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arXiv:2302.00476 · Quantum Information Processing

Breaking Landauer's bound in a spin-encoded quantum computer

Frank Z. Wang

Published Nov 12, 2022• 2 citations
arXivHTMLPDF

Abstract

Abstract It is commonly recognized that Landauer's bound holds in (irreversible) quantum measurement. In this study, we overturned this common sense by extracting a single spin from a spin–spin magnetic interaction experiment to demonstrate that Landauer’s bound can be broken quantitatively by a factor of $$10^{4} \sim 10^{10}$$ 10 4 ∼ 10 10 via quantum spin tunneling. It is the quantum limit ( $$\hbar /2 \approx 10^{ - 34} \;{\text{J}} \cdot {\text{s}}$$ ħ / 2 ≈ 10 - 34 J · s ), rather than Landauer’s bound, that governs the performance of a spin qubit. An optically-manipulated spin-encoded quantum computer is designed, in which energy bound well below $$k_{B} T$$ k B T to erase a spin qubit at the expense of a long spin relaxation time is theoretically sensible and experimentally verified. This work may represent the last piece of the puzzle in quantum Landauer erasure in terms of a single spin being the smallest and the closest to the quantum limit.

DOI
10.1007/s11128-022-03707-2

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Abstract It is commonly recognized that Landauer's bound holds in (irreversible) quantum measurement. In this study, we overturned this common sense by extracting a single spin from a spin–spin magnetic interaction experiment to demonstrate that Landauer’s bound can be broken quantitatively by a factor of 104∼101010^{4} \sim 10^{10}104∼1010 10 4 ∼ 10 10 via quantum spin tunneling. It is the quantum limit ( ℏ/2≈10−34  J⋅s\hbar /2 \approx 10^{ - 34} \;{\text{J}} \cdot {\text{s}}ℏ/2≈10−34J⋅s ħ / 2 ≈ 10 - 34 J · s ), rather than Landauer’s bound, that governs the performance of a spin qubit. An optically-manipulated spin-encoded quantum computer is designed, in which energy bound well below kBTk_{B} TkB​T k B T to erase a spin qubit at the expense of a long spin relaxation time is theoretically sensible and experimentally verified. This work may represent the last piece of the puzzle in quantum Landauer erasure in terms of a single spin being the smallest and the closest to the quantum limit.
Abstract Landauer’s bound is applicable to irreversible quantum operations. In this study, we showcased that the Doppler temperature manifests the existence of Landauer’s bound, which does not block a spin from (irreversibly) flipping with a tiny amount of energy via quantum tunneling. Verified by a spin–spin magnetic interaction experiment, this (energy) amount was determined to be only 1.25 times the theoretical value of Landauer’s bound. Based on Heisenberg’s principle, we defined information from a measuring perspective: one bit of information corresponds to the smallest error when quantifying the product of the measured energy uncertainty ( ΔE\Delta EΔE Δ E ) and the measured time duration ( Δt\Delta tΔt Δ t ). We then illustrate an optically manipulated, spin-encoded, near-Landauer-bound, near-Heisenberg-limit quantum computer that encompasses this new definition of information. This study may represent the last piece of the puzzle in understanding both quantum Landauer erasure and Heisenberg’s quantum limit since a single spin is the smallest information carrier.
Quantum computing is fundamentally limited by the Planck constant ( h=6.63 ×10−34 J⋅sh = 6.63\ \times {{10}^{ - 34}}\ J \cdot sh=6.63 ×10−34 J⋅s ) through the Heisenberg Limit. The energy consumption over a given time, or the speed of processing information with a specific energy budget, is a core research focus in quantum computing. To date, the smallest action (the energy-time cost) achieved is approximately 10−29J⋅s{{10}^{ - 29}}J \cdot s10−29J⋅s , using a giant spin qubit composed of 20 spins. In our study, we achieved an action of 1.66 ×10−34 J⋅s1.66\ \times {{10}^{ - 34}}\ J \cdot s1.66 ×10−34 J⋅s to reversibly manipulate a single spin qubit through a spin-spin magnetic interaction experiment. By adhering to the principle of least action, our theoretical and experimental results establish the minimal action required. Our findings highlight the potential of spin-qubit quantum computers as accelerators for computation-intensive applications, such as AI and Post-Quantum Cryptography, since they exhibit several unique advantages: 1. High energy efficiency (by approaching the Heisenberg limit as well as the Landauer bound); 2. High-density integration (with just an atom/ion per qubit); 3. Long coherence times (tens of seconds); 4. High-fidelity (98%); and 5. Fault tolerance (through decoherence-free subspaces).
Abstract Landauer’s principle introduces a symmetry between computational and physical processes: erasure of information, a logically irreversible operation, must be underlain by an irreversible transformation dissipating energy. Monitoring micro- and nano-systems needs to enter into the energetic balance of their control; hence, finding the ultimate limits is instrumental to the development of future thermal machines operating at the quantum level. We report on the experimental investigation of a lower bound to the irreversible entropy associated to generalized quantum measurements on a quantum bit. We adopted a quantum photonics gate to implement a device interpolating from the weakly disturbing to the fully invasive and maximally informative regime. Our experiment prompted us to introduce a bound taking into account both the classical result of the measurement and the outcoming quantum state; unlike previous investigation, our entropic bound is based uniquely on measurable quantities. Our results highlight what insights the information-theoretic approach provides on building blocks of quantum information processors.
Irreversibility has a fundamental thermodynamic cost, erasing information inevitably generates heat. This connection is quantified by the Landauer bound, which gives the minimum dissipation needed to erase a single bit of information. While this bound applies in both classical and quantum settings, it is saturated only in idealised limits of infinite resources. Here, we provide a unified first principles description of finite-resource erasure in both classical and quantum systems. We begin by proving the communal folklore that in the idealised regime the erasure cost of a bit encoded in a quantum or classical system is the same. Despite this, we show that their practical implementation differs substantially: achieving comparable erasure quality in quantum systems requires more control, larger accessible energy gaps and longer operation times. Classical protocols can achieve the erasure of a comparable quantum protocol under far weaker constraints which we expose in trade-off relations. Our results explain why practical erasure schemes fall short of Landauer's bound and show that classical systems enjoy several fundamental thermodynamic advantages.