  Technical means for identifying when tampering occurs is a critical part of
many containment and surveillance technologies. Conventional fiber optic seals
provide methods for monitoring enclosed inventories, but they are vulnerable to
spoofing attacks based on classical physics. We address these vulnerabilities
with the development of a quantum seal that offers the ability to detect the
intercept-resend attack using quantum integrity verification. Our approach
represents an application of entanglement to provide guarantees in the
authenticity of the seal state by verifying it was transmitted coherently. We
implement these ideas using polarization-entangled photon pairs that are
verified after passing through a fiber-optic channel testbed. Using binary
detection theory, we find the probability of detecting inauthentic signals is
greater than 0.9999 with a false alarm chance of $10^{-9}$ for a 10 second
sampling interval. In addition, we show how the Hong-Ou-Mandel effect
concurrently provides a tight bound on redirection attack, in which tampering
modifies the shape of the seal. Our measurements limit the tolerable path
length change to sub-millimeter disturbances. These tamper-indicating features
of the quantum seal offer unprecedented security for unattended monitoring
systems.
