  We outline a contact-tracing strategy based on proximity sensing using mobile
devices. We discuss what an ideal system should look like and what it can do.
We show that, when adopted sufficiently broadly, such a contact-tracing
strategy can bring COVID-19 under complete control, end the need of social
distancing, and return the society to full normalcy. We also review some of the
challenges faced by the current generation of proximity-sensing technologies,
including Bluetooth Low Energy used by phones, and consider both interim and
longer-term solutions. Our main contribution is that we reason through why such
a contact-tracing strategy is likely to achieve the stated goal of returning to
full normalcy. Using probabilistic models, we show that universal adoption is
not necessary to achieve the stated goal, thus there is some room for
exceptions; however, the adoption rate needs to be very high, e.g., above
$95\%$ depending on the disease parameters. With more vigilance in disease
surveillance to detect mild cases earlier, the number may be brought down to
about $90\%$. The results call for deployment effort to be led by public
authorities at the state or federal level so that the required adoption rate
can be reached and the tracing coverage is wide enough to be relevant for
disease control.
