A “donor” atom propelled by an ion beam is inserted very precisely in microseconds into an industry-standard silicon substrate.
The donor atom — in this case, antimony (Sb) —carries one more electron (five) than a silicon atom (four). Because electrons pair up, the odd Sb electron remains free.
Instruments monitor the free electron to determine if, under pressure from an electromagnetic field, it faces up or down, a property called “spin.” Electrons in this role, called qubits, signal “yes” or “no” from the subatomic scale, and so act as the information bearers of a quantum computer.
The ability to precisely place a donor atom in silicon means that it should be possible to insert a second donor atom just far enough away, in the “Goldilocks” zone where communication is neither lost through distance nor muffled by too-close proximity. Sandia will try to do this later this year, said lead researcher Meenakshi Singh, a postdoctoral fellow. Qubits “talking” to each other are the basis of quantum computing circuits.
The successful Sandia first step makes use of electromagnetic forces provided by a neighboring quantum dot pre-embedded in the silicon. The quantum dot — itself a tiny sea of electrons — contains a variety of energy levels and operates like a transistor to block or pass the qubit.
While components of this experiment have been demonstrated before, this is the first time all have worked together on a single chip, with researchers knowing accurately the vertical and horizontal placement of each qubit, instead of mere statistical approximations.
Applied Physics Letters - Electrostatically defined silicon quantum dots with counted antimony donor implants
Deterministic control over the location and number of donors is crucial to donor spin quantum bits(qubits) in semiconductor based quantum computing. In this work, a focused ion beam is used to implant antimony donors in 100 nm × 150 nm windows straddling quantum dots. Ion detectors are integrated next to the quantum dots to sense the implants. The numbers of donors implanted can be counted to a precision of a single ion. In low-temperature transport measurements, regular Coulomb blockade is observed from the quantum dots. Charge offsets indicative of donor ionization are also observed in devices with counted donor implants.
SOURCES - Sandia, Applied Physics Letters