Logical qubit processor
A miltestone in quantum computing
Hey there!
I'm thrilled to share two exciting milestones with you. Firstly, The Quantum Vibe community has now grown to over a hundred readers, and I'm deeply grateful for your support. Secondly, there's a groundbreaking development in the quantum world — the realization of a quantum circuit with logical qubits. This breakthrough, led by Mikhail Luskin from Harvard, promises to accelerate fault-tolerant quantum computing and reshape the landscape of quantum processors. The research is published in nature (available as accelerated preview, read here). In this issue, let's dive into the key elements of this discovery and explore its potential impact on the quantum computing landscape.
“Nature isn't classical, dammit, and if you want to make a simulation of nature, you'd better make it quantum mechanical.” -Richard Feynman 1981
Quantum computing can unlock never seen details of nature, potentially transforming our universe. However, implementing a quantum computer is extremely difficult. To understand why it is so, let’s break down the elements of a quantum circuit:
Qubits: The basic unit of information in a quantum circuit. Unlike classical bits, which can only be 0 or 1, qubits can exist in a superposition of both states simultaneously. This allows for powerful computing that can explore multiple possibilities at once.
Quantum gates: These are the operations that manipulate the states of qubits.
Entanglement: A special connection between two qubits where they become correlated and share information instantaneously, regardless of distance.
Readout: The process of extracting information from qubits.
The machinery is simple, isn’t it? Then, what is making things difficult? Actually, things go wrong at every step. While superposition and entanglement enable the power of quantum computing, they are extremely fragile and easily disrupted. For example:
Qubits are sensitive, and extremely prone to environment which leads to decoherence of quantum states manifesting as errors in information.
Quantum gates are not perfect, and thus operations to manipulate the qubits may result in unwanted correlations.
Entanglement is prone to state preparation errors, i.e., if the initial entangled state is not prepared correctly, it can lead to errors throughout the computation.
Readout is another tricky process which relies on the collapse of quantum state. For instance, a qubit can in general be in a superposition state of 0 and 1, however, during read out superposition is destroyed and quantum state collapses to 0 or 1. Given the probabilistic nature of quantum states, even small imperfections in readout can give unexpected outputs.
Moreover, limited coherence time of quantum states and imperfect quantum hardware are critical at all stages of quantum computing.
So, how does quantum computation work with so many errors? The answer is Quantum Error Correction (QEC): a powerful technique in quantum computing that aims to detect and correct errors which are inevitable in fragile quantum machinery. The basic idea of QEC is to encode the information stored in a physical or data qubit into a larger set of qubits which together form a logical qubit. These extra qubits act as backup and carry redundant information about the data qubit. In other words, logical qubits are abstractions built upon multiple physical qubits. They represent the ideal, error-free quantum information that we want to manipulate and process.
Wouldn’t it be nice if a quantum circuit could be realized with logical qubits? That’s what the team led by Mikhail Lukin did! They built a quantum circuit with 48 logical qubits. Not only they implemented the key elements of logical processing but also demonstrated practical utility of encoding methods for improving sampling of complex scrambling circuits. Rydberg atoms played a pivotal role, providing easily manipulated physical qubits due to their indistinguishability which helped them execute well controlled entanglement and readout by moving around the qubits. This is in contrast with widely used superconducting qubits which are distinguishable, i.e., exchanging two superconducting qubits won’t result in the same quantum circuit. The team also anticipates that Rydberg qubits might play an important role in scaling the number of logical qubits.
We are in the dawn of the quantum era. However, there is a complex puzzle to be solved to get the sun above the horizon, and the logical qubit quantum processor can certainly be one of the missing pieces.
So that’s that from this issue. Until next time, stay curious.