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Quantum mechanics has a reputation for being incredibly difficult and in many ways counterintuitive, so we’ve compiled a series of answers to our most frequently asked questions below, including the fundamental features of quantum mechanics, the reason for the increasing focus on quantum and some key questions regarding our technologies.

Why Quantum?

Quantum mechanics was developed and refined during the first half of the last century, as the branch of physics required to understand matter at the atomic, nuclear and fundamental particle levels, and to understand light at the level of single quanta, or photons. It has since proved to be an invaluable tool in understanding the building blocks of all the information technologies (IT) we use in the modern world, such as: electronic circuits that underpin computers and all the other devices we rely on every day; lasers and optoelectronic components that underpin communications technologies and numerous home, consumer and personal devices. Indeed, quantum mechanics has aided the repeated improvements of all these technologies to the state-of-the-art from which we benefit today.

The fundamental features of quantum physics, which determine how atoms and photons behave, run counter to our intuition and everyday experience. Quantum systems can be in multiple states at the same time – this is termed superposition (of states). Furthermore, quantum systems, even when separated by large distances, can be correlated more strongly than any correlations familiar to us – such systems are termed entangled. Finally, when any of us try to measure or interact with a quantum system to learn about what it is doing, we inevitably and irreversibly disturb it. This relationship between disturbance and information gained is fundamental. It is not something that we will overcome by building better measurement devices and probes in the future – it is built into Nature.

These fundamental features of quantum mechanics were appreciated in the early stages of its development. However, study of these features was viewed as a somewhat academic pursuit – undertaken only to understand the basis of this part of physics. They didn’t affect our macroscopic and “classical” (meaning non-quantum) lives. Fifty years ago there was no inkling at all that these quantum behaviours would ever contribute to our daily lives. Even today, the electronic, optical and other components that comprise our conventional IT do not exhibit these counter-intuitive behaviours. However, things are now changing. Over the last few decades – initially just theoretically or abstractly, but increasingly now as actual prototype devices – it has been realised that these fundamental features of quantum physics can play centre stage in completely new technologies. These new “quantum technologies” have the potential to outperform our conventional IT, or even achieve some tasks impossible with our usual stuff. The benefits could arise across IT: in computing and processing, in sensing and imaging, and in communications.

What is Quantum Mechanics?

Everything you see on a day to day basis can be explained using the classical laws of mechanics and electromagnetism – due to Newton, Faraday, Maxwell and many others. However, quantum mechanics is required to explain how matter behaves at the (sub-)atomic and fundamental level and also to understand the behaviour of light at the level of single photons or quanta. Interestingly, the fundamental features of quantum physics, including entanglement, superposition and the uncertainty principle, run completely counter to our intuition and everyday experience. 

Quantum mechanics was developed and refined during the first half of the last (20th) century – by Planck, Schroedinger, Heisenberg, Dirac and many others. It has since proved to be an invaluable tool in understanding the building blocks of all the information technologies (IT) we use in the modern world, such as: electronic circuits that underpin computers and all the other devices we rely on every day; lasers and optoelectronic components that underpin communications technologies and numerous home, consumer and personal devices such as mobile phones. Indeed, quantum mechanics has aided the repeated improvements of all these technologies to the state-of-the-art from which we benefit today.

What is entanglement?

Entanglement is the property that gives correlations – stronger than any with which we are familiar in everyday life – between two or more quantum systems, even when these are separated by large distances. This can mean that if you observe the state of one system, you automatically know the state of the other.

What do we mean by superposition in quantum physics?

Superposition is the principle describing the ability of a quantum system to be in two or more states at the same time. A potentially helpful analogy is a musical note, which generally comprises many different fundamental tones or harmonics playing together at the same time.

What do we mean by disturbance in quantum physics?

When we try to measure or interact with a quantum system to learn about what it is doing, we inevitably and irreversibly disturb it, except in the very specific case when we just happen to “disturb” it back into the state that it was already in before we looked.

What is wave-particle duality?

The wave-particle duality principle describes the capacity of objects to behave as both particles and waves (as is the case, for example, with photons – the tiny particles of light). An often useful generality is that quantum objects exhibit particle-like properties when they are emitted or detected, but propagate from place to place as if they are waves.

What is the Uncertainty Principle?

The uncertainty principle (also known as Heisenberg’s uncertainty principle) is a feature of quantum states whereby the values of two complementary properties of the system (such as the position and momentum of a particle) cannot both be precise at the same time. Measuring one property precisely inevitably adds disturbance, or uncertainty, to the other. This is particularly important for the development of quantum communications. Alice sends to Bob a sequence of quantum states chosen (randomly) to be precise states of one or other complementary properties (in this case certain polarisations of light). Any eavesdropper doesn’t know which complementary property will be well defined for the next arriving state, so she will inevitably disturb some of the transmission if she tries to measure. This disturbance alerts Alice and Bob to the hacking, or eavesdropping.

How does conventional cryptography work?

There are basically two forms of conventional cryptography: symmetric and asymmetric (the latter of which provides our current public-key infrastructure (PKI)).  Both approaches utilise cryptographic keys at the transmitter (“Alice”) and receiver (“Bob”) ends of the communication, along with a known mathematical algorithm for Alice to encrypt the data with her key and Bob to decrypt it with his key. Once encrypted, the data is unintelligible during transit to anyone who intercepts it. In symmetric cryptography Alice and Bob use the same key, which has to be kept secret from everyone else if their information is to remain secure, so they also need a mechanism to securely share this key. Asymmetric cryptography uses pairs of keys: Alice uses a public key (“public” because it is not a secret and available to everybody) to encrypt the data, whereas Bob uses a private key (secret and known only to him) to decrypt. Current real-world internet and other communications often rely upon a combination of the two approaches: asymmetric PKI first, to establish shared secret keys that are then used symmetrically to secure the communications or transaction. This two stage approach is clearly essential if Alice and Bob have never corresponded before, because in this case they will not have any shared secret key from which to begin.

