  The collimation of relativistic jets launched from the vicinity of
supermassive black holes (SMBHs) at the centers of active galactic nuclei (AGN)
is one of the key questions to understand the nature of AGN jets. However,
little is known about the detailed jet structure for AGN like quasars since
very high angular resolutions are required to resolve these objects. We present
very long baseline interferometry (VLBI) observations of the archetypical
quasar 3C 273 at 86 GHz, performed with the Global Millimeter VLBI Array, for
the first time including the Atacama Large Millimeter/submillimeter Array. Our
observations achieve a high angular resolution down to $\sim$60 ${\rm \mu}$as,
resolving the innermost part of the jet ever on scales of $\sim 10^5$
Schwarzschild radii. Our observations, including close-in-time High Sensitivity
Array observations of 3C 273 at 15, 22, and 43 GHz, suggest that the inner jet
collimates parabolically, while the outer jet expands conically, similar to
jets from other nearby low luminosity AGN. We discovered the jet collimation
break around $10^{7}$ Schwarzschild radii, providing the first compelling
evidence for structural transition in a quasar jet. The location of the
collimation break for 3C 273 is farther downstream the sphere of gravitational
influence (SGI) from the central SMBH. With the results for other AGN jets, our
results show that the end of the collimation zone in AGN jets is governed not
only by the SGI of the SMBH but also by the more diverse properties of the
central nuclei.
