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Conditional vs. Absolute Security
In theory, quantum cryptography seems to be a successful turning point in the information security sector. However, no cryptographic method can ever be absolutely secure. In practice, quantum cryptography is only conditionally secure dependent on a key set of assumptions.

Single Photo Source Assumption

The theoretical basis for quantum key distribution assumes a single photon source. However, single photo sources are difficult to construct, and most real-world quantum cryptography systems use faint laser sources as a medium for information transfer. These multi-photon sources open a pathway for eavesdropper attacks, particularly a photo splitting attack. An eavesdropper, Eve, is able to split the multi-photon source and retain one copy for herself. The other photons are then transmitted to Bob without any measurement or trace that Eve captured a copy of the data. Scientists believe they can retain security with a multi-photon source by using decoy states that test for the presence of an eavesdropper.

Identical Detector Efficiency Assumption

In practice, multiple single photon detectors are used in quantum key distribution devices, one for Alice and one for Bob. These photodetectors are tuned to detect an incoming photon during a short window of only a few nanoseconds. Due to manufacturing differences between the two detectors, their respective detection windows will be shifted by some finite amount. An eavesdropper, Eve, can take advantage of this detector inefficiency by measuring Alice's qubit and sending a "fake state" to Bob. Eve first captures the photon sent by Alice and then generates another photon to send to Bob. Eve manipulates the phase and timing of the "faked" photon in a way that prevents Bob from detecting the presence of an eavesdropper. The only way to eliminate this vulnerability is to eliminate differences in photodetector efficiency, which is difficult to do given finite manufacturing tolerances that cause optical path length differences, wire length differences, and other defects.

Advantages of Quantum Cryptography
Cryptography is the strongest link in the chain of data security. However, interested parties cannot assume that cryptographic keys will remain secure indefinitely. Quantum cryptography has the potential to encrypt data for longer periods than classical cryptography. Using classical cryptography, scientists cannot guarantee encryption beyond approximately 30 years, but some stakeholders could use longer periods of protection. Take, for example, the healthcare industry. As of 2017, 85.9% of office-based physicians are using electronic medical record systems to store and transmit patient data. Under the Health Insurance Portability and Accountability Act, medical records must be kept secret. Typically, paper medical records are shredded after a period of time, but electronic records leave a digital trace. Quantum key distribution can protect electronic records for periods of up to 100 years. Also, quantum cryptography has useful applications for governments and military as, historically, governments have kept military data secret for periods of over 60 years.