Quantum Sources Research Articles

Classical imaging with undetected photons using four-wave mixing in silicon core fibers

Undetected-photon imaging allows for objects to be imaged in wavelength regions where traditional components are unavailable. Although first demonstrated using quantum sources, recent work has shown that the technique also holds with classical beams. To date, however, all the research in this area has exploited parametric down-conversion processes using bulk nonlinear crystals within free-space systems. Here, we demonstrate undetected-photon-based imaging using light generated via stimulated four-wave mixing within highly nonlinear silicon fiber waveguides. The silicon fibers have been tapered to have a core diameter of ∼ 915 nm to engineer the dispersion and reduce the insertion losses, allowing for tight mode confinement over extended lengths to achieve practical nonlinear conversion efficiencies ( ∼ − 30 dB ) with modest pump powers ( ∼ 48 mW ). Both amplitude and phase images are obtained using classically generated light, confirming the high degree of spatial and phase correlation of our system. The high powers ( > 10 nW ) and long coherence lengths ( > 4 km ) associated with our large fiber-based system result in high contrast and stable images.

Photon pair generation in lithium niobate waveguide periodically poled by femtosecond laser

Reliable generation of single photons is of key importance for fundamental physical experiments and quantum protocols. The periodically poled lithium niobate (LN) waveguide has shown promise for an integrated quantum source due to its large spectral tunability and high efficiency, benefiting from the quasi-phase-matching. Here we demonstrate photon-pair sources based on an LN waveguide periodically poled by a tightly focused femtosecond laser beam. The pair coincidence rate reaches ∼8000 counts per second for average pump power of 3.2 mW (peak power is 2.9 kW). Our results prove the possibility of application of the nonlinear photonics structure fabricated by femtosecond laser to the integrated quantum source. This method can be extended to three-dimensional domain structures, which provide a potential platform for steering the spatial degree of freedom of the entangled two-photon states.

Birefringence compensation utilizing quarter-wave plates in cavity-enhanced spontaneous parametric down-conversion process

Single-photon source is an essential element in quantum information processing, and extensively used in the proof-in-principle demonstration in quantum physics, quantum imaging, quantum cryptography, etc. Considering the operating temperature and system complexity, it is a favorable option to choose spontaneous parametric down-conversion (SPDC) combined with the enhancement effect of a cavity. When generating significant single-photon source via the cavity-enhanced type-II spontaneous parametric down-conversion method, there appears inevitable birefringence effect which obviously influences the resonance condition. In order to compensate for birefringence effect, different approaches have been used such as introducing compensating crystal, placing a half-wave plate, tuning the temperature of the nonlinear crystal, customized conjoined double-cavity structure, and cluster effect. In this work, two quarter-wave plates, with an angle of 45° between the optical axis and the crystal axis, are placed in the cavity to ensure the double resonance of signal photon and idler photon. In the process, the signal photon and idler photon generated simultaneously have different polarizations perpendicular to each other through the type-II nonlinear crystal. Considering horizontally polarized photon, its polarization is changed into left circular polarization by the first quarter-wave plate and then returns as vertical polarization. After traversing a long optical path, it shifts to right circular polarization through the second quarter-wave plate. When the photon passes through the same quarter-wave plate again, the polarization state is originally converted into horizontal polarization state. Then the photon completes a round-trip. The other photon with vertical polarization experiences the same process. As a result, the signal photon and idler photon travel identical optical path. The general explanation is described by the Jones matrices, with the emphasis on the transformation of the polarizations of photons. This method can effectively compensate for birefringence effect, achieving double resonance by using a relatively simple device under the condition of smaller intra-cavity loss and more flexible for adjustment. The signal (idler) photon has a sub-natural linewidth of <inline-formula><tex-math id="Z-20230611154134">\begin{document}$1.01( 1.08 )\;{\rm{MHz}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20230422_Z-20230611154134.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20230422_Z-20230611154134.png"/></alternatives></inline-formula>, demonstrating the feasibility of the proposed technique. This introduced compensating method paves the way to the realization of single-photon quantum source applied to the research of single-photon-single-atom quantum information processing, quantum interface and quantum network node with a single cesium atom confined in the strongly coupled cavity quantum electrodynamics system.

A Tolerance‐Enhanced Spontaneous Parametric Downconversion Source of Bright Entangled Photons

AbstractSpontaneous parametric down‐conversion (SPDC), a primary resource of photonic quantum entangled states, strongly depends on the intrinsic phase matching condition. This makes it susceptible to changes in factors such as the pump wavelength, crystal temperature, and crystal axes orientation. Such intolerance to changing environmental factors prohibits deployment of SPDC‐based sources in non‐ideal environments outside controlled laboratories. Here, a novel system architecture based on a hybrid linear and non‐linear solution that is shown to make the source tolerance‐enhanced without sacrificing brightness. This linear solution is a lens‐axicon pair, judiciously placed, which is tested together with two common non‐linear crystals, quasi‐phase‐matched periodically‐poled KTiOPO4, and birefringent‐phase‐matched BiB3O6. This approach has the benefit of simultaneous tolerance to the environment and high brightness, which is demonstrated by using the proposed architecture as a stable entangled photon source and a spectral brightness as high as 22.58 ± 0.15 kHz mW−1 with a state fidelity of 0.95 , yet requiring a crystal temperature stability of only ±0.8 °C, a 5 × enhanced tolerance as compared to the conventional high brightness SPDC configurations is reported. This solution offers a new approach to deployable high‐brightness quantum sources that are robust to their environment, for instance, in satellite‐based quantum applications.

