# Quantum Computing and Communications, and The Quantum Internet

**Travelling Towards a Quantum Internet at Light Speed**

** - Overview**

Today’s internet connects us globally. It sends packets of information that carry our communications in classical signals -- sent by bursts of light through optical fibers, electrically through copper wire, or by microwaves to make wireless connections. It is fast and reliable. So why develop a quantum internet that uses single photons -- the smallest possible quantum of light -- to carry information instead?

Because there are new scientific domains to explore. Quantum physics governs the domain of the very small. It allows us to understand – and use to our advantage – uniquely quantum phenomena for which there is no classical counterpart. We can use the principles of quantum physics to design sensors that make more precise measurements, computers that simulate more complex physical processes, and communication networks that securely interconnect these devices and create new opportunities for scientific discovery.

A quantum machine could one day drive big advances in areas like artificial intelligence and make even the most powerful supercomputers look like toys.

**- A New Kind of Computing **

We experience the benefits of classical computing every day. However, there are challenges that today’s systems will never be able to solve. For problems above a certain size and complexity, we don’t have enough computational power on Earth to tackle them.

To stand a chance at solving some of these problems, we need a new kind of computing. Universal quantum computers leverage the quantum mechanical phenomena of superposition and entanglement to create states that scale exponentially with number of qubits, or quantum bits.

Quantum computers are making all the headlines these days, but quantum communication technology may actually be closer to practical implementation. The building blocks for these emerging technologies are more or less the same. They both use qubits to encode information -- the quantum equivalent to computer bits that can simultaneously be both 1 and 0 thanks to the phenomena of superposition. And they both rely on entanglement to inextricably link the quantum states of these qubits so that acting on one affects the other. But while building quantum computers capable of outperforming conventional ones on useful problems will require very large networks of qubits, you only need a handful to build useful communication networks.

**- How Do Quantum Computers Work?**

Normal computing operations use a bit (also known as a binary bit), which is the smallest unit of data in a computer and takes on a value of 0 or 1. Quantum computing uses a similar system of bits, called qubits. However, qubit values can be superimposed, so they can't actually take on two values, but one of three: 0, 1, and a 0 or 1 value. Thus, unlike conventional computers, this makes it possible to operate on two values at the same time.

Quantum computers perform calculations based on the probability of an object's state before it is measured - instead of just 1s or 0s - which means they have the potential to process exponentially more data compared to classical computers. Classical computers carry out logical operations using the definite position of a physical state. These are usually binary, meaning its operations are based on one of two positions. A single state - such as on or off, up or down, 1 or 0 - is called a bit.

In quantum computing, operations instead use the quantum state of an object to produce what's known as a qubit. These states are the undefined properties of an object before they've been detected, such as the spin of an electron or the polarisation of a photon. Rather than having a clear position, unmeasured quantum states occur in a mixed 'superposition', not unlike a coin spinning through the air before it lands in your hand. These superpositions can be entangled with those of other objects, meaning their final outcomes will be mathematically related even if we don't know yet what they are.

The complex mathematics behind these unsettled states of entangled 'spinning coins' can be plugged into special algorithms to make short work of problems that would take a classical computer a long time to work out... if they could ever calculate them at all. Such algorithms would be useful in solving complex mathematical problems, producing hard-to-break security codes, or predicting multiple particle interactions in chemical reactions.

**- Potential Strengths**

Quantum computers certainly have potential. In theory, they can solve problems that classical computers cannot handle at all, at least in any realistic time frame. Take factorization. Finding prime factors for a given integer can be very time consuming, and the bigger the integer gets, the longer it takes. Indeed, the sheer effort required is part of what keeps encrypted data secure, since decoding the encrypted information requires one to know a “key” based on the prime factors of a very large integer. In 2009, a dozen researchers and several hundred classical computers took two years to factorize a 768-bit (232-digit) number used as a key for data encryption. The next number on the list of keys consists of 1024 bits (309 digits), and it still has not been factorized, despite a decade of improvements in computing power. A quantum computer, in contrast, could factorize that number in a fraction of a second – at least in principle.

Other scientific problems also defy classical approaches. A chemist, for example, might know the reactants and products of a certain chemical reaction, but not the states in between, when molecules are joining or splitting up and their electrons are in the process of entangling with each other. Identifying these transition states might reveal useful information about how much energy is needed to trigger the reaction, or how much a catalyst might be able to lower that threshold – something that is particularly important for reactions with industrial applications. The trouble is that there can be a lot of electronic combinations. To fully model a reaction involving 10 electrons, each of which has (according to quantum mechanics) two possible spin states, a computer would need to keep track of 210 = 1024 possible states. A mere 50 electrons would generate more than a quadrillion possible states. Get up to 300 electrons, and you have more possible states than there are atoms in the visible universe.

Classical computers struggle with tasks like these because the bits of information they process can only take definite values of zero or one (1 or 0), and therefore can only represent individual states. In the worst case, therefore, states have to be worked through one by one. By contrast, quantum bits, or qubits, do not take a definite value until they are measured; before then, they exist in a strange state between zero and one, and their values are influenced by whatever their neighbours are doing. In this way, even a small number of qubits can collectively represent a huge “superposition” of possible states for a system of particles, making even the most onerous calculations possible.

### - Optical Quantum Networks

Optical quantum networks for distributing entanglement between quantum machines will enable distributed quantum computing, secure communications and new sensing methods.

These networks will contain quantum transducers for connecting computing qubits to travelling optical photon qubits, and quantum repeater links for distributing entanglement at long distances.

Quantum optical networks are similar to classical networks, but they use qubits to transmit information. Quantum networks exploit quantum phenomena such as superposition, unclonability, and entanglement. These phenomena cause particles, such as photons of light, to appear in a superposition state. This means they can represent many combinations of 1's and 0's at the same time.

Quantum networks can use entanglement to create connections rather than passing data directly through the network. They can also detect with certainty whether communications have been intercepted.

NIST (National Institute of Standards and Technology) is studying the architecture of quantum optical networks and how to integrate them with classical networks. They are also working on the management and control plane software stack.

Toshiba Europe's Cambridge Research Laboratory demonstrated optical fiber quantum communications over 600 kilometers in length.

**<More to come ..>**