Can single photons from a quantum dot secure a link between cities?
A quantum dot that emits one photon at a time in the telecom band produced secure keys between two cities with a very low error rate for more than a day.
Source
High-rate intercity quantum key distribution with a semiconductor single-photon source
Study at a glance
- Design
- Other — Physics experiment: a quantum dot single-photon source sent polarisation-encoded photons through lab fibre spools and a deployed 79 km fibre between Hannover and Braunschweig, with key rates computed from measured photon statistics.
- N
- No participant sample; the system is one quantum-dot source measured in the lab and over one deployed fibre link.
- Population
- An InAs/InGaAs quantum dot in a circular Bragg grating emitting telecom C-band single photons, tested over lab fibre spools and an intercity fibre
- Outcome
- Quantum bit error ratio, secret key bits per pulse and secret key rate, single-photon purity g(2)(0), and maximum tolerable loss
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What they did
The researchers used a semiconductor quantum dot inside a bullseye-shaped optical cavity that emits single photons at telecom wavelengths. They encoded bits in the photons' polarisation using the standard BB84 protocol and first tested the system through lab fibre spools of increasing length. They then sent photons over a 79 km installed fibre from Hannover to Braunschweig, measuring error rates, single-photon purity and secret key rates over many hours.
What they found
Over the intercity link the average quantum bit error ratio was about 0.65% and secure keys were sustained for 35 hours. The average finite-key secret key rate was about 5.35 kbit per second, and the source kept high single-photon purity during a 12-hour measurement. Lab measurements and simulations suggested positive keys up to a loss of 28.11 dB, around 144 km of standard fibre.
The limits
What it doesn't show
The polarisation states were prepared statically with motorised waveplates rather than chosen randomly at high speed, so this is not yet a complete real-time QKD system; the authors model, rather than demonstrate, a system with a fast modulator. Temporal filtering was applied to lower errors, and some performance numbers (such as a 311.86 km reach) are projections under improved parameters. It is a single source and a single link, and the count rate saturates at high clock rates because of the dot's emission lifetime.
Key terms
- Quantum key distribution (QKD)
- A way for two parties to create a shared secret key whose security rests on quantum physics, since measuring photons disturbs them.
- BB84 protocol
- The first QKD scheme: bits are encoded in one of two sets of photon polarisations, and only measurements made in the matching basis are kept.
- Quantum dot single-photon source
- A nanoscale semiconductor island that, when excited, emits photons one at a time on demand.
- g(2)(0)
- The second-order correlation at zero delay; values near zero mean the source rarely emits two photons at once.
- Quantum bit error ratio (QBER)
- The fraction of sifted key bits that disagree between sender and receiver; lower means more key can be extracted.
- Finite-key regime
- Security analysis that accounts for the statistical uncertainty of using a limited number of detected bits, giving lower but more realistic key rates.
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Quiz yourself
Which QKD protocol was implemented?
Common questions
Why use a true single-photon source instead of a dim laser?
Dim laser pulses sometimes contain two or more photons that an eavesdropper could split off; a single-photon source avoids this without the extra complexity of decoy states.
Why does emitting in the telecom C-band matter?
Standard optical fibre has its lowest loss in the telecom C-band, so these photons can travel much further than the 780 to 900 nm photons most quantum dots emit.
Is this a working encrypted link?
It generates key rates good enough in principle for encrypting speech, but the polarisation encoding was static, so a fully random, real-time system still has to be built.
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