Spin Control: Squeezing More Out of Quantum Signals

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Author: Denis Avetisyan

Researchers have successfully transferred squeezed microwave signals to an electron spin ensemble, enhancing the potential for stable quantum information storage and transmission.

Microwave signals interacting with a spin-resonator hybrid demonstrate squeezing levels that are tunable through resonance conditions, achieving squeezing below the vacuum level-a phenomenon further enhanced by preliminary spin saturation via resonant microwave pulses, and accurately modeled using coupled parameters representing effective coupling strengths and decay rates as described by $Eq.4$ .
Microwave signals interacting with a spin-resonator hybrid demonstrate squeezing levels that are tunable through resonance conditions, achieving squeezing below the vacuum level-a phenomenon further enhanced by preliminary spin saturation via resonant microwave pulses, and accurately modeled using coupled parameters representing effective coupling strengths and decay rates as described by $Eq.4$ .

A spin-resonator hybrid system achieved 61% quantum transfer efficiency using squeezed microwave signals, improving prospects for quantum memory and key distribution.

Efficiently storing and retrieving quantum information remains a central challenge in realizing scalable quantum technologies. This limitation motivates research into solid-state spin ensembles as promising quantum memory candidates, explored in ‘Probing an electron spin ensemble with squeezed microwave signals’. Here, researchers demonstrate a 61% transfer efficiency of squeezed microwave photons into a spin-resonator hybrid system, achieving a significant step towards realizing practical quantum memories for GHz signals. Could this approach enable robust, long-lived storage essential for future quantum communication and computation networks?

The Fragile Promise of Quantum Communication

Quantum networks represent a paradigm shift in communication and computation, promising unconditionally secure data transmission and computational capabilities exceeding those of classical computers. However, a significant obstacle to realizing these benefits lies in the inherent fragility of quantum signals. Unlike classical bits, quantum bits, or qubits, are susceptible to loss as they travel through optical fibers or air, a phenomenon that drastically limits the distance over which quantum information can be reliably transmitted. This signal attenuation isn’t simply a matter of weakening; each lost quantum particle carries valuable information, making direct amplification-a standard solution in classical communication-impossible. Any attempt to amplify a quantum signal inevitably disturbs its delicate quantum state, destroying the very information it carries. Consequently, the development of technologies capable of overcoming this fundamental limitation – specifically, methods for extending the range of quantum communication without compromising the integrity of the quantum state – is crucial to unlocking the full potential of this emerging field.

The fundamental challenge in establishing quantum networks lies in the fragile nature of quantum information itself. Unlike classical signals which can be amplified to overcome transmission loss, any attempt to amplify a quantum state inevitably destroys the very information it carries, due to the no-cloning theorem. This necessitates a fundamentally different approach: the quantum repeater. Rather than amplifying the signal, a quantum repeater utilizes quantum entanglement and quantum memory to extend the reach of quantum communication. These memories act as intermediate nodes, storing quantum information – qubits – long enough to establish entanglement over longer distances. The process involves dividing the communication channel into segments, creating entanglement between adjacent nodes, and then performing entanglement swapping to connect distant qubits – effectively ‘repeating’ the signal without directly measuring or copying it. The realization of practical, long-distance quantum networks, therefore, hinges on the development of robust and efficient quantum memory capable of preserving delicate quantum states for extended periods, a pursuit that remains at the forefront of quantum engineering.

The development of a functional quantum memory represents a critical bottleneck in the advancement of quantum networks. These memories aren’t simply storage devices; they must preserve the delicate quantum states – the $Qubits$ – long enough to allow for error correction and reliable transmission of information. Achieving this requires physical platforms exhibiting both extended coherence times – the duration a qubit maintains its quantum properties – and strong coupling to microwave signals. Microwave photons serve as ideal carriers of quantum information within these networks, necessitating efficient interfaces between the quantum memory and these signals. Materials under investigation range from trapped ions and superconducting circuits to solid-state defects like nitrogen-vacancy centers in diamond, each facing unique challenges in balancing these crucial parameters. Successfully engineering a platform that maximizes both coherence and coupling will unlock the potential for scalable and long-distance quantum communication.

