A simplified quantum computer can be made with off-the-shelf components

Quantum computers could one day blow boring, old classical computers out of the water, but their so far intricacy limits their usefulness. Engineers at Stanford University have now demonstrated a relatively simple new design for a quantum computer in which a single atom is entangled with a series of photons to process and store information.

Quantum computers take advantage of the strange world of quantum physics to perform calculations much faster than conventional computers can handle. Where today’s machines store and manipulate information in bits, as ones and zeros, quantum computers use qubits, which can exist as one or zero or a superposition of both one and zero at the same time. This means that their power increases exponentially with each added qubit, allowing them to tackle problems that are beyond the capabilities of classical computers.

Of course, quantum computers bring their own challenges. First, quantum effects operating on them are sensitive to perturbations such as vibration or heat, so quantum computers must remain at temperatures close to absolute zero. As such, its complexity is proportional to the computing power of the machine, getting physically larger and more complex as more processing power is added.

But the Stanford team says their new design is deceptively simple. It’s an optical circuit made using a few components already available — a fiber optic cable, a packet splitter, two optical switches and an optical cavity — and it can reduce the number of physical logic gates required.

says Ben Bartlett, lead author of the study. “While with this design we only need a few relatively simple components, the machine does not increase in size with the size of the quantitative program you want to run.”

The new design consists of two main parts: a ring that stores photons and a scattering unit. Photons represent qubits, with the direction in which they move around the ring to determine if their value is one or zero — or both if they travel in both directions simultaneously, thanks to quirks about quantum superposition.

To encode information on the photons, the system can direct them out of the loop into the scattering unit, where they enter a cavity containing a single atom. When a photon interacts with an atom, they become entangled, a quantum state where the two particles cannot be described separately, and changes made to one will affect its partner, no matter how great the distance separates them.

In practice, after the photon is returned to the storage ring, it can be “written” to it by manipulating the atom with a laser. The team says that a single atom can be reset and reused, manipulating many different photons in a single ring. This means that the power of the quantum computer can be increased by adding more photons to the ring, rather than having to add more rings and scattering units.

“By measuring the state of the atom, you can transfer the remote processes to the photons,” Bartlett says. “So we only need one controllable atomic qubit that we can use as an alternative to indirectly manipulating all the other optical qubits.”

Importantly, this system must be able to power a variety of quantitative processes. The team says they can run different programs on the same circuit, by writing new code to change how and when the atom and photons interact.

“For many optical quantum computers, gates are physical structures that photons pass through, so if you want to change the running software, that often involves physically reconfiguring the hardware,” Bartlett says. “Whereas in this case, you don’t need to change hardware—just give the machine a different set of instructions.”

Even better, optical quantum computer systems can operate at room temperature, removing the bulk added by extreme cooling systems.

Publish the research in the journal optics.

Source: Stanford University

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