China launches world’s first quantum communications satellite, marks an achievement in the realm of higher technology, a milestone in space and cryptography. Quantum Experiments at Space Scale (QUESS), nicknamed Micius after the philosopher, lifted off from Jiuquan Satellite Launch Center at 1:40 AM local time (August 15, 2016 in the U.S.) and is currently maneuvering itself into a sun-synchronous orbit at 500 km.

QUESS is an experiment in the deployment of quantum cryptography — specifically, a prototype that will test whether it’s possible to perform this delicate science from space. Inside QUESS is a crystal that can be stimulated into producing two photons that are “entangled” at a subatomic, quantum level. Entangled photons have certain aspects — polarization, for example — that are the same for both regardless of distance — in fact, the satellite will test that at 1,200 km, which will set a new record. The trouble with this tech is that photons are rather finicky things, and tend to be bounced, absorbed, and otherwise interfered with when traveling through fibers, air, and so on. QUESS will test whether sending them through space is easier, and whether one of a pair of entangled photons can be successfully sent to the surface while the other remains aboard the satellite.

If this proves possible, the satellite will attempt quantum key distribution via these entangled photons. When measured, a photon will show its observers a random polarization state — but critically, entanglement means the other photon will always show the same random state. These correlated polarizations can be the basis of a cryptographic key known only to the observers.

The best thing about this is that apart from the original distribution of the photons, there is no transmission involved, or at least not one we understand and can intercept. Whatever links the two photons is intangible and undetectable — you can’t entangle a third one to listen in, and if even if you managed to interfere with the process, it would be immediately noticed by the observers of the original entangled photons, which would cease to be perfectly correlated. As you can imagine, an undetectable and perfectly secure channel for digital communications is of enormous potential value for an endless list of reasons. China is early to the game with QUESS, but they’re not the only ones playing. Other quantum satellites, though none quite so advanced, are in the ether right now, and more are sure to come. The experiments from the whole set will definitely be interesting — if anyone can find a way to explain what’s going on in them.

**Quantum communication**

Quantum communication is a field of applied quantum physics closely related to quantum information processing and quantum teleportation. It’s most interesting application is protecting information channels against eavesdropping by means of quantum cryptography. The most well known and developed application of quantum cryptography is quantum key distribution (QKD). QKD describes the use of quantum mechanical effects to perform cryptographic tasks or to break cryptographic systems. The principle of operation of a QKD system is quite straightforward: two parties (Alice and Bob) use single photons that are randomly polarized to states representing ones and zeroes to transmit a series of random number sequences that are used as keys in cryptographic communications. Both stations are linked together with a quantum channel and a classical channel. Alice generates a random stream of qubits that are sent over the quantum channel. Upon reception of the stream Bob and Alice — using the classical channel — perform classical operations to check if an eavesdroper has tried to extract information on the qubits stream. The presence of an eavesdropper is revealed by the imperfect correlation between the two lists of bits obtained after the transmission of qubits between the emitter and the receiver. One important component of virtually all proper encryption schemes is true randomnessm which can elegantly be generated by means of quantum optics.

Quantum communication is thus the art of transferring a quantum state from one place to another. Traditionally, the sender is named Alice and the receiver Bob. Quantum information science is an area of study based on the idea that information science depends on quantum effects in physics. It includes theoretical issues in computational models as well as more experimental topics in quantum physics including what can and cannot be done with quantum information. The term quantum information theory is sometimes used, but it fails to encompass experimental research in the area.

Subfields include:

- Quantum computing, which deals on the one hand with the question how and whether one can build a quantum computer and on the other hand, algorithms that harness its power.
- Quantum complexity theory
- Quantum cryptography and its generalization, quantum communication
- Quantum error correction
- Quantum communication complexity
- Quantum entanglement, as seen from an information-theoretic point of view
- Quantum dense coding

Quantum networks form an important element of quantum computing and quantum cryptography systems. Quantum networks allow for the transportation of quantum information between physically separate quantum systems. In distributed quantum computing, network nodes within the network can process information by serving as quantum logic gates. Secure communication can be implemented using quantum networks through quantum key distribution algorithms.

Optical quantum networks using fiber optic links or free-space links play an important role transmitting quantum states in the form of photons across large distances. Optical cavities can be used to trap single atoms and can serve as storage and processing nodes in these networks.

Quantum teleportation is a well-known quantum information processing operation, which can be used to move any arbitrary quantum state from one particle (at one location) to another.

In quantum information theory, a quantum channel is a communication channel which can transmit quantum information, as well as classical information. An example of quantum information is the state of a qubit. An example of classical information is a text document transmitted over the Internet.

More formally, quantum channels are completely positive (CP) trace-preserving maps between spaces of operators. In other words, a quantum channel is just a quantum operation viewed not merely as the reduced dynamics of a system but as a pipeline intended to carry quantum information.