Why do we need new encryption and decryption methods?

Conventional asymmetric cryptography underpins the world’s public-key infrastructure (PKI) and is very widely used in current cyber security. In conventional asymmetric cryptography (see previous Q&A), data is encrypted by Alice using a public key and decrypted by Bob using a separate private key, that is known only to him. This works because although there is a mathematical relationship between the keys, it is essentially impossible to deduce the private key from the public key. At least until now… Unfortunately, it is now known that current PKI could be cracked in future by quantum computers, so private keys could be computed from public keys. Therefore new encryption and decryption methods are needed that are not vulnerable to quantum computer attack. One approach is to utilise shared secret symmetric keys (see previous Q&A) – for encryption and decryption – and cryptographic techniques that are “quantum safe” (safe in a future quantum world where all manner of quantum technologies exist). Then the security of communications is determined by the security of the key distribution mechanism. This is where quantum technology comes in – it provides a secure method of quantum key distribution (QKD).

What is Quantum Key Distribution (QKD)?

QKD is a provably secure method of distributing encryption keys. In QKD, encryption keys are physically distributed using a sequence of quantum light signals, or photons, whose quantum states are each assigned randomly to represent a 0 or a 1. This physical approach means the key cannot be cracked mathematically. It is also impossible to copy or steal the key in transit, since quantum mechanics dictates that any observation will disturb the quantum state – which can be detected by the receiver.

How does Quantum Key Distribution (QKD) work?

To share an encryption key by QKD, the basic idea is that the transmitter – usually called Alice – sends a long sequence of quantum light pulses to the receiver – usually called Bob. These may be sent down an optical fibre or through free space, whichever technology provides the best solution, but either way any adversary – usually called Eve – can only gain information on the transmitted light signals by measuring them in some way. Quantum physics dictates that Eve cannot avoid introducing disturbance to some of these signals through her measurements, so she cannot avoid exposing her eavesdropping. Clearly Bob also has to measure the quantum light signals that he receives in order to establish a key shared with Alice. Nevertheless, the really clever thing with QKD is that Alice and Bob can afterwards identify a subset of shared data to keep. Without exposing the actual data values, they can identify the specific light signals that Bob should not have disturbed by his measurements. They can locate and correct errors in the data that they keep, and then mathematically compress it down to a final shared secret key. What’s more, all these subsequent communications do not have to be encrypted (although they could be) – the security of the final key is not compromised even if Eve overhears all this discussion.

When will quantum communications technologies become the norm?

The first quantum technology revolution has already taken place and we are all the beneficiaries of devices reliant on quantum mechanics principles and using semiconductor chips, lasers etc. Nevertheless, all our current conventional IT stores, processes and communicates information according to familiar, classical laws. The quantum contribution is just to make the components better and improve the performance of our IT.

However, we are now entering the second quantum technology revolution, which represents a much bigger “quantum leap”.  A whole new generation of quantum devices is being developed and this time these devices are not simply leveraging quantum effects to improve their performance, they are operating in a fundamentally different manner, according to the laws of quantum physics. It is this feature that provides new quantum technologies with the potential to outperform conventional IT, or achieve tasks impossible with conventional IT. 

Quantum communications devices such as QKD boxes and quantum random number generators (QRNGs) are already available to purchase. However, these machines are generally rather costly, bulky and cumbersome – so currently more suitable to research and development settings and specific high-value applications, rather than everyday consumer use. Some prototypes are also being trialled on wider markets, for example, smartphones containing new miniature QRNG technology.  However, further work is needed to properly commercialise these prototype devices, by bringing down costs, ensuring integration capabilities with existing electronics and IT, and creating and adopting new industry standards. All this will make these products both properly regulated and widely available. So it will be a while before quantum communications technologies are fully integrated within our communications infrastructure. The Quantum Communications Hub is working on the migration of these technologies out of the lab and into real world demonstration situations, the next step towards commercialisation and new markets.

What are the dangers associated with quantum communication technologies?

As with any new technology, there are some concerns regarding the misuse of quantum communications technologies by various hostile actors. However, governments are seeking to mitigate against this by putting in place appropriate regulations regarding import, export and use, analogous to the regulations that already exist for conventional security technologies. It is widely acknowledged that the benefits of these new technologies very much outweigh the risks.

Will quantum communications technologies replace conventional communications technologies altogether?

Quantum communications technologies will be integrated within our current communications infrastructure but will not replace it, they will augment existing technologies. Clearly the objective is high-speed, flexible, transparent, user-friendly communications that are secure in a future quantum-enabled world, so quantum and conventional communications technologies will be deployed in combination to provide the best possible solution for this.

What is post-quantum cryptography?

Post-quantum cryptography, sometimes referred to as quantum-proof or quantum-resistant cryptography, is the name given to mathematical cryptographic techniques which are immune to conventional and known quantum computer attacks, and thought to be immune to new quantum hacking algorithms that may emerge in the future. “Quantum safe” is another frequently used term, although this is normally used to mean safe in a future fully quantum-enabled world, so “quantum-safe” solutions can include both quantum communications and post-quantum cryptography.

What is the UK National Quantum Technologies Programme?

The UK National Quantum Technologies Programme is a ten-year, £1 billion public and private investment established by the UK Government, to ensure the successful translation of quantum technologies from laboratory to industry. The programme aims to create a coherent government, industry and academic quantum technology community that gives the UK a world-leading position in the emerging multi-billion-pound new quantum technology markets.

More information on the UKNQTP can be found on the UK National Quantum Technologies Programme website.