New quantum dot could make quantum communications possible

A new form of quantum dothas been developed by an international team of researchers that can produce identical photons at will, paving the way for multiple revolutionary new uses for light. Many upcoming quantum technologies will require a source of multiple lone photons with identical properties, and for the first time these researchers may have an efficient way to make them. With these quantum dots at their disposal, engineers might be able to start thinking about new, large-scale quantum communications networks.

The reason we need identical photons for quantum communication comes back to the non-quantum idea of key distribution. From a mathematics perspective, it’s trivially easy to encrypt any message so that nobody can read it, but very hard to encrypt a message so only some select individuals can read it, and nobody else. The reason is key distribution: if everybody who needs to decrypt a message has the associated key needed for decryption, then no problem. So how do you get the key to everyone who needs to decrypt it?

This Stanford invention helps handle entangled photons, but does it introduce vulnerabilities in the process?

Quantum key distribution uses the ability of quantum physics to provide evidence of surveillance. Rather than making it impossible to intercept the key, and thus decrypt the message, quantum key distribution simply makes it impossible to secretly intercept the key, thus giving the sender of the message warning that they should try again with a new key until one gets through successfully. Once you’re sure that your intended recipient has the key, and just as importantly that nobody else has it, then you could send the actual encrypted file via smoke signal if you really wanted to — at that point, the security of the transmission itself really shouldn’t matter.

There has been some promising research in this field — it’s not to be confused with the much more preliminary work on using quantum entanglement to transfer information in such a way that it literally does not traverse the intervening space. That may come along someday, but not for a long, long time.

Regardless, one of the big problems with implementing quantum key distribution is that the optical technology necessary to get these surveillance-aware signals from sender to recipient just aren’t there. In particular, the wavelength of photons changes as they move down an optical fiber — not good, since creating photon with precise attributes is the whole source of quantum security.

An Optalysys optical computer, on a desktop

So, unless you’re less than one quantum dot’s range away from the person you want to talk to, quantum security wouldn’t work; a theoretical quantum repeater would insert too much uncertainty about the wavelength of any light it ferried along. With this technology, it could be possible ferry quantum cryptographic information across real-world distances, across or even between continents in the networked way of regular digital internet traffic.

These quantum dots basically achieve perfect single-photon emission by super-cooling the quantum dots so the emitting atoms do not fluctuate. These fluctuations results in very slightly different emission wavelengths, so by slowing them with cryogenic temperatures, they reduce the signal noise. This should allow the re-emission of quantum key information in a reliable-enough form to preserve the quantum security setup.

Of course, quantum security isn’t perfect. You can still listen in on either the sender or receiver directly, or perhaps even find a way to surveil these quantum dots themselves, reading each photon as it’s absorbed and reemitted. Potential attackers could install optical splitters so they get and invalidate one copy of the key, while another arrives unmolested at its destination.

Short of telepathy, there will never be perfect communication security — not even quantum physics can change that.

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