Electron and nuclear spin phase rotations reveal the off-diagonal elements of the density matrix.
Scientists from Oxford University have made a significant step towards an ultrafast quantum computer by successfully generating 10 billion bits of quantum entanglement in silicon for the first time – entanglement is the key ingredient that promises to make quantum computers far more powerful than conventional computing devices.
The researchers used high magnetic fields and low temperatures to produce entanglement between the electron and the nucleus of an atom of phosphorous embedded in a highly purified silicon crystal. The electron and the nucleus behave as a tiny magnet, or ‘spin’, each of which can represent a bit of quantum information. Suitably controlled, these spins can interact with each other to be coaxed into an entangled state – the most basic state that cannot be mimicked by a conventional computer.
Entanglement is the quintessential quantum phenomenon. It is a necessary ingredient in most emerging quantum technologies, including quantum repeaters, quantum information processing and the strongest forms of quantum cryptography. Spin ensembles, such as those used in liquid-state nuclear magnetic resonance have been important for the development of quantum control methods. However, these demonstrations contain no entanglement and ultimately constitute classical simulations of quantum algorithms. Here we report the on-demand generation of entanglement between an ensemble of electron and nuclear spins in isotopically engineered, phosphorus-doped silicon. We combined high-field (3.4 T), low-temperature (2.9 K) electron spin resonance with hyperpolarization of the 31P nuclear spin to obtain an initial state of sufficient purity to create a non-classical, inseparable state. The state was verified using density matrix tomography based on geometric phase gates, and had a fidelity of 98% relative to the ideal state at this field and temperature. The entanglement operation was performed simultaneously, with high fidelity, on 10^10 spin pairs; this fulfils one of the essential requirements for a silicon-based quantum information processor.
‘The key to generating entanglement was to first align all the spins by using high magnetic fields and low temperatures,’ said Stephanie Simmons of Oxford University’s Department of Materials, first author of the report. ‘Once this has been achieved, the spins can be made to interact with each other using carefully timed microwave and radiofrequency pulses in order to create the entanglement, and then prove that it has been made.’
The work has important implications for integration with existing technology as it uses dopant atoms in silicon, the foundation of the modern computer chip. The procedure was applied in parallel to a vast number of phosphorous atoms.
‘Creating 10 billion entangled pairs in silicon with high fidelity is an important step forward for us,’ said co-author Dr John Morton of Oxford University’s Department of Materials who led the team. ‘We now need to deal with the challenge of coupling these pairs together to build a scalable quantum computer in silicon.’
In recent years quantum entanglement has been recognised as a key ingredient in building new technologies that harness quantum properties. Famously described by Einstein as “spooky action at distance” – when two objects are entangled it is impossible to describe one without also describing the other and the measurement of one object will reveal information about the other object even if they are separated by thousands of miles.
Creating true entanglement involves crossing the barrier between the ordinary uncertainty encountered in our everyday lives and the strange uncertainties of the quantum world. For example, flipping a coin there is a 50% chance that it comes up heads and 50% tails, but we would never imagine the coin could land with both heads and tails facing upwards simultaneously: a quantum object such as the electron spin can do just that.
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