Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

arXiv:2411.15362 · Physical Review Letters

Unwanted Couplings Can Induce Amplification in Quantum Memories despite Negligible Apparent Noise

Faezeh Kimiaee Asadi, Jitender Kumar, Jiawei Ji, Khabat Heshami, Christoph Simon

Published Jul 18, 2025• 1 citation
arXivHTMLPDF

Abstract

Theoretical quantum memory design often involves selectively focusing on certain energy levels to mimic an ideal Λ configuration, a common approach that may unintentionally overlook the impact of neighboring levels or undesired couplings. While this simplification may be justified in certain protocols or platforms, it can significantly distort the achievable memory performance. Through numerical semiclassical analysis, we show that the presence of unwanted energy levels and undesired couplings in an absorptive memory based on a nitrogen-vacancy center can significantly amplify the signal, resulting in memory efficiencies exceeding unity, a clear indication of unwanted noise at the quantum level. Strikingly, this effect occurs even when the apparent noise, i.e., output in the absence of an input field, is negligible. We then generalize our results using semianalytical estimations to analyze this amplification, and propose a strategy to reduce its effect. Our findings extend to memory platforms beyond nitrogen-vacancy centers; as an example, we also analyze a cavity-based rubidium memory that experiences the same issue.

DOI
10.1103/pz34-47pw

Related papers

From quantum storage to amplification: the effect of unwanted couplings and an additional level in cavity-based ensemble quantum memories

Jia-Wei Ji, Christoph Simon

Quantum-memory models often reduce complex level structures to an idealized $Λ$ system, potentially missing nearby levels and unwanted couplings that can qualitatively alter the predicted performance. Here, we study an extension of a cavity-based $Λ$-type ensemble memory, a four-level model with unwanted couplings from both the control field and signal, using a fully quantum treatment. We derive explicit expressions for the single-photon storage efficiency, retrieval efficiency, and fidelity, and on this basis identify three distinct dynamical regimes: stable, threshold, and unstable. Within the stable regime, we additionally discriminate between two qualitatively different sub-regimes. Applying the theory to warm-vapor-inspired parameters, we determine the conditions under which the system can still operate as a high-quality quantum memory. More generally, our results provide a practical framework for distinguishing genuine memory operation from amplification and for optimizing realistic quantum memories beyond idealized models.

quant-pharXiv:2605.00446•May 4, 2026•0 citations

Effects of entanglement in an ideal optical amplifier

J. D. Franson, R. A. Brewster

In an ideal linear amplifier, the output signal is linearly related to the input signal with an additive noise that is independent of the input. The decoherence of a quantum-mechanical state as a result of optical amplification is usually assumed to be due to the addition of quantum noise. Here we show that entanglement between the input signal and the amplifying medium can produce an exponentially-large amount of decoherence in an ideal optical amplifier even when the gain is arbitrarily close to unity and the added noise is negligible. These effects occur for macroscopic superposition states, where even a small amount of gain can leave a significant amount of which-path information in the environment. Our results show that the usual input/output relation of a linear amplifier does not provide a complete description of the output state when post-selection is used.

arXiv:1608.08821•Physics Letters A•Jan 31, 2018•11 citations

Efficient, low noise, mode-selective quantum memory

Sarah E. Thomas

Photonic quantum information processing is a key element for scalable quantum technologies, and has applications in secure long-distance quantum communication and connecting nodes of a quantum computation network. However, logical photon-photon gates and state-of-the-art single photon sources rely on probabilistic processes. Quantum memories are devices that enable storage and on-demand recall of quantum states of light, and have been highlighted as a vital component in photonic networks to overcome the scaling problem by synchronising probabilistic processes. The Raman memory has a large storage bandwidth and high synchronising capacity, and is an ideal candidate for local synchronisation. However, previous demonstrations of the Raman memory suffer from four-wave mixing noise, which prohibits quantum level operation. In this thesis I investigate methods to increase the signal to noise ratio in the Raman memory. I investigate increasing the light-matter coupling strength to boost the memory efficiency, and then explore two different methods to suppress four-wave mixing noise. I demonstrate that operating the Raman memory in a cavity is successful in reducing four-wave mixing, but it is technically challenging to maintain a high memory efficiency. I investigate a new method of noise suppression by introducing an absorption feature at the frequency of the unwanted noise field. This technically simple method is successful in reducing the noise by an order of magnitude, and will be applicable to many quantum memory protocols. In the final section of this thesis I explore the temporal mode properties of the Raman memory. I demonstrate that the Raman memory is single mode and can be used to separate and manipulate temporal modes of light. This positions the Raman memory as a key device for enabling high-dimensional photonic quantum information processing, and enhancing light-matter interactions. These results pave the way towards an efficient, low-noise, mode-selective quantum memory.

