Security in encrypted communication is a top priority because of our highly connected and mobile society’s increasing reliance on the internet. Engineers at Department of Electrical Engineering and Computer Science and the Research Laboratory of Electronics, MIT, have developed a new protocol for high-speed communication between two parties with security vouchsafed by the laws of quantum physics. The protocol can also be used to distribute cryptographic keys, as in quantum key distribution (QKD) at much higher secure key rates than existing QKD methods.

Quantum illumination (QI) is a common encryption technique that uses multimode entangled light beams to secure communication against a passive eavesdropper, who is allowed to collect all light that is lost in propagation between the two communicating parties. QI faces issues that keep it from being applicable, namely: (1) it is not secure against an active attack in which an eavesdropper injects their own light into the sender to learn about the message; (2) its protocol has limitations on its secure data rate and on the distance between the sender and receiver. The engineers developed a new approach that overcomes these barriers and provides for much higher secure communication rates with longer distances.

Consider two communicators, Party 1 and Party 2. In this new QI method, Party 1 generates a broadband noise source and sends a small amount to Party 2, who encodes the message on that light using binary phase shift keying. Party 2 then sends the modulated light through an amplifier that helps the message bits overcome transmission losses. The amplifier injects a very significant amount of noise — thousands of times stronger than the message signal strength — that masks the message from a passive eavesdropper. Party 1 receives this noisy signal and combines it with a retained local oscillator (LO) that they derived from the broadband noise source. Homodyne reception allows Party 1 to decode the message at a low bit error rate, while the noise from Party 2’s amplifier precludes the eavesdropper from getting that information because they lack the LO that Party 1 possesses.

To thwart an active attack from an eavesdropper, Party 1 employs a multimode entanglement source in which they randomly choose to send to Party 2 the signal beam of entangled signal and idler with the same bandwidth as the broadband noise source. The idler beam is sent to a single-photon counter to monitor and record its photon flux and detection times. Party 2 taps part of the incoming light and similarly sends it to their single-photon counter. Both parties must maintain desired levels of photon flux and coincidences in their photon detection times. An active eavesdropper who injects their own light into the communication channel will necessarily disrupt level of coincidences between the Parties’ photon detection times and alert them to the active attack.

Some of the key advantages of this technology are: It uses commercial off-the-shelf components for easy implementation, broadband LO can be amplified to allow long-distance transmission without degradation, and Homodyne reception with broadband LO does not have data bandwidth limitation.

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