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Keeping a Secret
A laser for quantum encryption

Context: Even the best communication security can’t prevent an unauthorized party from intercepting and attempting to decode a message. Quantum encryption harnesses a feature of quantum mechanics to solve this problem, making it impossible to observe (or tap into) a system without fundamentally disturbing it, and thus being discovered. Designs for quantum encryption systems have proven simple and elegant but so far impossible to build. Now a team led by Hong-Gyu Park at the Korea Advanced Institute of Science and Technology has moved one step closer to finding this Holy Grail, creating a microlaser capable of transmitting quantized light waves that may one day carry messages with greater security.

Methods and Results: The laser was fabricated out of the semiconducting material indium gallium arsenide phosphide, chosen because it can be fashioned to emit photons when a current passes through it. The critical lasing component is a “photonic crystal,” a perforated disc of the semiconductor that traps photons, ensuring that they are emitted at a single, constant wavelength. The crystal rests on a narrow post large enough to ensure good electrical activity but small enough not to disrupt the crystal’s structure. For the first time, the Korean researchers have demonstrated that an electrically activated microlaser can meet these competing needs.

Why it matters: For quantum cryptography to work, the creator of a message must be able to encode information in single photons and send them at set time intervals. Intercepting a photon destroys the information, revealing the presence of an eavesdropper. However, if a given bit of information requires more than one photon, the message can be intercepted without being detected. Consequently, the laser in a quantum encryption system must reliably convert an electrical pulse into a single photon at a prescribed wavelength. This work falls short of that goal, as it converts each electrical input into multiple photons. But it is an important step toward building new photon sources for optical communication.

Source: Park, H. G. et al. (2004) Electrically driven single-cell photonic crystal laser. Science 305:1444-7.

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