Isolation Conversation
Calcium-43 ions queuing for groceries at a socially responsible separation of 10 micrometres. Image courtesy of Oxford University’s Ion Trap Group
Normally the atoms that make up the world we live in interact with each other very frequently, from the moment they get up in the morning to the time when they go to bed. Even after their bedtime, many atoms don’t stop socialising, and stay up partying all night. Usually this is not a problem, and indeed it’s often very useful (most chemical reactions would never take place without it), but if you want to build a quantum computer it’s a disaster. Quantum computing relies on the ability to keep track of delicate quantum states of selected atoms known as qubits, but when another atom in an unknown state comes along and shakes hands, there is a chance that our qubit will become infected such that the information encoded on it changes in an unpredictable way – a process called decoherence. Errors are introduced into the quantum computation which are difficult to detect and, if allowed to accumulate, will destroy the ability to perform calculations. It is therefore important that qubits curb their social tendencies to preserve the information stored on them.
Of course qubits that never talk to each other aren’t much use for computing, which requires some controlled interactions to perform logical operations. Our socially isolating qubit will need to go to the shops occasionally, or walk the dog. This is not easy to manage – we must find a way that they can interact with shop assistants or exercise Fido but at the same time prevent them from socialising with random strangers. Cafes and bars are definitely out. Even if the qubit succeeds in avoiding others, they are likely to come into contact with objects that an infected qubit has touched or sneezed on, and will need to wash their hands regularly.
If we can manage the movements of the qubits successfully, the controlled interactions between them result in what we call quantum entanglement. Entanglement between qubits forms a large scale collective state that is greater than the sum of its parts and is at the heart of the power of quantum computing. A qubit society, if you like.
One way to control the social life of qubits is to build them nice houses to live in which minimise unwanted external influences. There are a few ways to do this, and which is the best is hotly debated. For instance a house might involve a high vacuum to reduce the flow of visitors to one every couple of days, or a material such as diamond which has incredible soundproofing by virtue of its structure and composition. Many qubit houses have to be kept very cold in order to work, but placing a house in a refrigerator adds obvious practical complications.
To ensure good isolation, qubit houses also tend not to have front doors – the qubits are essentially held at home by force(s). To build a house big enough to accommodate the number of qubits we might need for a really useful calculation, which could be in the billions, looks nightmarishly difficult. So how can we realise our computer?
One possible answer comes by thinking a bit more deeply about how the computer program will work. At the most basic level of operations some qubits will need to interact with each other very frequently, so those qubits can share a house and talk to each other on a regular basis. We can keep track of the state of a small family of qubits in a shared house quite well, so the risk of decoherence is low. The family can order shopping online so that they rarely see anyone beyond a delivery driver, and, importantly, can communicate with other families via a telecommunications link with little or no risk of infection.
This architecture for qubit society is known as network-based quantum computing, and is one of the core themes of the UK’s research programme. At present the communications links between houses are very primitive, more like a crackly phone line than a seamless virtual hangout space, so improving these links is where a lot of the work is focused. One day, some people think, the links will be good enough that a single such network will span the globe, and families of qubits sitting in their homes in London, New York, Beijing and Johannesburg will be connected constantly with each other in one big quantum state. A quantum internet. A global quantum computer. A lot more work is required to realise this vision, but in the meantime here we are, socially isolating yet entangled with each other. Are we all qubits now?
Quantum computing and the UK programme to develop the technology
