Spin quantum computing, spin quantum cognition

TL;DR

The paper contrasts Kane-style silicon-based nuclear-spin qubits with Fisher's Posner-molecule proposal for quantum cognition, outlining how long-lived nuclear spins could enable robust information storage and potential biological readout. It analyzes control and readout mechanisms in engineered spin systems (A- and J-gates, hyperfine and exchange interactions, SET readout) alongside proposed biological readout pathways via rotational states and chemical binding, while noting uncertainties around readout and entanglement in Posner clusters. By highlighting shared design principles and open questions, the work proposes a bidirectional exchange: quantum computing concepts can illuminate quantum biology, and biological models can inspire new quantum-processing strategies. The overall message is that cross-disciplinary dialogue could yield practical advances in stabilizing and translating quantum information across silicon devices and neural substrates.

Abstract

Over two decades ago, Bruce Kane proposed that spin-half phosphorus nuclei embedded in a spin-zero silicon substrate could serve as a viable platform for spin-based quantum computing. These nuclear spins exhibit remarkably long coherence times, making them ideal candidates for qubits. Despite this advantage, practical realisation of spin quantum computing remains a challenge. More recently, physicist Matthew Fisher proposed a hypothesis linking nuclear spin dynamics, specifically those of phosphorus nuclei within the spin-zero matrix of calcium phosphate molecules, to neural activation and, potentially, cognition. The theory has generated both interest and scepticism, with some fundamental questions remaining. We review this intersection of quantum computing and quantum biology by outlining the similarities between these models of quantum computing and quantum cognition. We then address some of the open questions and the lessons that might be learned in each context. In doing so, we highlight a promising bidirectional exchange: not only might quantum computing offer tools for understanding quantum biology, but biological models may also inspire novel strategies for quantum information processing.

Figures (2)

• Figure 1: Simple schematic diagram of a nuclear-spin-based quantum computer. The Kane quantum computer employs phosphorus-31 atoms embedded in isotopically pure silicon, where each donor nucleus serves as a qubit. Each phosphorus atom has a weakly bound electron whose spin couples to the nuclear spin via the hyperfine interaction. Control is achieved using surface electrodes: A-gates tune the hyperfine coupling between a donor nucleus and its electron, while J-gates adjust the overlap of neighbouring electrons to mediate interactions between qubits. Readout relies on transferring nuclear spin information to the donor electron and detecting resulting charge shifts with a single-electron transistor (not shown), exploiting the Pauli exclusion principle to distinguish singlet and triplet states kanesimmons2003dzurak1.
• Figure 2: Simplified illustration of entangled Posner molecules. Each Posner molecule consists of nine calcium ions (blue spheres), eight of these on the corners of a cube, with one in the centre of the cube. On the faces of the cube are the six phosphates, with each phosphate consisting of one phosphorus (red) and four oxygen (yellow) ions. Entanglement between Posner molecules is conferred by the entanglement of phosphorus nuclear spins in different Posner molecules. In this illustration the black circle and arrows shows one entangled phosphorus pair in a singlet state (antiparallel spins).