Each logical qubit gets encoded into three qubits, and ancilla qubits check to see if one of the phases has flipped. Shor found that he could correct for phase errors using a similar principle to the one he used for bit flips. It sets up a situation where the correct answer to a calculation constructively interferes and is therefore amplified, while the incorrect answer gets suppressed by destructive interference.īut if an error causes the phase to flip, then destructive interference can switch to constructive interference, and the quantum computer will start amplifying the wrong answer. But if the waves are out of phase, then when one wave is at its peak, the other is at its nadir, and they will cancel each other out.Ī quantum algorithm takes advantage of this phase relationship among its qubits. If they collide, they will constructively interfere, merging into a single wave double the size. When two waves are in phase, their ripples are synchronized. You can think of this phase as a wave - it tells you the location of the wave’s peaks and troughs. The relative “phase” between qubits matters too. Superpositions are the key to quantum computing, but it’s not just the value of the qubit that’s important. But qubits have one more potential source of error.
#Harlow quantum error correction code
This code protects against a bit flip, which is the only possible error that can occur in classical computing. By measuring the states of these ancillary qubits, you learn if the three information-containing qubits are in identical states without disturbing the state of any of them. The first of these compares the first and second physical qubits the other compares the second and third. Shor’s code replaces the scales with two extra “ancilla” qubits. If the scales are imbalanced both times, the ball that stayed still is the culprit. If it was balanced only once, then one of the replaced balls is the odd one out. If the scale was balanced both times, then all balls are identical. The answer is to first pick two balls and compare their weights, then replace one of the balls with the remaining ball and check again.
What measurements will let you determine whether there is an oddball in the mix, and if so, which one it is? You’re given three balls that look identical, but one of the balls might have a different weight. The task is not unlike solving a simple logic puzzle. If one of the qubits was different, it would indicate that an error had occurred. Instead, he found a way to tell if the three physical qubits were in the same state as one another. Since measuring a quantum state would destroy the superposition, there wasn’t a straightforward way to check to see whether an error had occurred. The essential power of quantum computation comes from the fact that qubits can exist in a “superposition” of being in a combination of 0 and 1 at the same time. Shor’s quantum repeater code couldn’t be exactly the same as the classical version, though. He used three individual “physical” qubits to encode a single qubit of information - the “logical” qubit. If one of the bits is different from the others, the computer can correct the error and continue the calculation. Shor modeled his protocol after the classical repeater code, which involves making copies of each bit of information, then periodically checking those copies against each other.
#Harlow quantum error correction crack
So how did Shor crack the conundrums he faced? He used the added complexity of quantum mechanics to his advantage. But at the beginning of October, researchers led by Chris Monroe, a physicist at the University of Maryland, reported that they had demonstrated many of the ingredients necessary to run an error-corrected circuit like Shor’s. “We won’t be able to scale up quantum computers to the degree that they can solve really hard problems without it,” said John Preskill, a physicist at the California Institute of Technology.Īs with quantum computing in general, it’s one thing to develop an error-correcting code, and quite another to implement it in a working machine. Most physicists see it as the only path to building a commandingly powerful quantum computer.
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