(The following article by our very own Phil Merchant originally appeared on the Dead Cat Live Cat blog, and is being shared here with thanks to Dead Cat Live Cat.)
In a much deserved recognition of their hard work, Physicists Alain Aspect, John Clauser and Anton Zeilinger have been awarded the 2022 Nobel Prize in Physics. The prize has been granted for their research into one of the most intriguing ideas in 20th century Physics – Quantum Entanglement.
The work of the trio focused on a series of experiments that confirmed the existence of the phenomenon, but research into its implementation and applications continue today. Indeed, it is an exciting time to be an innovator in quantum technology - scientists and industry are pushing at the boundaries of technologies that implement and apply quantum entanglement, with the aim of developing commercial products that will improve our daily lives (from satellite communications to storage devices). But why is everyone so interested?
Quantum Entanglement – what is it, and is it real?
In a classical system, particles may exist in any one state (for example, determined by the energy or momentum of the particle). In quantum mechanics, however, it is possible for a particle to exist in multiple states simultaneously, in what is known as ‘quantum superposition’. When the particle is measured, the quantum state will collapse into one of the possible states.
Quantum entanglement complicates this situation further by envisioning a group of particles that exist not only in a state of quantum superposition, but in a condition where the state of each particle is dependent upon the other particles in the group. Thus, a measurement of one particle will instantly collapse the state not only for the measured particle, but for all particles of the system.
This is a fascinating phenomenon in its own right, but it leads to an interesting thought experiment. Suppose that we entangled two particles, and then separate them by millions of kilometres. Once suitably separated, we then measure the state of one of the particles. This will cause the quantum state to instantaneously collapse - not only for the local particle, but also for the entangled particle millions of kilometres away. Thus, we seem to have been able to “transmit” the result of a local experiment to a remote particle at a speed much faster than the speed of light. However, according to special relativity, the speed of light is the speed limit of the universe, so this instantaneous communication should not be possible.
This made some physicists very uncomfortable, not least eminent scientists Einstein, Podolsky and Rosen. They called it “spooky action at a distance”. In an attempt to reconcile the two branches of physics, the three formulated the idea of “Hidden Variables”. In this formulation, when an entangled state was created it forms a state described by variables that are hidden, but whose values will determine the outcome of any future measurement. Thus, a local measurement of one particle is simply revealing a pre-existing variable already present in the particle. Under this formulation, the remote particle already carries with it information on its final observed state – thus, measurement of the local state is not itself causing the remote particle to collapse its state.
The existence or not of hidden variables may be determined by experiment. In such experiments, pairs of entangled particles are created, separated and then measured using quantum state detectors at each remote location. When the detectors are randomly configured in different measurement configurations, Hidden Variables theory and Quantum Mechanics predict different outcomes in how often these separate measurements will agree.
The experiments of Aspect, Clauser and Zellinger were progressively more robust versions of these experiments, and they showed that it is Quantum Mechanics, and not Hidden Variable Theory, that accurately describes an entangled system. There is, indeed, “spooky action at a distance”.
So what now for quantum entanglement?
The phenomenon is fascinating in its own right, and leads to interesting philosophical discussions – not only about the nature of the universe, but the on-going attempts to reconcile quantum mechanics and classical physics. However, quantum entanglement is not just for lovers of science; everyday users of technology can be excited about the possibilities it can provide. Advances have been postulated, and are currently being pursued, in a variety of fields that may improve our everyday lives.
One immensely useful application is in the field of quantum computing. Bits of information in a classical computer can be in only one of two states (“0” or “1”). By contrast, and due to quantum superposition, quantum particles can exist in more than two states. When the particles are entangled, a quantum operation on one particle will immediately affect the entangled particles (but without collapsing the state) – thus, processing speeds can be significantly improved as compared to a classical computer. Exciting advances are being made in this field, such as the recent development of a six-qubit quantum processor in silicon.
Entanglement may also be deployed in cryptography, by forming part of a scheme for “Quantum Key Distribution”, or QKD. In QKD, a quantum channel is used to transmit a cryptographic key between users, and entangled particles may be used to transmit the key information. Given that a measurement of any one entangled particle will collapse the state of all entangled particles, an attempt by an eavesdropper to intercept the key will destroy the entanglement and warn the users that the key has been compromised. Rapid progress is also being made in this field – in 2020 Chinese researchers demonstrated entanglement-based quantum cryptography over a distance of 1120 kilometres.
These are just two examples – for further reading and other applications, please see other articles on the Dead Cat Live Cat blog, including: Quantum Ghost Imaging, which may be used to image an object with very low levels of light; Quantum Sensing, for accurate measurement of quantities, or Quantum Teleportation, where quantum information may be “teleported” from one location to another.
It is clear that Quantum entanglement is providing a rich landscape for new technological developments, from which we will all ultimately benefit. We at DeadCatLiveCat all salute the work of Aspect, Clauser and Zellinger, and congratulate them on their achievement.