Superposition and entanglement:
Two different features of particles behaviour at the atomic and subatomic level that quantum technologies use to unlock new capabilities.
A spinning coin is, in a way, both heads and tails until it falls one way or another. Similarly, according to quantum mechanics, atoms, electrons and particles of light (photons) can be in two (or more) states at once, until they are measured. This state is known as superposition. In quantum computing, for example, a qubit (a unit of quantum information) can be a combination of both a zero and one at the same time, until it is measured. QKD also uses this property to securely share an encryption key. When the key is sent in a quantum state, any attempt to copy, intercept or eavesdrop would introduce detectable errors.
When particles (such as two photons) are entangled, they form such a strong bond that the behaviour of one can determine the exact behaviour of the other. This effect occurs regardless of how far away they are from each other 75. Quantum technologies use this property to help securely share information, or to take ultra-precise measurements and images at the atomic level, even at a distance, in low light, or outside line of sight.
Chapter one
Quantum magnetic sensors: These exploit quantum properties to detect tiny changes in magnetic fields.
Quantum gravity sensors: Rather than using radio frequencies like conventional radar, a certain class of these sensors use falling clouds of cold atoms in two states (superposition) to detect tiny changes in the Earth’s gravitational field caused by objects 76.
Optical clocks: Current atomic clocks (eg those used on GPS satellites) measure the frequency of microwaves needed to change how electrons move around the nucleus of atoms. This gives a highly precise measurement of time 77. Developments in optical clocks, a new version of an atomic clock, use lasers instead of microwaves, which give the possibility of even more precise measurements 78.
Quantum key distribution: This is a way of securely sharing encryption keys using quantum phenomena. The key is sent as a quantum light pulse, which changes when measured. So, any attempt to copy, intercept or eavesdrop when sending the encryption key would introduce detectable errors 79.
Fully homomorphic encryption: This is one of three types of homomorphic encryption. Homomorphic encryption allows you to perform computations on encrypted information without first decrypting it. Fully homomorphic encryption has no limitations in terms of the types of operations it supports or their complexity. However, the more complex the operation, the more resource and time may be required 80.
Chapter four
Quantum random number generation: This technique involves using the principles of quantum physics to generate “truly random” numbers for classical encryption keys. It is one of several techniques designed to be more resistant against the capabilities of a future quantum computer.
Hybrid cryptography schemes: These combine classical and post-quantum cryptography approaches.
Cryptographically relevant quantum computer: A quantum computer that can efficiently solve the mathematical problems that underpin existing public key cryptography. Expert timescale estimates vary for when such a computer will emerge, if it ever does.
Encryption: “Information is encrypted and decrypted using a secret key. (Some algorithms use a different key for encryption and decryption)… In practical terms it will take such a long time to find the right key—ie many millions of years, depending on the computing power available and the type of key—that it becomes effectively impossible” 81.
Asymmetric cryptography (such as Rivest–Shamir–Adleman, or RSA encryption): “Asymmetric encryption uses the notion of a key pair: a different key is used for the encryption and decryption process. One of the keys is typically known as the private key and the other is known as the public key. The private key is kept secret by the owner and the public key is either shared amongst authorised recipients or made available to the public at large. Data encrypted with the recipient’s public key can only be decrypted with the corresponding private key. Data can therefore be transferred without the risk of unauthorised or unlawful access to the data.” 82
Asymmetric cryptography has two common functions:
- to securely share cryptographic keys before encrypting a message, including before encrypting a message using symmetric cryptography (key agreement); and
- for digital signatures, a way of authenticating or proving who someone or a device is before they enter into a transaction, share keys, join a network or download a security update 83.
Symmetric cryptography: “In symmetric encryption the same key is used for encryption and decryption. It is therefore critical that a secure method is considered to transfer the key between sender and recipient.” 84 It currently common practice for the sender to share the key using asymmetric cryptography to ensure the key remains secret/confidential. A quantum computer would be able to break the encryption protecting the secret key, and therefore use the key to decrypt the secure transmission. According to NIST, this puts the information these keys protect (including communications and stored information) at risk of exposure or “undetected modification” 85.
Digital signatures: These are commonly used to establish that an “entity” (such as a computer, mobile or IoT device) seeking access to an online network or to exchange cryptographic keys is who they claim to be, and should be allowed access to the network. They are also commonly used to authenticate financial transactions and “can provide a level of trust that an email has not been intercepted or spoofed” 86.
Zero knowledge proof: This is a privacy enhancing technology. It involves using a set of instructions to prove information without disclosing it. For example, a person could prove their age or whether they are financially solvent, without needing to disclose that information 87.
75 NQCC webpage on Quantum features
76 UK National Quantum technologies programme article: Gravity sensors see underground (2021)
77 NASA article: What is an atomic clock?
78 University of Birmingham media release: Next generation atomic clocks are a step closer to real world applications (2022)
79 Quantum Communications Hub article on QKD
80 ICO PETs guidance: Homomorphic encryption
81 ICO encryption guidance: What is encryption?
82 ICO guidance on encryption: What types of encryption are there?
83 NCSC whitepaper on preparing for quantum-safe cryptography (2020)
84 ICO guidance on encryption: What types of encryption are there?
85 NIST cybersecurity white paper on getting ready for post-quantum cryptography: Exploring challenges associated with adopting and using post-quantum cryptographic algorithms (2021)