•Apr 1, 2019•0 citations

Theory of noise suppression in Λ -type quantum memories by means of a cavity

Joshua Nunn, J. H. D. Munns, Sarah E. Thomas +10 more

Quantum memories, capable of storing single photons or other quantum states of light, to be retrieved on demand, offer a route to large-scale quantum information processing with light. A promising class of memories is based on far-off-resonant Raman absorption in ensembles of $\mathrm{\ensuremath{\Lambda}}$-type atoms. However, at room temperature these systems exhibit unwanted four-wave mixing, which is prohibitive for applications at the single-photon level. Here, we show how this noise can be suppressed by placing the storage medium inside a moderate-finesse optical cavity, thereby removing the main roadblock hindering this approach to quantum memory.

arXiv:1601.00157•Physical Review A•Jul 31, 2017•41 citations

Impact of Magnetic Field and Noises on Rabi Oscillation of Spin Qubit in Nitrogen-Vacancy Center in Diamond

Islam Jubaidul, Amsyar Rahim, C. Wei

Quantum computing holds the potential to outperform classical computing in certain complicated tasks by leveraging qubits and their manipulation through quantum gates. A compelling platform to physically realize qubits and their control is the Nitrogen-Vacancy (NV) center in diamond, where Rabi oscillation of spin qubit enables effective quantum operations. However, a significant challenge lies in mitigating qubit decoherence caused by external factors. Hence, this paper reports on the experimental demonstration of Rabi oscillation of spin qubit in NV center in diamond, focusing on investigating the impact of magnetic field and noises. Experiments conducted using CIQTEK Diamond Quantum Computer for Education (CIQTEK is a China quantum company) reveal that the magnetic field significantly impacts the resonant frequency which must align with the microwave pulse to observe clear Rabi oscillations. However, despite careful frequency tuning, noises from several sources lead to qubit decoherence, disrupting Rabi oscillations and causing them to decay over time. These results provide insights into key factors affecting qubit performance, paving the way in developing strategies to minimize decoherence.

International Conference on Communications, Circuits and Systems•May 23, 2025•0 citations

Possibility of inhomogeneous coupling leading to decoherence in an electromagnetically-induced-transparency quantum-memory process

Xiong-Jun Liu, Zheng-Xin Liu, Xin Liu +1 more

We study the effect of inhomogeneous coupling between atoms and external light fields on quantum memories that employ electromagnetically induced transparency (EIT). By introducing atomic subensembles to describe the present inhomogeneous coupling case we find that there is a nonsymmetric dark-state subspace that allows the EIT quantum-memory technique to function perfectly. This shows that such a memory scheme can work ideally even if the atomic state is very far from being a symmetric one.

