• Physics 18, 126
The efficiency of a quantum cryptography scheme can be improved by replacing conventional attenuated lasers with single-photon quantum-dot sources.
Quantum physics enables communication methods that guarantee the ultimate security of messages sent over public channels. But taking advantage of this level of secrecy is not as simple as picking up the phone: Before any information is sent, the communicators must go through the slow process of creating a shared quantum key that is used to encrypt and decrypt the message. Researchers at the University of Science and Technology of China (USTC) have now made that process more efficient by generating the key using a semiconductor quantum dot rather than a conventional laser [1]. The team demonstrated the method by sharing a quantum key between buildings on the USTC campus.
Making the exchange of a message invulnerable to eavesdropping doesn’t strictly require quantum resources. All you need to do is to encrypt the message using a one-use-only random key that is at least as long as the message itself. What quantum physics offers is a way to protect the sharing of such a key by revealing whether anyone other than sender and recipient has accessed it.
Imagine that a sender (Alice) wants to send a message to a recipient (Bob) in the presence of an eavesdropper (Eve). First, Alice creates a string of random bits. According to one of the most popular quantum communication protocols, known as BB84, Alice then encodes each bit in the polarization state of an individual photon. This encoding can be performed in either of two orientations, or “bases,” which are also chosen at random. Alice sends these photons one at a time to Bob, who measures their polarization states. If Bob chooses to measure a given photon in the basis in which Alice encoded its bit, Bob’s readout of the bit will match that of Alice’s. If he chooses the alternative basis, Bob will measure a random polarization state. Crucially, until Alice and Bob compare their sequence of measurement bases (but not their results) over a public channel, Bob doesn’t know which measurements reflect the bits encoded by Alice. Only after they have made this comparison—and excluded the measurements made in nonmatching bases—can Alice and Bob rule out that eavesdropping took place and agree on the sequence of bits that constitutes their key.
The efficiency and security of this process depend on Alice’s ability to generate single photons on demand. If that photon-generation method is not reliable—for example, if it sometimes fails to generate a photon when one is scheduled—the key will take longer to share. If, on the other hand, the method sometimes generates multiple photons simultaneously, Alice and Bob run the risk of having their privacy compromised, since Eve will occasionally be able to intercept one of those extra photons, which might reveal part of the key. Techniques for detecting such eavesdropping are available, but they involve the sending of additional photons in “decoy states” with randomly chosen intensities. Adding these decoy states, however, increases the complexity of the key-sharing process.
The usual method for generating single photons under the BB84 protocol is surprisingly low-tech: A laser is shone through an attenuating filter so that almost all the photons are absorbed. Whether the transparency of the filter is optimized for security (minimizing multiple-photon events) or efficiency (minimizing zero-photon events), the fraction of these “weak coherent pulses” that contain single photons is constrained by the statistical properties of coherent-light sources. The theoretical maximum fraction is 1/e, or about 37%, but real-world setups typically achieve values of less than 10%.
The USTC researchers sought to raise this ratio by replacing the attenuated laser with a more reliable single-photon source. They fabricated a quantum dot comprising indium arsenide on gallium arsenide and coupled it to a microcavity that enhanced the quantum dot’s emission via the Purcell effect. By exciting the quantum dot using a carefully shaped laser pulse, the researchers achieved a single-photon emission rate of 71% and a multiphoton emission rate of 2%. They then tested the concept by transmitting a quantum key over free space, bouncing the signal off a pair of mirrors more than 100 m away from the laboratory. After accounting for errors and photons lost to scattering (which eat up most of the key sequence), they measured a key rate of 1.08 × 10–3 bits per pulse―nearly twice the rate that they would expect to achieve if the same link used weak coherent pulses from a conventional laser.
“I think the experiment is quite a feat,” says University of Toronto physicist Hoi-Kwong Lo, who was not involved in the study. Lo is one of the researchers behind the so-called decoy-state quantum key distribution protocol, a version of the BB84 scheme with enhanced security. “This is the first time that a single-photon source has been used in quantum key distribution to beat the decoy-state-protocol key rate,” Lo says.
Despite its improved reliability, the quantum-dot photon source falls short of doing away with the need for decoy states entirely, since it still produces rare multiphoton events. “If the source could emit single photons with 100% reliability, then decoy states would not be required,” says Jian-Wei Pan, who leads the USTC team. “Although the quantum-dot source can emit single photons with a better probability than weak coherent pulses, it is not perfect, so decoy states are still helpful.”
–Marric Stephens
Marric Stephens is a Corresponding Editor for Physics Magazine based in Bristol, UK.
References
- Y. Zhang et al., “Experimental single-photon quantum key distribution surpassing the fundamental weak coherent-state rate limit,” Phys. Rev. Lett. 134, 210801 (2025).
