors, quantum computation, nanoelectronic devices, spintronics, nanofabrication, donors in silicon.

An exchange-based surface-code quantum computer architecture in silicon

“…The 2D qubit plane is sandwiched between the top and bottom layout of wires forming source and drain. The exponential decay of the exchange interaction with the separation between the donor atoms is well known in the literature, as is the sensitivity of the interaction to valley interference effects [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35]. This results in a tension between donor separation and exchange strength to design a fast CNOT gate while maintaining sufficient distance between the atoms to allow for control wires, also known as pitch problem.…”

“…The current state-of-the-art scanning tunnelling microscope (STM) based atomic-precision fabrication technology [5] has demonstrated donor placement with ±𝑎 0 accuracy, where 𝑎 0 is the lattice constant of silicon. However, even such small variations in the donor position may lead to considerably large variations in exchange interaction [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35], placing stringent requirement on uniformity assumptions in the design of control schemes for large-scale architectures [30,35]. In the past, strategies have been developed to mitigate the impact of exchange variations, which include the design of robust composite pulse schemes such as BB1 [36], exchange char- acterisation [30], the application of electric fields [37] and the placement of donor atoms along the [110] crystal direction [34].…”

Hybrid Solid-State Qubits: The Powerful Role of Electron Spins

“…Although it is possible to convert a superconducting qubit state into cavity microwave photon (3), achieving a useful strong coupling between such a photon and a single electron spin appears beyond capabilities of current technology 5 . Instead, the spin-cavity coupling (which scales as √ N for N spins) can be enhanced by placing a larger ensemble of spins within the mode volume of the cavity, thus ensuring that a microwave photon in the cavity is absorbed into a collective excited state of the ensemble.…”

Maximum density of quantum information in a scalable CMOS implementation of the hybrid qubit architecture

“…Implementations of charge and spin qubits in silicon have been explored in both quantum dots sytems [4,5,6,7] and single dopant atom transistors [8,9,10,11,12,13,14,15,16]. Coherent manipulation of quantum states have been demonstrated in both atomic systems [17,18], as well as in semiconductor quantum dots (QD), which take advantages from more relaxed bounds on the device dimensions [19,20].…”