Autonomous quantum error correction method greatly increases qubit coherence times

April 29, 2016 by Lisa Zyga feature
One possible implementation of the logical qubit. The qubits are in the blue boxes and the resonators are in the red boxes. Credit: Kapit. ©2016 American Physical Society

(—It might be said that the most difficult part of building a quantum computer is not figuring out how to make it compute, but rather finding a way to deal with all of the errors that it inevitably makes. Errors arise because of the constant interaction between the qubits and their environment, which can result in photon loss, which in turn causes the qubits to randomly flip to an incorrect state.

In order to flip the qubits back to their correct states, physicists have been developing an assortment of quantum techniques. Most of them work by repeatedly making measurements on the system to detect errors and then correct the errors before they can proliferate. These approaches typically have a very large overhead, where a large portion of the computing power goes to correcting errors.

In a new paper published in Physical Review Letters, Eliot Kapit, an assistant professor of physics at Tulane University in New Orleans, has proposed a different approach to quantum error correction. His method takes advantage of a recently discovered unexpected benefit of quantum noise: when carefully tuned, quantum noise can actually protect qubits against unwanted noise. Rather than actively measuring the system, the new method passively and autonomously suppresses and corrects errors, using relatively simple devices and relatively little computing power.

"The most interesting thing about my work is that it shows just how simple and small a fully error corrected quantum circuit can be, which is why I call the device the 'Very Small Logical Qubit,'" Kapit told "Also, the error correction is fully passive—unwanted error states are quickly repaired by engineered dissipation, without the need for an external computer to watch the circuit and make decisions. While this paper is a theoretical blueprint, it can be built with current technology and doesn't require any new insights to make it a reality."

The new passive error correction circuit consists of just two primary qubits, in contrast to the 10 or more qubits required in most active approaches. The two qubits are coupled to each other, and each one is also coupled to a "lossy" object, such as a resonator, that experiences photon loss.

"In the absence of any errors, there are a pair of oscillating photon configurations that are the 'good' logical states of the device, and they oscillate at a fixed frequency based on the circuit parameters," Kapit explained. "However, like all qubits, the qubits in the circuit are not perfect and will slowly leak photons into the environment. When a photon randomly escapes from the circuit, the oscillation is broken, at which point a second, passive error correction circuit kicks in and quickly inserts two photons, one which restores the lost photon and reconstructs the oscillating logical state, and the other is dumped to a lossy circuit element and quickly leaks back out of the system. The combination of careful tuning of the resonant frequencies of the circuit and adding photons two at a time to correct losses ensures that the passive error correction circuit can operate continuously but won't do anything to the two good qubits unless their oscillation has been broken by a photon loss."

The new method can correct photon loss errors at rates up to 10 times faster than those achieved by active, measurement-based methods. In addition, the passive method can partially suppress noise, so that there are fewer in the first place. In its current version, the method can correct only one error at a time, so if a second photon loss occurs before the correction is complete, the method cannot fix the resulting error.

All of this error correction leads to a significant increase in the qubit coherence time. The new method can improve this time by a factor of 40 or more compared to without any error correction, and this improvement is greatly needed in order to construct quantum computers. As Kapit explains, qubit coherence times are currently so short that millions of would be required to build a useful quantum computer. Increasing the coherence times can reduce this number to something more feasible.

In the future, Kapit plans to integrate the new passive method with active, measurement-based methods to create a hybrid strategy, and investigate how the two methods might work together.

He is also currently working with an experimental team to try to build, test, and optimize the device in the next few years.

Explore further: Scientists track quantum errors in real time

More information: Eliot Kapit. "Hardware-Efficient and Fully Autonomous Quantum Error Correction in Superconducting Circuits." Physical Review Letters. DOI: 10.1103/PhysRevLett.116.150501

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not rated yet Apr 30, 2016
eh, i'm having trouble understanding the science and the definition for noise, i.e.this is a quantum bit. There are given number of states given the field relaxation time; anyway, just define the device in the frequency domain. You were only trying to define a single state. But I see you don't know what state you are in and the allowed states of the system. In other words, if you're building a quantum computer, surely that is more logical. However defining the state as quanta, i.e. a specific e field at a point, instead of making it random, why not define all states. But the idea of noise? There is no noise, everything can traced back to a set of particles in a specific state simply based upon the spectra. If you do not fully understand the spectra, where is your understanding of quanta. The probability, i.e. close to one, the state is defined. My set point, my modulation, how about from a drum to a hyper-neutrino? It's all definable! Your error would be in your precision.
not rated yet Apr 30, 2016
jeez, you guys don't see the time frequency-response. Time from -infinity to +infinity, space, field, e undefined; therefore, see only the field and these points in a spacetime, with our space being any space. The space of the particle any particle, within a crystal, each point is primarily defined by it's near space and use superposition about the point you are inquiring. You define where you are, when you are, where the charges are ... then build a quantum computer. So this paper may be refined. You should have an array defining any particle by using only the + and -. Your question then, with this sensitivity, you can hear an electron move, so the question is "What moved?" and all its iterations, calculable about the space being queried. Surely there is a set you may read without error.
not rated yet Apr 30, 2016
Then, using the computer to calculate "what moved" or the defined state and whether a match can be found just by swapping plys or creative search algorithms using only two images. Don't forget, everything is moving, not Dr. E I think QM is a set of blinders. Yes, it is all waves; but make "It", the waves, definable by design, not guessing. I would seek to know all the atomic perturbations, wow! What a set of data. Not noise! We just have to learn to play with a bundle of + and - points in space-time. These describe Space. We look at that state as a point in 4D.
Apr 30, 2016
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not rated yet Apr 30, 2016
Fascinating: First time I've seen random frequency quantum switching elements tied to a very "un-random" resonating time dependent element.
Its so surprising that the almost classical resonator elements in such intimate contact with a quantum system don't do more harm than good.
not rated yet May 01, 2016
Not the discontinuity vs continuity argument again? You get all those Planck limits and Heisenberg nonsense.

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