With the advances in quantum computers, the end of encryption as we know it is near. Current cryptosystems, such as RSA (Rivest, Shamir, and Adleman) has a security based on computational hardness assumptions which can potentially be broken with a powerful quantum computer in practical time. A private key can (will) be computed using the public key using a Quantum brute force approach.

The solution is to use Symmetric key systems but one of their drawbacks is the **rate at which the keys are generated vs the digital communication rates**, which are currently is significantly lower.

As a potential solution to this issue, a research paper from Duke’s University proposes to implement a mechanism to increase the rate at which a secret key is generated in QKD (Quantum key distribution) systems using **time-bin quantum photonic states**.

**Time-bin encoding** is a technique used in Quantum information science to encode a qubit of information on a photon.

Time-bin encoding is done by having a **single-photon** go through a Mach-Zender interferometer (MZ).

Mirrors positioning or the path shaped by an optical fiber in addition to polarizing cubes will lead the photons to take different paths, characterized by different lengths and thus leading to **different states**.

Time-bin encoding technique is **very robust against decoherence**, where particules, such as the photons in this case, may loose their phase relation. In a nutshell, coherence in the quantum world ensures that if a particule is modified, the linked particules are also modified. In this perspective this ensures that there will be **no loss of information from a system into the environment**.

Using commercial off-the-shelf components, the paper presents the results of an experiment where **high-dimensional quantum states (Qudits)** vs Qubits are used to transmit more than one secret bit per received photon.

Qudits are quantum objects for which the **number of possible states is greater than two**. Included among them are qutrits, which have three potential states, and ququarts, which boast four. Thanks to those additional states, it takes fewer qudits than qubits to do the same amount of processing.

Qubits can have two states, such as the polarization of a single photon: vertical polarization and horizontal polarization. In the quantum world, the property of **superposition** allows the photon for instance to be in both states at the same time.

Using optical fibers, the experiment was able to achieve an extractable secret key rate of **26.2 megabits/s** with a channel loss of 4 dB (equivalent to a **20-km-long optical fiber**), the highest secret key rate reported at this quantum channel loss. For comparison, you may refer to the table underneath where the highest rate of 23 Mbps was achieved only achieved with a simulated **0 Km** distance (0.1 dB loss).

The high QKD transmission rates resulting from these tests improve the feasibility of implementing QKD on large distance networks (**20 to 80 km**).

**Table 1**. Comparison of some notable high-rate QKD systems. (Click to enlarge)

**Sources: **

**Provably secure and high-rate quantum key distribution with time-bin qudits**, Nurul T. Islam, Charles Ci Wen Lim, Clinton Cahall, Jungsang Kim, Daniel J. Gauthier. in Sci. Adv. 2017;3: e1701491.- Wikipedia.

**Photo Credits**: Micro-focussing an Argon-ion laser onto a graphene sample. University of Exeter. / FlickR. CCC License.