The processor can perform a few simple tasks, which have been demonstrated before with single nuclei, atoms and even photons, but this is the first time that such tasks have been performed in an all-electronic device that looks and feels much like a regular microprocessor.
A qubit, or "quantum bit," is analogous to a bit in standard computing that can be assigned a value of 1 or 0, but thanks to a quantum mechanics property known as "quantum entanglement," a qubit can actually be in more than one state at once, which enhances the speed and performance of a quantum processor exponentially.
The researchers built two qubits with approximately one billion aluminum atoms each, and made them act as a single entity that can very rapidly switch from one state to another. They also used the chip to successfully run elementary quantum computing algorithms, and particularly one known as Grover's search, demonstrating quantum information processing on a chip.
Prior to the team's findings, scientists couldn't get solid-state qubits to perform algorithms mainly because they couldn't get the entanglement state to last long enough: while previous research in the field only made the entanglement state last for about a nanosecond, the Yale research team managed to make it last it 1,000 times longer, enough to run simple algorithms that don't require too many operations.
"The qubits are physically robust but the quantum superpositions of their two energy levels persist only for 1-3 microseconds," Prof. Girvin, a theoretical physicist who was part of the research group, told Gizmag via email.
"The main problem is that like real atoms, [qubits] suffer spontaneous emission. The excited state can decay to the ground state by emitting a photon that can travel away guided by the wires of the circuit or it can be absorbed by some source of dissipation in the circuit. This is partially understood but still needs more work."
A second issue, Prof. Girvin told us, is that various perturbations can cause the transition frequency between the two qubit states to lose its stability. "If this is not rock stable, the superposition can lose memory of the phase it is supposed to have." However, the team has largely overcome this problem through engineering design of the qubit itself.
To perform the operations, the qubits communicate with one another using a "quantum bus" made of photons that send and receive information through wires connecting the qubits — a mechanism that was previously developed by the same group.
The key that made the two-qubit processor possible was getting the qubits to switch on and off very suddenly, so that they exchange information quickly and in an highly controllable fashion. The search algorithm demonstrated on the chip performed a total of 10 quantum operations in 100 nanoseconds, or about one tenth of the lifetime of the qubit superpositions, meaning the fidelity of the result was particularly high.
The team now plans to focus on increasing the amount of time the qubits maintain their states, so that more complex algorithms can be run, as well as increasing the number of qubits connected to the quantum bus to further improve the computation speed.