AWGs for QKD – Quantum Key Distribution –

Quantum key distribution (QKD) is a secure communication method for exchanging encryption keys only known between shared parties.
It uses properties found in quantum physics to exchange cryptographic keys in such a way that is provable and guarantees security.

QKD enables two parties to produce and share a key that is used to encrypt and decrypt messages. Specifically, QKD is the method of distributing the key between parties.
Key distribution on a conventional scale relies on public key cyphers that use complicated mathematical calculations requiring a prohibitive amount of processing power to break. The viability of public key ciphers, however, faces several issues, such as the constant implementation of new strategies used to attack these systems, weak random number generators and general advances in computing power. In addition, quantum computing will render most of today’s public key encryption strategies unsafe.

QKD is different from conventional key distribution because it uses a quantum system that relies on basic and fundamental laws of nature to protect the data, rather than relying on mathematics.

QKD works by transmitting many light particles, or photons, over fiber optic cables between parties. Each photon has a random quantum state, and collectively, the photons sent make up a stream of ones and zeros. This stream of ones and zeros are called qubits and they are the equivalent of bits in a binary system. When a photon reaches its receiving end, it travels through a beam splitter, which forces the photon to randomly take one path or another into a photon collector. 

The receiver then responds to the original sender with data regarding the sequence of the photons sent and the sender then compares that with the emitter, which would have sent each photon.

Photons in the wrong beam collector are discarded; what’s left is a specific sequence of bits. This bit sequence can then be used as a key to encrypt data; any errors and data leakage are removed during a phase of error correction and other post-processing steps. 

To do this, Alice sends photons, the smallest part of light, to Bob. Since photons are not only particles but also waves, they oscillate. They can oscillate in different directions; when they oscillate in only one direction, they are called polarized.
With polarizing filters, we can filter photons, then, for example, only the photons that oscillate up and down can pass through; these are then called vertically polarized.

With the help of quantum mechanics it is possible to send single photons: Alice sends polarized photons to Bob and if Bob holds his filter the same way as the photons polarized by Alice, the light passes through.
If, on the other hand, he holds his filter crosswise to the direction of polarization, no photon passes through and each photon can be measured only once.

What does it happen if Bob turns his filter just a little bit to Alice’s polarization direction, diagonally?  Sometimes a photon can pass through the filter and sometimes not; if both polarize diagonally in the same direction, the photons all pass through again. If, on the other hand, they polarize in different diagonal directions, the photons no longer pass through Bob’s filter.

Quantum Key Exchange (QKD)

Alice makes notes of the polarization with which she sent the photons off and Bob makes a note of how he held his filter and whether light was received or not.
Now the two can talk publicly about how Alice polarized her photons and how Bob held his filter.Everyone can hear this.

Whenever one person used the filter diagonally and the other vertically or transversely, that part gets deleted; from the remaining ones they build their key. If an attacker (man-in-the-middle) called Eve tries to read along a photon with a polarization filter, she has to send a new photon to Bob afterwards.
Eve does not know if she has held the filter correctly: if she doesn’t see any light, she could have held her filter crosswise to Alice’s filter or just slightly differently, but the photon didn’t get through.

If she does see light, it could still be because the photon came through with the filter slightly rotated, so she may still have held the filter incorrectly. Eve doesn’t know if holding the filter diagonally was correct or not.

Photons cannot be copied and only be measured once, so Eve has to guess quite often what she sends on to Bob.
Alice and Bob only talk later about how they used their filtersand which parts they can use for their key.
Eve, however, has to decide beforehand whether she was correct with diagonal or non-diagonal, but without the public conversation, she has to guess how to proceed, and thus often makes mistakes.
Bob makes as many mistakes as Eve, but his are simply deleted, before Alice and Bob build their key, they compare individual digits.
They don’t use them for the key afterwards, but if they don’t match they know that someone has been listening, so the quantum encryption is used to agree on a key.
Since the method is based on the randomness of quantum mechanics, whether photons can pass through slightly twisted filters or not, it is considered unbreakable.