arXiv:quant-ph/0510141•Physical Review A•Jan 30, 2006•9 citations
Theoretical quantum memory design often involves selectively focusing on certain energy levels to mimic an ideal Λ configuration, a common approach that may unintentionally overlook the impact of neighboring levels or undesired couplings. While this simplification may be justified in certain protocols or platforms, it can significantly distort the achievable memory performance. Through numerical semiclassical analysis, we show that the presence of unwanted energy levels and undesired couplings in an absorptive memory based on a nitrogen-vacancy center can significantly amplify the signal, resulting in memory efficiencies exceeding unity, a clear indication of unwanted noise at the quantum level. Strikingly, this effect occurs even when the apparent noise, i.e., output in the absence of an input field, is negligible. We then generalize our results using semianalytical estimations to analyze this amplification, and propose a strategy to reduce its effect. Our findings extend to memory platforms beyond nitrogen-vacancy centers; as an example, we also analyze a cavity-based rubidium memory that experiences the same issue.
Quantum-memory models often reduce complex level structures to an idealized ΛΛΛ system, potentially missing nearby levels and unwanted couplings that can qualitatively alter the predicted performance. Here, we study an extension of a cavity-based ΛΛΛ-type ensemble memory, a four-level model with unwanted couplings from both the control field and signal, using a fully quantum treatment. We derive explicit expressions for the single-photon storage efficiency, retrieval efficiency, and fidelity, and on this basis identify three distinct dynamical regimes: stable, threshold, and unstable. Within the stable regime, we additionally discriminate between two qualitatively different sub-regimes. Applying the theory to warm-vapor-inspired parameters, we determine the conditions under which the system can still operate as a high-quality quantum memory. More generally, our results provide a practical framework for distinguishing genuine memory operation from amplification and for optimizing realistic quantum memories beyond idealized models.
In an ideal linear amplifier, the output signal is linearly related to the input signal with an additive noise that is independent of the input. The decoherence of a quantum-mechanical state as a result of optical amplification is usually assumed to be due to the addition of quantum noise. Here we show that entanglement between the input signal and the amplifying medium can produce an exponentially-large amount of decoherence in an ideal optical amplifier even when the gain is arbitrarily close to unity and the added noise is negligible. These effects occur for macroscopic superposition states, where even a small amount of gain can leave a significant amount of which-path information in the environment. Our results show that the usual input/output relation of a linear amplifier does not provide a complete description of the output state when post-selection is used.
Photonic quantum information processing is a key element for scalable quantum technologies, and has applications in secure long-distance quantum communication and connecting nodes of a quantum computation network. However, logical photon-photon gates and state-of-the-art single photon sources rely on probabilistic processes. Quantum memories are devices that enable storage and on-demand recall of quantum states of light, and have been highlighted as a vital component in photonic networks to overcome the scaling problem by synchronising probabilistic processes. The Raman memory has a large storage bandwidth and high synchronising capacity, and is an ideal candidate for local synchronisation. However, previous demonstrations of the Raman memory suffer from four-wave mixing noise, which prohibits quantum level operation. In this thesis I investigate methods to increase the signal to noise ratio in the Raman memory. I investigate increasing the light-matter coupling strength to boost the memory efficiency, and then explore two different methods to suppress four-wave mixing noise. I demonstrate that operating the Raman memory in a cavity is successful in reducing four-wave mixing, but it is technically challenging to maintain a high memory efficiency. I investigate a new method of noise suppression by introducing an absorption feature at the frequency of the unwanted noise field. This technically simple method is successful in reducing the noise by an order of magnitude, and will be applicable to many quantum memory protocols. In the final section of this thesis I explore the temporal mode properties of the Raman memory. I demonstrate that the Raman memory is single mode and can be used to separate and manipulate temporal modes of light. This positions the Raman memory as a key device for enabling high-dimensional photonic quantum information processing, and enhancing light-matter interactions. These results pave the way towards an efficient, low-noise, mode-selective quantum memory.
Quantum memories, capable of storing single photons or other quantum states of light, to be retrieved on demand, offer a route to large-scale quantum information processing with light. A promising class of memories is based on far-off-resonant Raman absorption in ensembles of $\mathrm{\ensuremath{\Lambda}}$-type atoms. However, at room temperature these systems exhibit unwanted four-wave mixing, which is prohibitive for applications at the single-photon level. Here, we show how this noise can be suppressed by placing the storage medium inside a moderate-finesse optical cavity, thereby removing the main roadblock hindering this approach to quantum memory.
Quantum computing holds the potential to outperform classical computing in certain complicated tasks by leveraging qubits and their manipulation through quantum gates. A compelling platform to physically realize qubits and their control is the Nitrogen-Vacancy (NV) center in diamond, where Rabi oscillation of spin qubit enables effective quantum operations. However, a significant challenge lies in mitigating qubit decoherence caused by external factors. Hence, this paper reports on the experimental demonstration of Rabi oscillation of spin qubit in NV center in diamond, focusing on investigating the impact of magnetic field and noises. Experiments conducted using CIQTEK Diamond Quantum Computer for Education (CIQTEK is a China quantum company) reveal that the magnetic field significantly impacts the resonant frequency which must align with the microwave pulse to observe clear Rabi oscillations. However, despite careful frequency tuning, noises from several sources lead to qubit decoherence, disrupting Rabi oscillations and causing them to decay over time. These results provide insights into key factors affecting qubit performance, paving the way in developing strategies to minimize decoherence.