Quantum computers could one day blow boring old classical computers out of the water, but so far their complexity limits their usefulness. Engineers at Stanford have now demonstrated a new relatively simple design for a quantum computer where a single atom is entangled with a series of photons to process and store information.
Quantum computers tap into the weird world of quantum physics to perform calculations far faster than traditional computers can handle. Where existing machines store and process information in bits, as either ones and zeroes, quantum computers use qubits, which can exist as one, zero, or a superposition of both one and zero at the same time. That means their power scales exponentially with each added qubit, allowing them to tackle problems beyond the reach of classical computers.
Of course, quantum computers bring their own challenges. For one, the quantum effects they run on are sensitive to disturbances like vibration or heat, so quantum computers need to be kept at temperatures approaching absolute zero. As such, their complexity scales with the computing power of the machine, so they become physically larger and more cumbersome as more processing power is added.
But the Stanford team says their new design is deceptively simple. It’s a photonic circuit made using a few components that are already available – a fiber optic cable, a beam splitter, two optical switches and an optical cavity – and it can reduce the number of physical logic gates needed.
“Normally, if you wanted to build this type of quantum computer, you’d have to take potentially thousands of quantum emitters, make them all perfectly indistinguishable, and then integrate them into a giant photonic circuit,” says Ben Bartlett, lead author of the study. “Whereas with this design, we only need a handful of relatively simple components, and the size of the machine doesn’t increase with the size of the quantum program you want to run.”
The new design is made up of two main parts: a ring that stores photons, and a scattering unit. The photons represent qubits, with the direction that they travel around the ring determining whether their value is a one or a zero – or both if it travels in both directions at once, thanks to the quirks of quantum superposition.
To encode information on the photons, the system can direct them out of the ring into the scattering unit, where they enter a cavity containing a single atom. When the photon interacts with the atom, they become entangled, a quantum state where the two particles can no longer be described separately, and changes made to one will affect its partner, no matter how large a distance separates them.
In practice, after the photon is returned to the storage ring, it can be “written” to by manipulating the atom with a laser. The team says that the one atom can be reset and reused, manipulating many different photons in the one ring. That means the quantum computer’s power can be scaled up by adding more photons to the ring, rather than needing to add more rings and scattering units.
“By measuring the state of the atom, you can teleport operations onto the photons,” says Bartlett. “So we only need the one controllable atomic qubit and we can use it as a proxy to indirectly manipulate all of the other photonic qubits.”
Importantly, this system should be able to run a variety of quantum operations. The team says that different programs can be run on the same circuit, by writing new code to change how and when the atom and photons interact.
“For many photonic quantum computers, the gates are physical structures that photons pass through, so if you want to change the program that’s running, it often involves physically reconfiguring the hardware,” says Bartlett. “Whereas in this case, you don’t need to change the hardware – you just need to give the machine a different set of instructions.”
Better still, photonic quantum computer systems can operate at room temperature, removing the bulk added by the extreme cooling systems.
The research was published in the journal Optica.
Source: Stanford University
