Quantum computing at room temperature

Researchers from the US Army predict that quantum computing circuits that will no longer need extremely low temperatures to function will become a reality within about a decade

[Translation by Dr. Nachmani Moshe]

For years solid-state quantum technology that functions well at room temperature has seemed like a distant achievement. Although the application of transparent crystals with optical nonlinearity has emerged in the world of science as the most likely basis for this milestone, the likelihood of such a system has always remained low. Now, US Army scientists have officially verified the feasibility of this approach. Researcher Dr. Kurt Jacobs from the US Army Research Laboratory, in collaboration with researchers Dr. Mikkel Heuck and Professor Dirk Englund from the Massachusetts Institute of Technology, became the first to demonstrate the feasibility of a quantum logic gate composed of photonic circuits and optical crystals.

"If future devices that use quantum technologies need to be cooled to extremely low temperatures, this need will make them expensive, bulky and energy-hungry," said the lead researcher. "Our research focuses on the development of future photonic circuits that can utilize the mechanism of quantum entanglement to develop quantum devices that operate at room temperature." Quantum technology offers a variety of future advances in the fields of computing, communication and remote sensing.

In order to perform any operation, ordinary computers operate on the basis of completely defined information. The information itself is stored in many bits, each of which can be on (1) or off (0). A normal computer, receiving an input defined by bits, can process that input to generate an answer, which is also given as a number. A normal computer processes one input at a time. In contrast, quantum computers store information in the form of qubits (qubits, Wikipedia) that can be in a special state where they are both on and off at the same time. This mode allows a quantum computer to check the answers to a large number of input types simultaneously. Although such a computer cannot display all the answers at the same time, it can display the interrelationships between the different answers, a situation that allows certain problems to be solved much faster than on a normal computer.

Unfortunately, one of the significant drawbacks of quantum systems is the fragility of the special states of the qubits. Most of the hardware used in quantum technology must be kept at extremely cold temperatures - close to absolute zero - in order to prevent the destruction of these special states following a reaction with the environment in which the computer is located. "Any reaction of a qubit with anything else in its environment will destroy its quantum state," explains the researcher. "For example, if the environment is a gas of particles, then keeping the system at an extremely low temperature will cause the gas molecules to move slowly, so that they will not collide with great frequency in the quantum circuits."

Researchers have put a lot of effort into solving this problem, but a complete solution has yet to be found. Currently, photonic circuits incorporating non-linear optical crystals have emerged as the only plausible solution for developing quantum computing in conjunction with room-temperature solid-state systems. "Photonic circuits are a little like electrical circuits, except for the fact that they use light instead of electrical signals," notes the researcher. "For example, we can produce channels in a transparent material, channels in which the photons can move, similar to electrical signals moving along electrically conductive wires." Unlike quantum systems that use ions or atoms to store information, quantum systems that use photons can bypass the low temperature limit. However, the photons must still react with other photons in order to perform logical operations. This is where nonlinear optical crystals come into play.

Researchers can create cavities within the crystals and these cavities are able to temporarily trap photons inside. Using this method, the quantum system can achieve two possible states of a qubit: a cavity that stores a photon (on) and a cavity without a photon (off). These qubits in turn can create quantum logic gates that produce the infrastructure for the special states. In other words, researchers can exploit the temporary state of the cavity in the crystal (full of photons or empty) in order to represent a qubit. The logic gates function on the basis of two qubits together, creating a quantum entanglement between them. This entanglement is produced automatically in a quantum computer, and is the mechanism required to develop quantum approaches that can be used in sensing applications. However, scientists who support the idea of ​​creating quantum logic gates using non-linear optical crystals have only been based on hypothesis - until now. Although this idea shows great promise for the future, doubts still remain as to the ability of this method to lead to the development of practical logic gates. The application of nonlinear optical crystals was in question until researchers presented a way to implement a quantum logic gate using this approach based on existing quantum gate components. "The problem was that if a single photon moves in a channel, that photon carries behind it a 'wave packet' of a defined shape," explains the lead researcher. "For a quantum gate, it is necessary that the photonic wave packets remain the same even after the gate is activated. Since non-linearity destroys the wave packets, the question was whether we could fill the space in the wave packet, make them react non-linearly and then cause the photons to be emitted again while keeping the original shape of the wave packet." Once they designed the appropriate quantum logic gate, the researchers performed several computer simulations of the gate's operation in order to demonstrate that such a system could, in theory, function as required.

Practical construction of a quantum logic gate using this method will first of all require significant improvements in the quality of certain photonic components, the researchers explain. "Based on the progress achieved during the last decade, we predict that we will need about another decade in order to achieve the required improvements," said the researcher. "At the same time, the process of charging and emitting a wave packet without distorting its shape is the process that will allow us to achieve this technology, which is currently only theoretical."
The research findings have long been published in the scientific journal Physical Review Letters.

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