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The transfer of information, on both large and small scales, is the backbone of modern computing.
Inside a computer, data has to be passed back and forth among various processors. On the internet, information is transferred between computers all over the planet, typically using fiber optic cables.
A dependable exchange of information is also essential for quantum information technologies currently under development. The key to quantum data technologies, like quantum computers and quantum cryptography, is the use of quantum bits or "qubits" as the fundamental unit of data. Unlike classical bits, qubits are not limited to having a value of 0 or 1. This is because quantum particles can take on superposition states – making them capable of being 0 and 1 at the same time. On the one hand, quantum technology could be used to create extremely powerful computers that leverage superposition states to carry out faster, more efficient calculations than classical computers. Conversely, superposition states are also quite sensitive and cannot be transferred using conventional methods.
For computing purpose, the state of a stationary qubit has to be changed into a so-called "flying" qubit, such as a photon, and then back into a different stationary qubit. A few years ago, scientists announced the ability to transmit the quantum state of an atom in this manner. In June 2018, a team of Swiss scientists succeeded in transmitting quantum data, at the press of a button and with high fidelity, between two quantum bits approximately a meter apart.
A Quantum Breakthrough
The main breakthrough the recent study was the deterministic transmission of the quantum state, accomplished at the press of a button. In earlier studies, a transfer of quantum states was realized, but that transmission was probabilistic: It worked only occasionally. An effective transmission could, for example, be indicated by a "heralding photon," and whenever the transmission wasn't effective, it was simply attempted again. Hence, the effective transmission rate was less than desirable. For useful applications, deterministic processes like the one demonstrated in the new study are the preferred way to go.
To realize their breakthrough, the Swiss team linked two superconducting qubits using a coaxial cable, like the kind used to connect antenna terminals. The quantum state of the first qubit, defined by the quantity of superconducting electron pairs it holds, was first sent to a microwave photon of a resonator using highly-controlled microwave pulses. From that resonator, the photon was sent through the coaxial cable to a second resonator. There, microwave pulses translated its quantum state to the second qubit.
The entire procedure took less than a millionth of a second, and yet there is a substantial room for improvement. Quantum mechanical entanglement produces a close link between two quantum objects even across great distances, an attribute will likely be used for cryptography or even quantum teleportation.
The Road to Useful Quantum Computing
A logical next step would involve using two qubits, as transmitter and receiver, to make entanglement swapping between qubit pairs possible. Such a system is useful for cutting-edge quantum computers, which should be built in the coming years.
So far, quantum computers include just a handful of qubits. With bigger computers that include a few hundred qubits, engineers must figure out how to connect them most effectively to be able to realize the many benefits of building a quantum computer.
Scientists are also looking to created connected quantum computer modules like the clusters of conventional computers currently in use. However, the transmission distance, just a meter in the recent study, would have to be raised significantly. Scientists have shown that an extremely cold, and therefore superconducting cable could transfer photons over tens of metres while maintaining fidelity. Linking up a quantum computing center, therefore, appears to be quite possible in the near future.
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