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In much popular coverage, the surface code is implicitly assumed as the ‘standard model’ of quantum computation.

The missing gate can be reinstated by a technique known as magic state injection, though this introduces a significant overhead and is usually considered the likely bottleneck for practical calculations.

A wide variety of quantum gates can be implemented efficiently, though one short of that needed to constitute a universal set of quantum gates.

Interactions are only required between ‘neighbouring’ qubits in the 2D lattice.

Works for physical qubits and gates with errors below a threshold of about 1%.

Surface Code – logical qubits are built from multiple physical qubits in a 2D lattice.

This has been driven because the ability of leading experimentalists to build and control would-be quantum devices has started to overlap with the requirements of one leading family of protocols, a topologically based QECC known as the surface code. There has been a surge in interest in quantum computing over the last two years. Families of protocols compete in terms of how practical they are to implement and the processing overhead they introduce. Fault tolerant operations are used to perform quantum gates on the logical qubits. Carefully designed stabilizer checks are used to detect and correct errors without disturbing the logical qubit state. In a typical approach noisy physical qubits are encoded into a robust logical qubit using a QECC. Much subsequent work has explored a variety of approaches. Quantum computing was first conceived by Richard Feynman in 1982, but it only really started to attract mainstream attention following the revolutionary work of 1996 that showed reliable FTQC was in principle possible. If we naively try to check and correct their value the quantum calculation could collapse prematurely.Īs Andrew Landahl (Sandia National Labs) points out “We don’t actually have the quantum equivalent of the digital bit yet, for that we need a working logical qubit”. The physical qubits are not just 0 or 1, but an analogue superposition of both and are entangled with other qubits involved in a calculation. Quantum mechanics is intrinsically different.

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Calculations are arranged over a series of clock cycles, and after each step the circuit hardware kicks the bit back to the nearer of 0 or 1.

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But to realise the full potential of quantum software we need a mechanism to prevent errors piling up uncontrollably in longer calculations – we need fault tolerant quantum computation, FTQC.Ĭonventional computers have error correction built into their very conception by being digital instead of analogue.

Many groups will push to see what can be done with NISQ devices, trying to complete their calculations before noise overwhelms them. Quantum systems are typically very sensitive to external interference. If 50-70 qubits are thought to be sufficient to demonstrate quantum supremacy, why in practice do we think we need so many more? Indeed, around the world groups are working towards the long term goal of quantum computers containing millions of qubits. But it is also still a rapidly evolving discipline, and innovation here, as much as in hardware, will shape the future quantum computing roadmap. The field of quantum error correction offers some increasingly mature solutions. Quantum computers can only realise their full potential by defeating the noise that plagues their delicate qubits.

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