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arXiv:2409.11956 · Physical Review Research

Spin amplification in realistic systems

Ivan Iakoupov, V. M. Bastidas, Yuichiro Matsuzaki, Shiro Saito, William J. Munro

Published Jul 14, 2026• 0 citations
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Abstract

Spin amplification is the process that ideally increases the number of excited spins when one of them is excited initially. We show that by applying optimal control techniques to design classical drive pulse shapes, spin amplification can be achieved in a previously unexplored fast regime, with amplification times comparable to the intrinsic interaction timescale. This is an order of magnitude faster than the previous protocols and makes spin amplification possible even with significant decoherence and inhomogeneity in the spin system. The initial spin excitation can be delocalized over the entire ensemble, which is a more typical situation when a photon is collectively absorbed by the spins. We focus on the superconducting persistent-current artificial atoms and the Rydberg atoms as spins.

DOI
10.1103/4nqk-9t5s

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Spin amplification is the process that ideally increases the number of excited spins when one of them is excited initially. We show that by applying optimal control techniques to design classical drive pulse shapes, spin amplification can be achieved in a previously unexplored fast regime, with amplification times comparable to the intrinsic interaction timescale. This is an order of magnitude faster than the previous protocols and makes spin amplification possible even with significant decoherence and inhomogeneity in the spin system. The initial spin excitation can be delocalized over the entire ensemble, which is a more typical situation when a photon is collectively absorbed by the spins. We focus on the superconducting persistent-current artificial atoms and the Rydberg atoms as spins.
A long-lived qubit is usually well-isolated from all other systems and the environments, and so is not easy to couple with measurement apparatus. It is sometimes difficult to implement reliable projective measurements on such a qubit. One potential solution is spin amplification with many ancillary qubits. Here, we propose a spin amplification technique, where the ancillary qubits state change depending on the state of the target qubit. The technique works even in the presence of inhomogeneous broadening. We show that fast and accurate amplification is possible even if the coupling and frequency of the ancillary qubits is inhomogeneous. Since our scheme is robust against realistic imperfections, this could provide a new mechanism for reading out a single spin that could not have been measured using the previous approaches.
We present an experimentally feasible method to produce a large and tunable spin squeezing when an ensemble of many four-level atoms interacts simultaneously with a single-mode photon and classical driving lasers. Our approach is to simply introduce a time-dependent collective atom-photon coupling. We show that the maximal squeezing factor measured experimentally can be well controlled by both its driving magnitude and driving frequency. In particular, when increasing the driving magnitude, the maximal squeezing factor increases and thus can be rapidly enhanced. We also demonstrate explicitly in the high-frequency approximation that this spin squeezing arises from a strong repulsive spin-spin interaction induced by the time-dependent collective atom-photon coupling. Finally, we evaluate analytically, by using current experimental parameters, the maximal squeezing factor, which can reach 40 dB. This squeezing factor is far larger than previous ones.
The storage of quantum information in spin-ensembles is limited by practically unavoidable inhomogeneous broadening, and the macroscopic number of spins in such an ensemble makes the design of control solutions to increase the coherence time a challenging task. Together with a concurrently developed Krylov theory that allows us to treat the control problem efficiently, we design optimal cavity modulation for such spin ensembles that achieve an order of magnitude enhancement in qubit lifetime compared to the losses due to inhomogeneity and cavity decay.
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Spin ensembles show increasing promise for highly efficient quantum sensing and quantum technologies. However, protecting their coherent properties remains a significant challenge. Dynamical decoupling (DD) typically employs a sequence of fast pulses that aim to reduce the effect of unwanted system-environment interactions. However, the performance of the DD sequences typically suffers in spin ensembles, e.g., due to amplitude and frequency inhomogeneities and control errors. In this work, we compare the performance of an extensive number of DD protocols for coherence protection and quantum sensing in an ensemble of negatively charged NV centers under ambient conditions. We identify the optimal approach that uses rapid adiabatic passage pulses, which are robust to amplitude and frequency inhomogeneity, in combination with the variation of their phases. We experimentally demonstrate an improved signal contrast and an order-of-magnitude increase in memory time with the optimal sequence compared to the widely used XY8 with standard rectangular pulses. We also apply our approach for improved sensing of a weak oscillating field. While our particular demonstration is with NV centers in diamond, the optimized protocol is generally applicable to other spin ensembles and to systems with high amplitude and/or frequency inhomogeneities.
The absorption of traveling photons resonant with electric dipole transitions of an atomic gas naturally leads to electric dipole spin-wave excitations.For a number of applications, it would be highly desirable to shape and coherently control the spatial waveform of the spin waves before spontaneous emission can occur.This paper details a recently developed optical control technique to achieve this goal, where counterpropagating, shaped subnanosecond pulses impart subwavelength geometric phases to the spin waves by cyclically driving an auxiliary transition.In particular, we apply this technique to reversibly shift the wave vector of a spin wave on the D2 line of laser-cooled 87 Rb atoms by driving an auxiliary D1 transition with shape-optimized pulses, so as to shut off and recall superradiance on demand.We investigate a spin-dependent momentum transfer during the spin-wave control process, which leads to a transient optical force as large as ∼1hk/ns, and study the limitations to the achieved 70 ∼ 75% spin-wave control efficiency by jointly characterizing the spin-wave control and matter-wave acceleration.Aided by numerical modeling, we project potential future improvements of the control fidelity to the 99% level when the atomic states are better prepared and by equipping a faster and more powerful pulse shaper.Our technique also enables a background-free measurement of the superradiant emission to unveil the precise scaling of the emission intensity and decay rate with optical depth.