Quantum device could complete 2.6 billion years of computation in 4 minutes

Researchers demonstrated a Gaussian boson sampling system based on “beam splitters”. Beam splitters take the incoming photon stream from the laser beam and split it into two streams moving in different directions. Essentially, the system
is a linear optical quantum computer from a network of beamsplitters.

Beam splitter at 45 degree angle can be considered as a four-port device. If two identical photons hit the same beam splitter from two different ports, then both will come out of the same port. The paths of the photons become entangled. If a sufficiently large network of beamsplitters is constructed, then branched entangled states arise. The number of output states scales quickly with the number of inputs and beamsplitters.

In the current demo, the researchers used 50 inputs and a chip with the equivalent of 300 beamsplitters. The total number of possible output states is about 1030.

In less than four minutes, the researchers obtained results that would take a fast classical computer about 2.5 billion years to calculate.

This was followed by rigorous tests to ensure that the behavior of the system was indeed quantum. If you calculate what will happen to specific input states and compare the output states with the results of those calculations, then the measurement results match the predictions. It is also possible to calculate the power output of the network if the light is not in a quantum state or if the photons are not identical and these measurements do not match predictions. This convincingly proves that the result is due to quantum effects.

One laser emits 25 beams of the same intensity, each of which is associated with two crystals, and the crystals generate single photons. The crystals are connected to optical fibers, the outputs of which are connected to a microcircuit where the beam splitters are located. Fiber outputs must be aligned with photodetectors. The entire installation covers an area of ​​about 1.5 x 2.5 m and must be stabilized with high accuracy (about 10 nm).

At the moment, quantum computers are composed of a chain of ions. The ions are spaced far enough apart that they can be accessed separately, which means that information (in the form of quantum bits or qubits) can be stored and read from individual atoms. Computational operations can be performed using microwaves and ion movement. Another variant of the architecture of quantum computers is superconducting rings cooled by liquid helium.

Each ring is a qubit that is connected to the others through wires. The advantage of this approach is that the hardware is relatively easy to scale. But the quantum behavior of each qubit is much easier to break.

In contrast to both of these options, an optical quantum computer can be a chip-scale device that is powered by an array of laser diodes, read out by a series of single-photon detectors. None of them require ultra-low temperatures or vacuum (if photon counting detectors are required, liquid nitrogen can be used). Optical quantum computing requires thermal stability and a fairly sophisticated feedback system to get the lasers to work exactly as they want them to.

Source: Arstechnica