A spin-resonator system is experimentally set up to transfer squeezed microwave signals, generated by a Josephson parametric amplifier (JPA), to a spin ensemble via a superconducting resonator, allowing for Wigner tomography and investigation of coupling regimes by tuning an external magnetic field.
A spin-resonator system is experimentally set up to transfer squeezed microwave signals, generated by a Josephson parametric amplifier (JPA), to a spin ensemble via a superconducting resonator, allowing for Wigner tomography and investigation of coupling regimes by tuning an external magnetic field.

Harnessing Spin Ensembles: A Solid-State Pathway

Phosphorus donors in isotopically enriched Silicon-28 represent a compelling platform for quantum memory due to their extended coherence times. The use of Silicon-28, devoid of the nuclear spin $I=1/2$ isotope Silicon-29, significantly reduces magnetic field fluctuations that typically limit spin coherence. Phosphorus nuclei possess a nuclear spin $I=1/2$, but the dominant contribution to coherence arises from the electron spin. These electron spins exhibit relatively long $T_2$ coherence times, exceeding 100 $\mu$s in some implementations, making them suitable for storing quantum information. The low density of defects in isotopically purified silicon further minimizes decoherence pathways, enhancing the viability of this solid-state approach to quantum memory.

Achieving coherent control and readout of solid-state spin ensembles necessitates strong coupling to microwave cavities, forming what is termed a Spin-Resonator Hybrid system. This architecture enhances the interaction between the spin ensemble and the applied microwave fields, effectively increasing the signal-to-noise ratio and enabling precise manipulation of the spin states. The cavity concentrates the microwave field, boosting the effective magnetic field experienced by the spins, and provides a means to efficiently read out the spin state through detection of the cavity’s transmitted or reflected signal. The strength of this coupling, quantified by the cooperativity parameter $C = g^2 / \kappa \gamma_T$, directly influences the efficiency of both control and readout operations, where $g$ represents the spin-cavity coupling strength, $\kappa$ is the cavity loss rate, and $\gamma_T$ is the spin relaxation rate.

The Fermi Contact Interaction, a direct consequence of the hyperfine coupling between the electron spin and the nuclear spin, is central to controlling phosphorus donor spins in isotopically enriched silicon. This interaction arises from the probability of finding the electron at the nucleus, resulting in a coupling strength proportional to the electron spin density at the nuclear position. Specifically, the interaction energy is expressed as $H_{hf} = \mathbf{A} \cdot \mathbf{S}$, where $\mathbf{A}$ is the hyperfine tensor and $\mathbf{S}$ is the electron spin operator. By applying microwave fields resonant with the hyperfine interaction, coherent control over the electron spin states can be achieved, enabling manipulation and readout of the quantum information encoded within the spin ensemble. The strength of this interaction, and thus the efficiency of control, is maximized in isotopically enriched 28Si due to the absence of nuclear spins that would otherwise broaden the resonance.

Measured microwave reflection spectra of the spin-resonator hybrid closely match modeled predictions based on a coherent probe-resonator detuning, validating the theoretical framework for both resonant and off-resonant conditions.
Measured microwave reflection spectra of the spin-resonator hybrid closely match modeled predictions based on a coherent probe-resonator detuning, validating the theoretical framework for both resonant and off-resonant conditions.

Squeezed Light: Amplifying the Quantum Signal

Traditional amplification methods introduce noise at a rate that exceeds the signal strength in quantum systems, effectively obscuring the delicate quantum information. This is due to the inherent quantum fluctuations present in any amplifier. Consequently, conventional techniques are unsuitable for amplifying weak quantum signals without compromising their integrity. A solution lies in utilizing squeezed microwave signals, a non-classical state of light where the quantum uncertainty is redistributed; noise in one quadrature is reduced below the standard quantum limit at the expense of increased noise in the orthogonal quadrature. This noise reduction in the signal quadrature enables amplification without introducing excessive noise that would otherwise corrupt the quantum information, making squeezed light amplification a necessity for sensitive quantum measurements.

