So far molecules have not been used in quantum computing, despite the possibility of their potential to make the technology faster. Till today, smaller particles have been used, as molecules have rich internal structures which are complicated, delicate and unpredictable.
The ice was broken by the Harvard scientists who succeeded in trapping molecules to perform quantum operations. Qubits are the units of information, and for this experiment they used ultra-cold polar molecules. They were working on this for the last 20 years.
Quantum computing exploits the findings of quantum mechanics for computation, making it exponentially faster than classical computing. This system uses tiny particles trapped reliably to serve as qubits and constitute the logic gates. The Harvard research uses molecules to form an iSWAP gate — a key quantum circuit that creates entanglement. It is the entanglement that makes quantum computing so powerful.
The molecules used were of NaCs — sodium-cesium. They were trapped using optical tweezers (lasers). The environment was ultra-cold. The dipole-dipole interactions between the molecules were used to perform a quantum operation. The rotation of molecules was carefully controlled. That led to the entanglement of two molecules — creating a quantum state (known as two qubit Bell state with 94 per cent accuracy.)
As we know, information processing is done in logic gates in both quantum computers and traditional computers. Classical gates manipulate binary bits (0s and 1s). Quantum gates operate on qubits — which achieve superposition — by existing in multiple states simultaneously. In other words, quantum computers can do things which are impossible for traditional computers. They create entangled states and even perform operations in multiple computational states at once.
Quantum gates are reversible too. They can manipulate qubits with precision while preserving their quantum nature. In this experiment, they used iSWAP gate which swapped two qubits and applied a phase shift (an essential step to generate entanglement where the states of two qubits become correlated irrespective of the distance that sets them apart).
This trapped molecule technology is an important milestone and the last building block of a molecular quantum computer. Scientists can avail of the nuclear spins and nuclear magnetic resonance for quantum computing. So far molecules were considered unstable for use in quantum operations since their movements can interfere with coherence. Trapping them successfully, researchers have overcome this hurdle.
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