Physicists design a programmable optical quantum computer

In normal silicon computer chips built into your laptop or smartphone, the information is displayed in one of two states: 0 or 1. In a quantum computer, a quantum bit (or qubit) can also be stored and relayed in both states. exponentially more information.

Two qubits can perform operations on four values, three on eight and so on two powers – they can do it simultaneously, while the traditional bits must take a position. As such, a quantum computer could be billions of times faster than a normal silicon computer built on a von Neumann architecture. By analogy, imagine being able to read an entire library at once rather than one book at a time.

According to one estimate, a quantum computer with 50 quantum bits would be more powerful in solving quantum sampling problems than today's fastest supercomputer. Quantum computers can solve big data problems that involve finding optimal solutions from a large number of options, which is extremely important in many medical fields (protein folding, gene sequencing) and climate. the biggest selling points for quantum computers). An attacker can now copy your email without you knowing it. a hack of a quantum system is however held by the laws of physics to leave traces.

According to McKinsey & Company, in 2015, about 7,000 researchers worldwide worked in the field, including about $ 1.5 billion a year. But despite the magnitude of the industry's efforts to put a quantum computer on the market, progress is rather slow.

The companies most likely to succeed, such as IBM or Google, are a few dozen. Most of these machines work with superconducting wires, which are cooled near absolute zero. The problem is that the electron qubits interact essentially with everything and that it is extremely difficult to avoid quantum decoherence (collapse of the wave function and the qubit). However, in another approach to quantum computing, the atoms are held in place not by cold temperatures, but by lasers.

Unlike electrons, photons do not interact with the environment, so there are no coherence problems over time. In addition, photons can be manipulated with great precision and move as fast as possible, at the speed of light.

At the same time, the fact that photons just pass each other also prevents scientists from using a simple way to implement quantum operations as they would with superconducting machines.

Researchers at the University of Bristol may have found an elegant solution. They designed a silicon photonic chip that divides photons into different spatial modes, guiding them along defined trajectories. The chip is composed of many interferometers charged with dividing the photons into different modes, each mode passing through a specific waveguide. Basically, this labyrinth of interferometers acts as a single door.

To demonstrate the capabilities of their device, the researchers implemented 98 different two-qubit gates, performing approximately 1,000 measurements for each set. Operations were successfully completed in 93% of cases, which is comparable to other types of quantum computing in the world. Superconductive quantum computers, for example, have a completion rate of about 90% to 95%. The team even ran an optimization algorithm on the photon chip.

Remarkably, from a single laser light source, the chip generates pairs of photonic qubits. The challenge in the future will be how to generate many more identical and entangled photons. The hardware currently in use is suitable for two qubits, but for hundreds you would need a totally different configuration. For example, the photon detector was placed on the outside of the chip. In addition, installing enough phase shifters and phase separators to support hundreds of qubits seems confusing at the moment, but scientists believe they can do so.

The results appeared in the journal Photonic Nature

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