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Deterministic Atom Implant
Deterministic atom implantation (Fig 1) has been developed from the industry-standard ion implantation process used in the fabrication of large scale integrated silicon devices. Our chips contain an active substrate sensitive to single ion implants (Fig 2). With this technique we are working towards the development of a multi-qubit demonstrator quantum computer system in which fundamental 1-qubit and 2-qubit gate operations are possible with high gate fidelities. Our team has already shown that implanted atoms can be programmed for single-shot spin readout in a Si:P MOS qubit structure with a high readout fidelity.
The next challenge will be to assemble 2-P-donor devices, with coupling via the exchange (J) interaction, and SET readout devices on each qubit enabling tomography and/or direct Hamiltonian determination of the exchange coupling strength and control to high precision. To approach this goal we will focus on the development of high-precision extensions of the deterministic atom implantation technique to fabricate promising devices that are robust to donor positional accuracy, defect tolerance and other issues of fabrication accuracy.
Simple spin transport demonstrator devices, based on chains of 3-5 P atoms, will be assembled using deterministic implantation (with also the benefits from experience with the parallel program in precision scanned probe construction) and a scanned nanostencil (Fig 3) on our Colutron ion beam system (Fig 4). The ultimate goal of this work will be to fabricate a device to investigate candidate spin transport mechanisms including coherent transport by adiabatic passage (CTAP).
A crucial contribution to the success of these strategies will be the development of techniques to control the defects and composition in the materials from which the devices are fabricated and to monitor the fabrication process flow for quality. In collaboration with Dr Jeff McCallum the application of ultra-sensitive materials characterisation techniques such as Deep Level Transient Spectroscopy will be employed to monitor the silicon gate oxide interface trap density which governs the success, or otherwise, of single atom Si:P devices. This work will be extended to probe the environment of single implanted P ions in collaboration with Martin Brandt at the WSI, Germany.
Ultimately, the replication of these pilot devices in isotopically pure 28Si will be possible which will open the possibility of exploiting the exceptionally long nuclear spin states of implanted 31P atoms. In addition, the application of optical control methods are becoming possible based on single-P-donors coupled to a photonic cavity. We seek to use our techniques to insert single atoms into suitable cavities and also exploit near-band-gap photon absorption which leads to a neutral donor with a bound exciton. We aim to electrically probe these with an integrated SET device, to enable electrical readout of optically-created spin states. We will also explore optical control of Er dopants in silicon, inserted by ion implantation, which have an optical transition at the telecommunications wavelength of 1.5 μm.
With these devices, optical coupling between two spatially separated electron spin qubits represents a key building block of quantum repeater systems for long-range quantum communications and ultimately for distributed quantum computation: the “quantum internet”.