  Quantum key distribution allows remote parties to generate
information-theoretic secure keys. The bottleneck throttling its real-life
applications lies in the limited communication distance and key generation
speed, due to the fact that the information carrier can be easily lost in the
channel. For all the current implementations, the key rate is bounded by the
channel transmission probability, $\eta$. Rather surprisingly, by matching the
phases of two coherent states and encoding the key information into the common
phase, this linear key-rate constraint can be overcome---the secure key rate
scales with the square root of the transmission probability, $O(\sqrt{\eta})$,
as proposed in twin-field quantum key distribution [Nature (London) 557, 400
(2018)]. To achieve this, we develop an optical-mode-based security proof that
is different from the conventional qubit-based security proofs. Furthermore,
the proposed scheme is measurement device independent, i.e., it is immune to
all possible detection attacks. The simulation result shows that the key rate
can even exceed the transmission probability $\eta$ between two communication
parties. In addition, we apply phase postcompensation to devise a practical
version of the scheme without phase locking, which makes the proposed scheme
feasible with the current technology. This means that quantum key distribution
can enjoy both sides of the world---practicality and security.