Josephson Parametric Amplifiers (JPAs) generate squeezed states of microwave photons by exploiting the nonlinear inductance of Josephson junctions. A specific implementation utilizes a flux-driven JPA, where the applied magnetic flux modulates the junction’s inductance, creating parametric gain. This JPA is coupled to a coplanar waveguide resonator to enhance the interaction and achieve significant squeezing. Current devices demonstrate squeezing levels up to 5.3 dB below the vacuum fluctuation limit, meaning the quantum uncertainty is reduced in one quadrature of the electromagnetic field at the expense of increased uncertainty in the other, thereby improving signal-to-noise ratio for quantum measurements. This level of squeezing is quantified by $10 \log_{10}(1 – |\beta|^2)$, where $\beta$ is the squeezing parameter.

Following the generation of squeezed microwave signals, circulators and cryogenic amplifiers play a crucial role in signal fidelity and sensitivity. Circulators direct the squeezed signal towards the detection apparatus while isolating the input from reflected noise and amplified signals, preventing signal degradation and instability. These devices operate at cryogenic temperatures, typically around 4 Kelvin, to minimize thermal noise which would otherwise obscure the weak quantum signal. The amplified, noise-filtered signal is then processed using heterodyne detection, a technique that down-converts the microwave signal to an intermediate frequency for analysis, enabling precise measurement of the quantum state and significantly improving the signal-to-noise ratio.

Experimental characterization of the superconducting JPA demonstrates over 5.29 dB of squeezing at a pump frequency of 11.1 GHz, as evidenced by increased gain, surpassing the vacuum limit and yielding high signal purity.
Experimental characterization of the superconducting JPA demonstrates over 5.29 dB of squeezing at a pump frequency of 11.1 GHz, as evidenced by increased gain, surpassing the vacuum limit and yielding high signal purity.

Characterizing Quantum Memory Performance

Electron Spin Resonance (ESR) and Planck Spectroscopy were employed to characterize and refine the amplification chain used in the quantum memory system. ESR allowed for precise tuning of the microwave frequency and magnetic field to maximize signal detection from the spin ensemble, while minimizing noise contributions. Planck Spectroscopy was utilized to calibrate the thermal noise floor of the amplification chain, enabling accurate determination of the minimum detectable signal and optimizing the signal-to-noise ratio. This calibration process ensured that the amplification chain operated within its linear regime, preventing signal distortion and maintaining the fidelity of the quantum information stored within the spin ensemble.

Coherence time measurements are critical for characterizing quantum memory performance, as they define the duration for which quantum information is reliably stored. Utilizing Hahn Echo and Inversion Recovery methods, the quantum memory demonstrated a $T_2$ dephasing time of 2.16 milliseconds and a $T_1$ relaxation time of 85.49 seconds. The $T_2$ value represents the time after which phase coherence is lost due to interactions with the environment, while $T_1$ indicates the time required for the population of excited states to return to equilibrium. These values directly correlate to the maximum duration quantum information can be maintained within the memory, with longer coherence times enabling more complex quantum operations.

Wigner Tomography was employed to fully characterize the quantum state of the spin ensemble, providing a complete density matrix reconstruction. This technique involves measuring the quantum state along multiple quadrature axes and then performing a mathematical inversion to obtain the density matrix, which describes the probability distribution of the quantum state. Processing of the resulting data was accelerated using a Field-Programmable Gate Array (FPGA), enabling real-time state reconstruction and verification of successful quantum readout and manipulation. The reconstructed density matrix confirmed the fidelity of the prepared and measured spin states, validating the performance of the quantum memory.

Experimentally determined coherence times, T2 (a) via Hahn echo and T1 (b) via inversion recovery, characterize the Si:P ensemble's quantum properties.
Experimentally determined coherence times, T2 (a) via Hahn echo and T1 (b) via inversion recovery, characterize the Si:P ensemble’s quantum properties.

Towards a Quantum Internet: A Future Networked Reality

The foundation for a future quantum internet rests upon the ability to store and retrieve quantum information with high fidelity, a challenge this research directly addresses. Current limitations in transmitting quantum states over long distances necessitate quantum repeaters – devices that rely on quantum memories to extend the range of communication. This work demonstrates a significant advancement in creating such memories, offering a crucial building block for realizing long-distance quantum communication and, ultimately, distributed quantum computing. By enabling the storage of quantum bits, or qubits, for extended periods, this technology paves the way for complex quantum networks where geographically separated quantum computers can collaborate on tasks beyond the reach of even the most powerful classical computers. The implications extend beyond secure communication; it opens doors to advancements in fields like drug discovery, materials science, and fundamental physics, all powered by the interconnectedness of a quantum web.

