'Cat' cubes have made the quantum computer more fail-safe



Physicists from Amazon presented a scheme of fail-safe quantum computers, which implemented a new approach to quantum correction of errors. At the heart of the architecture of this computer are "cat" cubes (in the quantum state of Schrodinger's cat), which are highly resistant to flipping bits. The preprint of the article is published on arXiv.org.


Creating a quantum computer is not an easy task. The problem is that the quantum system inevitably comes into contact with the environment. At the same time, information about the quantum system initially encoded in the device seeps into the environment. This phenomenon is called decoherence. After de-conference, you can no longer access all the information about the device, watching only it. This means that there is an error and some of the information is lost. In order to build a reliable quantum computer, you need to learn how to minimize such errors.


One approach is to reduce the number of logical element errors using excess qubits or to actively quantum correct errors. This method is based on the natural assumption that if you compare multiple copies of the same state and they will be different from each other, then there was an error, and otherwise, everything is fine. Despite some difficulties (in quantum mechanics it is impossible to simply copy qubits), this approach is implemented in practice. But it has a disadvantage - the need to use additional qubits significantly increases their total number, which makes the task more difficult from the point of view of technical implementation.



Passive, or autonomous, quantum error correction is the opposite approach. To implement it develops physical computing systems that have internal resistance to errors.


Flea sphere for Schrodinger's cats / Chamberland et al. / arXiv.org, 2020
Flea sphere for Schrodinger's cats / Chamberland et al. / arXiv.org, 2020

A team of scientists from Amazon, led by Fernando Brandao, has developed a new quantum computer scheme that is error-resistant. They used "cat" cubes, or cubits in the quantum state of the cat Schrodinger(cat state), that is, in the superposition of coherent states with opposite phases. The idea is that after the "cat" qubit is stabilized, bugs with flip bits become extremely rare, and phase coup errors become more frequent. And in order to protect against phase coup errors, you can use an active error correction. In this case, the simplest X-based repetition code was used to actively correct the bugs, where X is the Pauli Matrix.


Storage cubes and auxiliary cubes (yellow and gray mugs in the picture) were encoded as Schrodinger's cat states localized acoustic mod resonators with phonon defects (PCDR). Two-photon recharge from the engineering tank (designated as RESVR in the picture) was used to stabilize the stabilization. The reading was made with the help of a transmon. / Chamberland et al. / arXiv.org, 2020
Storage cubes and auxiliary cubes (yellow and gray mugs in the picture) were encoded as Schrodinger's cat states localized acoustic mod resonators with phonon defects (PCDR). Two-photon recharge from the engineering tank (designated as RESVR in the picture) was used to stabilize the stabilization. The reading was made with the help of a transmon. / Chamberland et al. / arXiv.org, 2020

For logical operations on qubits are used quantum valves. Earlier work has shown that a universal set of valves, including valves X, q, CNOT, and Toffoli, can be performed on "cat" qubits using the same two-photon mechanism of arousal and loss. The key point is that these valves are not only universal but also do not lead to additional bits. The "cat" cubes between which the logical operation is carried out must be connected to a single feeding tank. The study authors found that this connection leads to correlated errors between them. If you connect more than four qubits to one tank, these interferences will not be compensated by an active error correction. This number defined the layout and ability to connect the architecture.


The idea of using Schrodinger's cat status together and actively correcting errors is not new. The difference between this study and previous studies is that the scientists performed a complete noise simulation, including an in-depth study of the rare bits flip processes. Using the architecture proposed in the work and active error correction, allows you to achieve a logical error of 2.7×10-8using just 9 qubits of data code, this is an improvement of more than five orders of magnitude compared to completely unprotected qubits. According to scientists, such a scheme ("cat" cubes and repetition codes) with just over two thousand superconducting components used for stabilization, could create a hundred logical qubits capable of reliably performing a thousand Toffoli valves.

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