The quantum entanglement of a large ensemble of spins in a semiconductor has been carried out at room temperature for the first time, by researchers in the US. The team entangled more than 10,000 copies of two-qubit entangled states in a commercial silicon-carbide (SiC) wafer at ambient conditions. SiC is widely used in electronics, so this latest achievement could be an important step towards the creation of sophisticated quantum devices that harness entanglement.
Entanglement is a purely quantum-mechanical phenomenon that allows two or more particles to have a much closer relationship than is allowed by classical physics, no matter how far apart they may be. The states of entangled particles are inextricably linked such that any change made to one particle instantly influences the state of the other. Entangled particles are seen as a key component of quantum computers, but for entanglement to be truly utilized in practical applications, researchers must be able to entangle quantum bits (qubits) at room temperature and preserve the entangled state.
Ordered states
To produce entanglement between particles, the system must initially be in a highly ordered state. This is normally only possible at cryogenic temperatures of around –270 °C and involves applying extremely large magnetic fields – conditions that are rather impractical. Entanglement becomes even more difficult when a large number of qubits are involved, for example in a solid-state ensemble.
Now, Paul Klimov and David Awschalom from the University of Chicago, together with colleagues at the University of California, Santa Barbara and the Argonne National Laboratory, have developed a new method that addresses these challenges. It uses a combination of infrared laser light with microwave and radio-frequency pulses to entangle nearly 10,000 two-qubit electron and neutron spin pairs. This is done in a macroscopic 40 µm3 volume of the commercial SiC wafer.
The electron–nuclear spin pairs are located at the intrinsic “colour centre” defects found in SiC. These are similar to the “nitrogen vacancy” centres found in diamond, which can also be used as qubits. While the team’s techniques could be applied to diamond, Awschalom told physicsworld.com that the team used SiC because of the important role it plays in high-power electronics, optoelectronic devices and sensors. The fabrication techniques developed in these fields will transfer directly to the development of sophisticated entanglement-harnessing devices, says Awschalom, adding that “creating sophisticated devices from diamond is generally much more difficult”.
Two-step process
Creating the entangled ensemble is a two-step process. The team first “initializes” or polarizes the system, in a very small magnetic field using infrared laser light. The entanglement that the researchers measure starts out highly coherent – up to 88% fidelity with respect to a maximally entangled state – and then decays within 300 ns. This initial “inhomogeneous” entanglement coherence can then be extended by applying sequences of radio-frequency and microwave pulses.
“Many of the microwave/radio-frequency pulse techniques have been developed over decades of nuclear magnetic resonance (NMR) research, and many of them are implemented in commercial magnetic resonance imaging (MRI) technologies,” says Awschalom. “We believe that the ultimate limit of this coherence will be of the order of 100 μs, however, we have not performed the measurements necessary to confirm this.”
The entanglement is confirmed by performing “quantum state tomography”, which is a measurement of the quantum state of the system. It can be very difficult to measure the quantum state of a specific ensemble when it is intertwined with many other quantum systems that are naturally present in the substrate. To overcome this challenge, the team developed a new tomography protocol for precisely measuring the state of a specific ensemble, even when there is significant noise.
Surprisingly, the team found that this entanglement works best at ambient conditions – at lower temperatures, the polarization of the system degrades slightly, and so the maximum possible entanglement fidelity is also lowered. “With more sophisticated polarization techniques, however, the entanglement fidelity at cryogenic conditions can be made to approach the entanglement fidelity that is possible at ambient conditions,” says Awschalom.
Practical scaling
The team managed to entangle nearly 10,000 spin pairs, but by tweaking the experimental apparatus, it could be possible to create hundreds of billions of two-qubit entangled states in a chip of material that is 0.5 mm × 3 mm × 3 mm (the approximate size of the sample). “The more copies of the system, the stronger its signal-to-noise, and the stronger it can couple to things like other ensembles or light, for example,” cautions Awschalom.
While it is far too early to tell if this technique will directly lead to a practical quantum computer, the entangled states created could be used as building blocks in a quantum computer many years in the future. But even this is only possible if it can be expanded beyond two-qubit entanglement to much larger entangled states. This, according to Awschalom, is probably the biggest challenge in scaling up any quantum system – at room or cryogenic temperatures – into a useful quantum technology.
On the flip side, the entangled spins could be used as quantum sensors. “Given that the entanglement works at ambient conditions and the fact that SiC is bio-friendly, one particularly exciting application is in vivo biological sensing,” explains Awschalom, adding that “future devices of this type could include entanglement-enhanced magnetic resonance imaging probes, which could have important biomedical applications”.
The research is published in Science Advances.
