  Continuous-variable systems protected by bosonic quantum error-correcting
codes have emerged as a promising platform for quantum information processing.
To date, design of codewords has centered on optimizing the occupation of basis
states in the error-relevant basis. Here, we propose utilizing the phase degree
of freedom in basis state probability amplitudes to devise codes that feature
destructive interference, and thus reduced overlap, between error codewords. To
showcase, we first consider the correction of excitation loss using single-mode
codes with Fock-space parity structure and show that, with a tailored
"two-level" recovery, altering the signs of probability amplitudes can
significantly suppress decoherence. We then study the joint channel of
excitation loss and Kerr effect, and show the critical role of nontrivial phase
for optimal quantum codes for such intricate yet important channels. The
principle is extended to improve bosonic codes defined in other bases and
multi-qubit codes, showing its wide applicability in quantum error correction.
