What Is Quantum Communication? BB84 vs. Entanglement Distribution, and Toshiba's Development History
Key distribution for the quantum internet broadly falls into two approaches: "BB84" and "entanglement distribution." Here's how each one works, and how entanglement swapping — the technology that connects them — works.
BB84: Key Distribution Using Polarization States
BB84 is the first practical quantum key distribution (QKD) protocol, proposed in 1984 by Charles Bennett and Gilles Brassard. The sender (Alice) randomly prepares a photon's polarization in one of four states — a rectilinear basis (vertical/horizontal) and a diagonal basis rotated 45 degrees from it (45°/135°) — and sends it to the receiver (Bob). Bob measures each photon in a randomly chosen basis, without knowing in advance which basis Alice used.
After transmission, Alice and Bob publicly compare (not the values themselves, but) which basis was used for each photon over a public channel, and keep only the measurement results where the bases matched (this step is called "sifting"). If an eavesdropper (Eve) measures the photons to intercept them, the principles of quantum measurement guarantee that the original polarization state is disturbed, allowing Alice and Bob to statistically detect the presence of eavesdropping by comparing a sample of results afterward. The decisive difference from classical cryptography is that security here rests not on computational hardness, but on the laws of quantum mechanics themselves — the disturbance of a state by measurement, and the no-cloning theorem.
The Difference from Entanglement Distribution
BB84 is a "prepare-and-measure" scheme, whereas the other major approach, entanglement distribution (represented by the E91 protocol, proposed by Artur Ekert in 1991), generates a pair of photons in an entangled state and sends one photon of the pair to Alice and the other to Bob. The results each party independently measures show a strong correlation arising from entanglement (a correlation that cannot be explained by any classical hidden-variable model — the so-called violation of Bell's inequality), and by verifying the strength of this correlation, eavesdropping can be detected because it weakens the correlation.
| BB84 (prepare-and-measure) | Generates and sends a polarization state from a single-photon source each time. Simpler to implement, and currently the approach with the most advanced commercial deployment |
|---|---|
| Entanglement distribution (e.g. E91) | Generates entangled photon pairs and distributes them to both parties. Reduces reliance on a trusted single-photon source, and is well suited to future quantum repeater networks |
Entanglement Swapping: The Heart of the Quantum Repeater
Photons in optical fiber attenuate exponentially with transmission distance, so simply continuing to send photons places a hard limit on the reach of quantum communication. Entanglement swapping is the key technology that overcomes this limit.
Here is the procedure. First, one entangled photon pair is generated independently for segment A-B, and another independently for segment C-D (A and D are the two endpoints, B and C are the relay point). Next, a Bell state measurement (BSM) is performed on the photons at B and C at the relay point. As a result, A and D — which have never directly interacted with each other — are projected into a newly entangled state, determined by the measurement outcome.
By repeating this and chaining it across successive relay points, entanglement can be "relayed" between A and D without ever directly transmitting a photon between them. This is the basic principle behind the quantum repeater, and it is positioned as the core technology that will let a future quantum internet overcome the physical distance limit imposed by photon attenuation.
Toshiba's History of Building Real BB84 Hardware
Toshiba is one of the companies that has pursued the practical implementation of BB84 earliest and most consistently. In 1999, it began quantum cryptography research at Toshiba Research Europe (Cambridge Research Laboratory) in Cambridge, UK, and has steadily built up technical milestones ever since.
| 1999 | Began quantum cryptography research at its Cambridge, UK site |
|---|---|
| 2003 | Announced the world's first QKD over more than 100km of optical fiber |
| 2010 | Achieved the world's first continuous key generation rate exceeding 1Mbit/s |
| 2017 | Achieved a continuous key generation rate exceeding 10Mbit/s |
| Recent years | Carried out QKD over a 254km segment on a commercial optical fiber network in Germany; partnered with Orange Business in France to launch a commercial quantum-safe network service; demonstrated continuous key generation combined with 800G encrypted transmission between data centers in Illinois and Indiana, USA (21.8km, over a commercial fiber network) |
Toshiba's QKD system uses an efficient BB84 protocol with a decoy-state approach and phase encoding, and its key error probability is said to be under 10⁻¹⁰ (equivalent to less than once every 30,000 years). These achievements demonstrate that BB84 is not merely "a theoretical scheme," but a technology that can actually be operated on commercial networks.
Summary
- BB84 is a scheme that prepares and sends a photon's polarization state each time, and is currently the most commercially advanced QKD scheme
- Entanglement distribution is a scheme that distributes entangled photon pairs, and is well suited to future quantum repeater networks
- Entanglement swapping is the core technology of the quantum repeater, relaying entanglement between two points that never directly interact
- Toshiba began research in 1999 and has since set multiple world-first records in both key generation speed and transmission distance, advancing all the way to commercial deployment
Learn About the Security Model of the Quantum Internet
The next article explains how each layer of the OSI model changes for quantum communication infrastructure.
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