- Sooraj Bopanna

# Quantum Communication Fundamentals

An important aspect of quantum computing is the ability to communicate the processed

information. Thus, quantum communication plays an important role in the quantum computing ecosystem.

The field of quantum communication considers potential advantages of communicating via the transmission of quantum states. Quantum communication is fundamentally different than classical communication with respect to encoding, conveying, and authenticating information. The following three basic concepts reflect the differences and potential promise of quantum communication.

The first is called super dense coding, also referred to as quantum dense coding (QDC). QDC is a basic quantum communications protocol that conveys two bits of classical information through the transmission of a single qubit. This protocol makes use of quantum entanglement as a resource and requires that the sender and receiver have access to pre-shared entangled qubits.

There are several challenges to QDC’s real-world implementation.

For example, distributing a Bell state (simplest state of quantum entanglement) between distant users, a requirement to begin QDC, is not trivial in practice. Ideally, photons can be entangled in such degrees of freedom as polarization, frequency, or time of arrival. Unfortunately, in practice, a single photon is extremely fragile and can therefore be corrupted or lost during transmission through free-space or optical fiber over distances of tens or hundreds of kilometers.

The second is the no-cloning theorem, which states that an arbitrary unknown quantum bit cannot be copied. It is important to note that this theorem only applies to unknown quantum states, for if the state is known, it can be identically prepared over and over again. However, given a qubit in an unknown state, there is no means to copy it and end up with two qubits in the same unknown state. This has far reaching consequences, one of the most direct of which is that quantum states used for quantum communication cannot be amplified, since an amplifier makes many copies of a state and sends them along the transmission path to amplify the signal. Efforts to overcome this limitation have resulted in significant research into what are called quantum repeaters, devices which potentially enable longer distance transmission of quantum states, though at a heavy technological price.

The third concept is that attempts to intercept or measure a quantum state are detectable, a

pillar of secure quantum communication. The no-cloning theorem rules out duplication of an

unknown qubit state. Therefore, a potential eavesdropper has no choice but to measure some aspect of a qubit to glean its information, and this measurement process always leaves a detectable signature. A sender and receiver therefore know when an eavesdropper is present. This inherently makes quantum communication secure, as the sender and receiver can re-transmit the message when the message has been tampered with.

[Image Credit]: Quanta Magazine