Controlling Quantum Behaviour of Individual Atoms
Building single-atom qubits under a microscope
Excerpts and salient points ~
+ This is the first time a single-atom qubit has been achieved using a Scanning Tunneling Microscope (STM), the Nobel Prize-winning IBM invention that allows atoms to be viewed and moved individually. This is an important breakthrough because the STM can image and position each atomic qubit to precisely control the arrangement of nearby qubit atoms. The microscope works by scanning the ultra-sharp needle tip near a surface to sense the arrangement of individual atoms, and the needle tip can pull or carry atoms into desired arrangements.
[O]ur team demonstrated the use of single atoms as qubits for quantum information processing. Quantum bits, or qubits, are the fundamental building blocks of a quantum computer’s ability to process information.
+ These single-atom qubits are extremely sensitive to magnetic fields so they can also be used as quantum sensors to measure the subtle magnetism of nearby atoms. We used this sensitivity to make qubits interact – or entangle – with each other and make a two-qubit device. This is a critical step toward the understanding of how to accomplish the ultimate goal of having many qubits interact so that we can take advantage of the quantum speedup in the processing power over conventional computers.
+ So, how can we coax a titanium atom into a chosen quantum superposition state? The answer is to apply high-frequency radio waves, called microwaves, to the atom. These microwaves, emanating from the microscope’s tip, steer the atom’s magnetic direction. When tuned to the right frequency, these microwaves lead the titanium atom to perform a “quantum dance,” as shown in the figure below. The atom holds still on the surface, but its magnetic north pole rapidly spirals around, ending in the desired direction.
+ This dance, called “Rabi oscillation,” is extremely fast, taking only about 20 nanoseconds to turn the qubit around, from pointing up to “0,” to pointing down to “1” or back again. At the end of the dance, the atom points to a designed direction—a “0” or a “1” or a superposition that lies in between—depending how long we apply the radio waves. The technical term of this key technique is pulsed electron spin resonance, and it can create any superposition state we want. We control and observe these spin rotations using the STM’s extreme sensitivity.
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