Optical isolators are crucial components in optical communication and quantum information processing. Traditional isolators rely on the Faraday magneto-optic effect, requiring strong magnetic fields, which limits their miniaturization and use in quantum applications. Magnetic-free alternatives have been proposed, but achieving noiseless operation in the quantum regime, essential for preserving the quantum characteristics of single photons, remains a challenge. Existing magnetic-free isolators often suffer from drive-induced noise, compromising the quantum properties of single photons. This research aims to overcome this limitation by demonstrating a noiseless, single-photon isolator using an incoherent optical pumping mechanism in hot atoms. The use of hot atoms, unlike cold atoms, offers advantages in terms of experimental simplicity and scalability. The incoherent nature of the pumping process minimizes noise, preserving the quantum properties of the single photons while achieving high isolation and low insertion loss. This approach offers significant advantages for quantum communication and information processing applications due to its scalability and compatibility with existing optical technologies.

Numerous theoretical and experimental efforts have explored magnetic-free optical nonreciprocity (ONR) using various mechanisms, including nonlinear effects, spatiotemporal modulation of permittivity, opto-mechanical interactions, moving Bragg lattices, chiral quantum systems, and random thermal motion of atoms. Most reported schemes operate in the classical regime with weak coherent light. However, quantum applications require isolators that function with genuine single photons while preserving their quantum properties. Previous experimental demonstrations of ONR with single photons have been limited by strong drive fields needed to overcome dephasing, introducing significant background noise. While some theoretical schemes have addressed the issue of single-photon ONR, experimental realizations of noiseless ONR with single photons have remained elusive due to difficulties in filtering noise from weak single-photon signals. Recent work has proposed a noiseless isolator using incoherent population transfer, reducing drive-induced noise but only demonstrated with weak coherent and pseudothermal light sources, not genuine single photons. This paper addresses the gap by experimentally demonstrating a noiseless isolator with genuine single photons.

The experiment uses a two-stage process. First, heralded single photons are generated via spontaneous four-wave mixing in a paraffin-coated hot ⁸⁷Rb vapor cell. Two counter-propagating coupling lasers create correlated photon pairs (Stokes and Anti-Stokes), with the Stokes photons used for heralding. The heralded Anti-Stokes photons are then directed into a second ⁸⁷Rb vapor cell to demonstrate the ONR. This cell employs an open V-type atomic system with a pump field that couples the atomic transitions. The pump field is red-detuned from the transition |5²S₁/₂, F=1⟩ → |5²P₃/₂, F=2⟩ by Δp = 2π × 157 MHz, creating an asymmetric atomic population in the ground state via velocity-selective incoherent population transfer. This asymmetry leads to nonreciprocal absorption, allowing signal photons to pass through with negligible loss in one direction, while strongly absorbing them in the opposite direction. The single-photon transmission is characterized by measuring coincidence counts. The quantum characteristics of the transmitted photons are assessed using the second-order cross-correlation function g⁽²⁾(τ) and the heralded autocorrelation parameter α, ensuring the preservation of the single-photon nature. The classical noise is measured by blocking the signal photons while keeping the pump field on. The Cauchy-Schwarz inequality R is also evaluated to assess nonclassical correlations. The reversibility of the nonreciprocity is demonstrated by tuning the frequency of the pump field while keeping the single-photon frequency constant. The experimental setup consists of a single-photon generation system (using spontaneous four-wave mixing), an ONR system (using the second Rb cell with a pump laser), and single-photon detectors. Careful filtering and polarization control minimize background noise, enabling the observation of the noiseless single-photon nonreciprocity.

The experiment successfully demonstrates a noiseless single-photon isolator with an ultralow insertion loss of 0.6 dB and high isolation of 30.3 dB. The bandwidth of the isolator reaches hundreds of megahertz. The nonreciprocal direction is reversibly controlled by simply adjusting the pump laser frequency detuning. Crucially, the quantum properties of the input single photons are preserved in the forward direction, as confirmed by measurements of g⁽²⁾(τ), α, and R, showing negligible additional quantum noise is introduced by the isolator. The measured g⁽²⁾(τ) of the forward signal photons remains close to the input value, demonstrating the preservation of the single-photon nature. The heralded autocorrelation parameter α remains close to 0.09 in the forward direction, indicating near-single-photon character preservation. The Cauchy-Schwarz inequality R value of 129 in the forward direction compared to 0.27 in the backward direction, confirms the preservation and destruction of nonclassical correlations respectively. The low background noise, less than 0.1%, validates the system's noiseless operation.

The results directly address the challenge of creating a noiseless single-photon isolator. The observed low insertion loss and high isolation, coupled with the preservation of the quantum characteristics of the single photons, signify a significant advancement. The reversibility of the nonreciprocity demonstrated here offers flexibility for reconfigurable optical systems. This scheme's simplicity and reliance on widely used optical technologies make it robust and potentially scalable for integration into quantum information processing devices. The use of hot atoms, unlike cold atoms, avoids the need for complex cooling systems, making the implementation more practical and less resource-intensive. The successful preservation of single-photon characteristics demonstrates the suitability of this technique for quantum applications, where maintaining the quantum state is crucial. This advancement could impact various fields, including quantum computing, quantum communication, and quantum sensing.

This work experimentally demonstrates a magnetic-free, noiseless, and reconfigurable single-photon isolator operating at room temperature. The use of incoherent optical pumping in hot atoms offers a simple, robust, and scalable approach to achieving high isolation and low insertion loss without introducing additional quantum noise. The ability to reverse the nonreciprocal direction by simply tuning the pump laser frequency makes this isolator highly versatile. This achievement holds significant potential for applications in quantum networks and integrated quantum information processing.

The current experimental setup might benefit from further optimization to increase the bandwidth and improve the isolation ratio. While the noise levels are low, exploring methods to further reduce them could enhance the device’s performance. The specific choice of rubidium atoms and the energy level transitions employed in this study might limit its immediate applicability to other systems. The extent of scalability to large-scale quantum networks needs further investigation. Future research could also focus on integrating this isolator onto a chip platform for practical applications in quantum information technologies.