Making Hacking Impossible – Quantum Cryptography

A better version of quantum key distribution. Highly sensitive data is abundant on the Internet. Advanced encryption techniques ensure that this sensitive data cannot be read or intercepted. High-performance quantum computers will be able to break these keys in seconds in the future.

It is fortunate, therefore, that quantum mechanical methods offer not only new, faster algorithms but also highly effective cryptography.

Quantum key distribution (QKD) is a jargon that says the device is protected against attacks on the communication channel, but not against manipulations or attacks on the devices. The devices could output keys that they had previously kept, which could be passed on to hackers.

With device-independent QKD, (abbreviated DIQKD), it’s a completely different story. Although this technology is theoretically known since 1990, it was only recently experimentally implemented by Harald Weinfurter from Ludwig Maximilian University of Munich and Charles Lim of the National University of Singapore (NUS).

There are many ways to exchange quantum mechanical keys. Either the transmitter transmits light signals to receivers or entangled quantum systems can be used. In the current experiment, scientists used two quantum mechanically entangled rubidium-atoms located 400m apart in two labs on the LMU campus.

These two facilities are connected by a 700-meter long fiber optic cable running under Geschwister Scholl Square, in front of the main building.

Scientists first stimulate each atom using a laser pulse to create entanglement. After this, the scientists stimulate each atom with a laser pulse. Each atom then releases a photon. Due to the conservation of angular momentum, the spin of an atom becomes entangled with its emitted photon’s polarization.

Two light particles travel across the fiber optic cable to a receiving station. A combined measurement of the photons will reveal atomic quantum memory.


Alice and Bob, as the parties are often referred to by cryptographers, measure the quantum states their respective atoms in order to exchange a key. This is done randomly in either two or four directions. If the directions match, the measurement results can be used to generate secret keys.

A so-called Bell inequality can be derived from the measurements. These inequalities were originally created by John Stewart Bell, a physicist who wanted to determine if nature could be described using hidden variables.

Weinfurter explains that DIQKD is used to “examine the possibility of manipulations to the devices, such as hidden measurement results not being saved in the devices before,”

The protocol that was implemented, developed by NUS researchers, has two settings for key generation.

Conventional QKD methods guarantee security only if the quantum devices are sufficiently well-characterized. Tim van Leent is one of the lead authors along with Kai Redeker and Wei Zhang of this paper.

He had doubts at first about the possibility of the experiment working. His team proved that his doubts were unfounded, and significantly improved the quality of the experiment, which he is happy to admit. A separate research group from Oxford also demonstrated the device-independent key allocation in conjunction with the collaboration project between LMU & NUS. The researchers created a system that included two entangled ions within the same laboratory to accomplish this feat.

Charles Lim says that these two projects are the foundation of future quantum networks in which absolute secure communication between distant locations is possible.

The next goal is to include more entangled atom pair pairs in the system. Van Leent says that this would enable many more entanglement states, which will increase the data rate and eventually the key security.

Researchers also want to expand the range. It was limited by approximately half of the photons lost in the fiber connecting the laboratories. Researchers were also able to convert the wavelength of photons into a low-loss region that is suitable for telecommunications. They were able to increase the distance of the quantum network connection from 33 km to 33 km by using a little more noise.

What is quantum cryptography?

Quantum cryptography uses quantum mechanics’ naturally occurring properties to secure data and transmit it in a way that cannot easily be hacked.

Cryptography is the art of protecting and encrypting data. Quantum cryptography differs from other cryptographic systems in that its security model is based on mathematics and physics.

Quantum cryptography protects the system from being compromised, regardless of who sent it. It is therefore impossible to view or copy data encoded in quantum states without being alerted by the receiver or sender. Quantum cryptography should be used to protect against quantum computing.

Quantum cryptography transmits data using individual photons or particles of light. Photons are binary bits. Quantum mechanics is the key to security in this system. These properties are the following:

Particles can exist in multiple states or places at once.

A quantum property can’t be observed without altering or disturbing it.

It is impossible to copy whole particles.

These properties make it impossible for anyone to measure the quantum states of any system without disturbing it.

Because photons have all the qualities required for quantum cryptography, they can be used because their behavior is well understood and they act as information carriers in optical fiber cables. Quantum key distribution (QKD) is a secure way to exchange keys. It’s one of the most well-known examples of quantum cryptography.

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Quantum Cryptography Working Nature

The theory behind quantum cryptography is that it follows a 1984 model.

This model assumes that Alice and Bob want to securely exchange messages. Alice sends Bob a key to initiate the message. The key is a stream of photons traveling in one direction. Each photon is a bit of data, either a 0, or 1. These photons, however, are not only linearly moving, but also oscillating or vibrating in a particular way.

The photons travel through the polarizer before Alice, and the sender initiates the message. A polarizer allows photons to pass through it in the same vibrations as others while allowing others to pass through it in a different state. You can choose from one of the following polarized states: vertical (1 bit), horizontal (0 bit), 45 degrees right (1 Bit), or 45 degrees left (1 Bit). One of the two polarizations that the transmission uses represent a single bit.

Now, the photons travel across optical fiber from Bob to the polarizer. The beam splitter reads each photon’s polarization. Bob doesn’t know the correct polarization so the photon key is given to Alice. Alice compares the key to determining which polarizer Alice used and tells Bob which one she used for each photon. Bob confirms that he used the correct type of polarizer. The photons that were read with an incorrect splitter are discarded and the rest of the sequence is taken as the key.

Let’s say Eve is an eavesdropper. Eve attempts to listen in but has the same tools and equipment as Bob. Bob is able to speak to Alice and confirm the type of polarizer used for each photon. Eve does not have this advantage. Eve renders the final key incorrectly.

Alice and Bob would know if Eve was listening in on them. Eve would observe the photon flow and change the positions of the photons that Alice and Bob expected to see.

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