The development of efficient and reliable quantum memories represents a significant stride toward realizing a truly secure Quantum Key Distribution (QKD) network. QKD relies on the principles of quantum mechanics to guarantee secure communication, but practical implementation demands the ability to store quantum information – qubits – for extended periods without decoherence. These quantum memories act as crucial nodes, enabling the storage of qubits until they can be successfully transmitted and received, overcoming the limitations imposed by signal loss over long distances. By enhancing both the storage time and fidelity of these memories, researchers are addressing a key bottleneck in QKD, paving the way for secure communication networks that are theoretically impervious to eavesdropping. This improved infrastructure will not only safeguard sensitive data but also facilitate the distribution of quantum keys across vast geographical areas, forming the backbone of a future quantum internet.

Future advancements center on transforming these initial quantum memory modules into a robust, scalable architecture capable of supporting a fully functional Quantum Network. This necessitates increasing the number of interconnected quantum bits, or qubits, while simultaneously maintaining their delicate quantum states – a process riddled with technical challenges related to decoherence and signal loss. Researchers are actively exploring novel materials and advanced control techniques to enhance qubit coherence times and improve the fidelity of quantum operations. Ultimately, the goal is to create a network where quantum information can be reliably stored, processed, and transmitted over vast distances, paving the way for secure communication protocols like Quantum Key Distribution and distributed quantum computing capabilities exceeding the limitations of classical systems. The successful integration of these scaled-up quantum memories will be a pivotal step towards realizing the full potential of a global quantum internet.

Numerical calculations demonstrate that optimal quantum state transfer efficiency, achieved at unit cooperativity, is highly sensitive to both external and internal coupling rates, as well as spin dephasing and spin-resonator coupling strengths.
Numerical calculations demonstrate that optimal quantum state transfer efficiency, achieved at unit cooperativity, is highly sensitive to both external and internal coupling rates, as well as spin dephasing and spin-resonator coupling strengths.

The pursuit of enhanced quantum transfer efficiency, as demonstrated by this research into spin-resonator hybrids, echoes a fundamental principle of responsible innovation. Any system striving for greater capacity – in this case, 61% transfer efficiency with squeezed microwave signals – must simultaneously address the ethical implications of that power. As Niels Bohr stated, “The opposite of trivial is not deep, it is dangerous.” A seemingly ‘efficient’ quantum system, divorced from considerations of equitable access or potential misuse, risks becoming a dangerous acceleration without direction. This work, while technically impressive, necessitates a parallel examination of the values embedded within its design and deployment.

Beyond the Squeeze

The demonstrated transfer of squeezed microwave states to a spin-resonator hybrid represents a technical achievement, yet the pursuit of increasingly efficient quantum transfer should not overshadow fundamental questions. Scalability without ethics leads to unpredictable consequences; a 61% transfer efficiency is merely a metric until coupled with robust error correction and, crucially, a clear understanding of the system’s limitations under realistic conditions. The fragility of these non-classical states demands not simply improved resonators, but a proactive consideration of decoherence pathways and their mitigation.

Future work will undoubtedly focus on extending coherence times and increasing cooperativity. However, a more pressing challenge lies in defining the purpose of this enhanced control. Quantum key distribution is an obvious application, but the true potential of such a system rests on its ability to interface with more complex quantum systems. This necessitates a shift in focus from simply doing more, to ensuring that what is done is aligned with demonstrably safe and beneficial outcomes.

Only value control makes a system safe. The creation of increasingly powerful quantum tools requires a concurrent development of ethical frameworks that govern their application. The next stage of this research should therefore prioritize not simply technological advancement, but the responsible integration of these systems into a broader technological landscape.

Original article: https://arxiv.org/pdf/2512.17490.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/

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2025-12-23 03:24