  Quantum cryptography holds the promise to establish an
information-theoretically secure global network. All field tests of
metropolitan-scale quantum networks to date are based on trusted relays. The
security critically relies on the accountability of the trusted relays, which
will break down if the relay is dishonest or compromised. Here, we construct a
measurement-device-independent quantum key distribution (MDIQKD) network in a
star topology over a 200 square kilometers metropolitan area, which is secure
against untrustful relays and against all detection attacks. In the field test,
our system continuously runs through one week with a secure key rate ten times
larger than previous result. Our results demonstrate that the MDIQKD network,
combining the best of both worlds --- security and practicality, constitutes an
appealing solution to secure metropolitan communications.