Types of QKD

There are many different types of QKD, but two main categories are prepare-and-measure protocols and entanglement-based protocols.
Prepare-and-measure protocols focus on measuring unknown quantum states. They can be used to detect eavesdropping, as well as how much data was potentially intercepted.

Entanglement-based protocols focus on quantum states in which two objects are linked together, forming a combined quantum state. The concept of entanglement means that measurement of one object thereby affects the other. If an eavesdropper accesses a previously trusted node and changes something, the other involved parties will know.

By implementing quantum entanglement or quantum superpositions, just the process of trying to observe the photons changes the system, making an intrusion detectable.
It is difficult to implement an ideal infrastructure for QKD. It is perfectly secure in theory, but in practice, imperfections in tools such as single photon detectors create security vulnerabilities. It is important to keep security analysis in mind.

Modern fiber optic cables are typically limited in how far they can carry a photon. The range is often upward of 100 km. Some groups and organizations have managed to increase this range for the implementation of QKD. The University of Geneva and Corning Inc. worked together, for example, to construct a system capable of carrying a photon 307 km under ideal conditions.
Another challenge of QKD is it relies on having a classically authenticated channel of communications established. This means that one of the participating users already exchanged a symmetric key in the first place, creating a sufficient level of security. A system can already be made sufficiently secure without QKD through using another advanced encryption standard. As the use of quantum computers becomes more frequent, however, the possibility that an attacker could use quantum computing to crack into current encryption methods rises, making QKD more relevant.

QKD attack methods

Even though QKD is secure in theory, imperfect implementations of QKD have the potential to compromise security. Techniques for breaching QKD systems have been discovered in real-life applications. For example, even though the BB84 protocol should be secure, there is currently no way to perfectly implement it.

The phase remapping attack was devised to create a backdoor for eavesdroppers. The attack takes advantage of the fact that one party member must allow signals to enter and exit their device. This process takes advantage of methods used widely in many commercial QKD systems.
Another attack method is the photon number splitting attack. In an ideal setting, one user should be able to send one photon at a time to the other user. However, most of the time, additional similar photons are sent. These photons could be intercepted without either party knowing. 

To combat this type of attack, an improvement to the BB84 protocol was implemented, called decoy state QKD, which uses a set of decoy signals mixed in with the intended BB84 signal while enabling both parties to detect if an eavesdropper is listening.

QKD implementation methods

There are two different approaches to implement QKD: one focuses on discrete variable (DV-QKD) and relies on single photons with encoded random data. The other one plays on the wave nature of light with information encoded in the quadrature of its electromagnetic fields, it is continuous variable (CV-QKD). Coherent homodyne or heterodyne detection is used to continuously retrieve the quadrature value of the signal to read the key into it.

In the market there are different modulation solutions for the transmitter side of the communication (Alice) and also for the receiver side (Bob) optical hybrid demodulators can be used.
One of the most technologically advanced intensity modulator is the Exail NIR-MX800: the intrinsic and unparalleled benefits of LiNbO3 modulation offers high bandwidth, high contrast and ease of use.

The Arb Rider AWG-7000 and AWG-5000 Series Arbitrary Waveform Generators allow controlling directly those kind of Electro-Optic Modulator and to generate very short optical pulses.
The unique features of generating pulse with 50 ps rise/fall time, 100 ps pulse width and 5Vpp amplitude offers the solution of driving the EOM without using an external amplifier.

The diagram above represents a typical connection of the Arbitrary Waveform Generator AWG-7000 with a first modulation block used to generate short optical pulses.

For example using Exail’s NIR-MX800 and the Active Technologies AWG-7000, very short optical pulses width from 100 ps can be achieved at 850 nm, 1310 nm and 1550 nm respectively.
It is important to note that in the connection diagram between the AWG-7000 and the Intensity Modulator are not used external amplifier, since the AWG-7000 is able to generate very narrow pulses of 100 ps width at full amplitude 5Vpp like in the pictures below.