Quantum key distribution (QKD) has always faced a stubborn limitation: to reach its theoretical promise, a system needs to use stable single-photon sources that produce exactly one photon on demand. Those sources are difficult to build and pricey to operate. In practice, most networks use faint laser pulses, which are good enough to demonstrate QKD, but vulnerable to multi-photon leakage and limited in range.
A research group at the Hebrew University of Jerusalem, working with Los Alamos National Laboratory, has developed a different approach. Instead of waiting for flawless emitters, the team used quantum dots, which are tiny semiconductor particles so small that their actions are controlled by quantum mechanics. To these quantum dots, the team applied two purpose-built protocols that make imperfect light sources work more predictably.
The result: secure key exchange over significantly longer distances than state-of-the-art laser systems, shown in both simulations and lab experiments and validated on a room-temperature testbed.
The research, led by Ph.D. students Yuval Bloom and Yoad Ordan under Professor Ronen Rapaport, appears in PRX Quantum. Their work found a solution to erratic photon transmitters: sub-Poissonian quantum-dot emitters paired with protocols that manage the two-photon events rather than trying to eliminate them. (To be sub-Poissonian refers to a more predictable or anti-bunched pattern, in contrast with the randomness of a Poisson distribution.)
Two Key Techniques
At the core of the research advance are two techniques. The first is a truncated decoy-state protocol, a rethink of a widely used QKD approach tuned for imperfect single-photon sources. By deliberately shaping the photon-number statistics and mixing in carefully chosen decoy settings, the system can expose potential eavesdropping that exploits extra photons. The second technique is known as a heralded purification protocol. This protocol effectively filters emissions in real time so that only the trustworthy single-photon detections contribute to the cryptographic key.
Together, these two techniques convert what used to be a liability—occasional two-photon outputs—into information the receiver can use to secure the channel.
The team’s research is significant because lasers distribute photons according to Poisson statistics (meaning they tend to be random). Even when dimmed, some pulses carry more than one photon, a weakness that sophisticated attackers can pry open. By engineering quantum dots with nanoantennas, the Hebrew University team moved the emission toward sub-Poissonian behavior and then used protocol-level controls to manage what remains.
In experiments, the approach extended the distance for secure key generation by more than three decibels compared with best-in-class laser-based QKD. This improvement translates into meaningfully longer links or higher key rates at the same distance.
What It Means: Quantum is More Accessible to Real World Uses
The larger significance of the team’s research: quantum protocols can be implemented with equipment already found in many labs, lowering the barrier to entry for quantum-secure pilots.
The team’s protocols are compatible with a range of non-ideal emitters, suggesting a path to multi-vendor, cost-sensitive deployments. Standards bodies will need to certify purified keys and decoy schedules for production networks. And carriers considering quantum overlays will need to understand how the extra three-plus decibels play across their installed networks.
The bottom line here is that the researchers have pushed QKD beyond the limits set by today’s faint lasers and made a convincing case that practical, affordable quantum security is within reach.
