  Since the start of the COVID-19 pandemic, technology enthusiasts have pushed
for digital contact tracing as a critical tool for breaking the COVID-19
transmission chains. Motivated by this push, many countries and companies have
created apps that enable digital contact tracing with the goal to identify the
chain of transmission from an infected individual to others and enable early
quarantine. Digital contact tracing applications like AarogyaSetu in India,
TraceTogether in Singapore, SwissCovid in Switzerland, and others have been
downloaded hundreds of millions of times. Yet, this technology hasn't seen the
impact that we envisioned at the start of the pandemic. Some countries have
rolled back their apps, while others have seen low adoption.
  Therefore, it is prudent to ask what the technology landscape of
contact-tracing looks like and what are the missing pieces. We attempt to
undertake this task in this paper. We present a high-level review of
technologies underlying digital contact tracing, a set of metrics that are
important while evaluating different contact tracing technologies, and evaluate
where the different technologies stand today on this set of metrics. Our hope
is two-fold: (a) Future designers of contact tracing applications can use this
review paper to understand the technology landscape, and (b) Researchers can
identify and solve the missing pieces of this puzzle so that we are ready to
face the rest of the COVID-19 pandemic and any future pandemics. A majority of
this discussion is focused on the ability to identify contact between
individuals. The questions of ethics, privacy, and security of such contact
tracing are briefly mentioned but not discussed in detail.