Measurement-device-independent quantum key distribution with passive time-dependent source side channels

While measurement-device-independent (MDI) quantum key distribution (QKD) allows two trusted parties to establish a shared secret key from a distance without needing to trust a central detection node, their quantum sources must be well characterized, with side channels at the source posing the greatest loophole to the protocol's security. In this paper, we identify a time-dependent side channel in a common polarization-based QKD source that employs a Faraday mirror for phase stabilization. We apply a recently developed numerical proof technique [Phys. Rev. A 99, 062332 (2019)] to quantify the sensitivity of the secret key rate to the quantum optical model for the side channel, and to develop strategies to mitigate the information leakage. In particular, we find that the MDI three-state and BB84 protocols, while yielding the same key rate under ideal conditions, have diverging results in the presence of a side channel, with BB84 proving more advantageous. While we consider only a representative case example, we expect the strategies developed and key rate analysis method to be broadly applicable to other leaky sources.

Quantum remote sensing with atom-light entangled interface

Quantum remote sensing utilizes quantum entanglement between the probe and the receiver to enhance the capability to sense a remote target. Quantum illumination is considered as a promising protocol to realize such a quantum technology in an environment of high loss and intense noise. However, the protocol requires an additional on-demand quantum memory, the imperfect performance of which diminishes the quantum advantage and limits the enhancement of sensing. In this paper, we propose a new protocol for quantum remote sensing based on quantum illumination with atom-light entangled interface. Compared to conventional light-only quantum illumination, the proposed protocol utilizes Raman coupling to create a long-lived atomic spin wave entangled with a Stokes light. The atomic spin wave, automatically built-in memory via the Raman coupling, acts as a local reference. The entangled Stokes light is used as a probe to irradiate a remote target. Meanwhile, the returned probe light from target is detected through coupling again to the atomic spin wave. A joint measurement on the returned probe light and spin wave is performed to discriminate the target. A 4 dB quantum enhancement over classical illumination is estimated. The atom-light entangled interface naturally integrates the quantum source, quantum memory, and quantum receiver in a single unit which exhibits great potential to develop highly compact and portable devices for quantum-enhanced remote sensing.

Two-Photon Interference of Single Photons from Dissimilar Sources

Entanglement swapping and heralding are at the heart of many protocols for distributed quantum information. For photons, this typically involves Bell-state measurements based on two-photon interference effects. In this context, hybrid systems that combine high rate, ultrastable, and pure quantum sources with long-lived quantum memories are particularly interesting. Here, we develop a theoretical description of pulsed two-photon interference of photons from dissimilar sources to predict the outcomes of second-order cross-correlation measurements. These are directly related to, and hence used to quantify, photon indistinguishability. We study their dependence on critical system parameters such as quantum state lifetime and emission frequency, and quantify the impact of time jitter, pure dephasing, and spectral wandering. We show that for a fixed lifetime of one of the two emitters, for each frequency detuning there is an optimal lifetime of the second emitter that leads to the highest photon indistinguishability. Expectations for different hybrid combinations involving III-V semiconductor quantum dots, color centers in diamond, atom-scale defects in two-dimensional materials and neutral atoms are quantitatively compared for real-world system parameters. Our work provides a theoretical basis for the treatment of dissimilar emitters and enables assessment of which imperfections can be tolerated in hybrid photonic quantum networks.

Quantum batteries in non-Markovian reservoirs.

In this Letter, we propose schemes to improve the performance of quantum batteries and provide a new, to the best of our knowledge, quantum source for a quantum battery without an external driving field. We show that the memory effect of the non-Markovian reservoir can play a significant role in improving the performance of quantum batteries, which originates from a backflow on the ergotropy in the non-Markovian regime, while there is no counterpart in Markovian approximation. We find that the peak for the maximum average storing power in the non-Markovian regime can be enhanced by manipulating the coupling strength between the charger and the battery. Finally, we find that the battery can also be charged by non-rotating wave terms without driving fields.

Direct Observation of Dynamically Localized Quantum Optical States.

Quantum-correlated biphoton states play an important role in quantum communication and processing, especially considering the recent advances in integrated photonics. However, it remains a challenge to flexibly transport quantum states on a chip, when dealing with large-scale sophisticated photonic designs. The equivalence between certain aspects of quantum optics and solid-state physics makes it possible to utilize a range of powerful approaches in photonics, including topologically protected boundary states, graphene edge states, and dynamic localization. Optical dynamic localization allows efficient protection of classical signals in photonic systems by implementing an analogue of an external alternating electric field. Here, we report on the observation of dynamic localization for quantum-correlated biphotons, including both the generation and the propagation aspects. As a platform, we use sinusoidal waveguide arrays with cubic nonlinearity. We record biphoton coincidence count rates as evidence of robust generation of biphotons and demonstrate the dynamic localization features in both spatial and temporal space by analyzing the quantum correlation of biphotons at the output of the waveguide array. Experimental results demonstrate that various dynamic modulation parameters are effective in protecting quantum states without introducing complex topologies. Our Letter opens new avenues for studying complex physical processes using photonic chips and provides an alternative mechanism of protecting communication channels and nonclassical quantum sources in large-scale integrated quantum optics.

Influence of multiphoton events on the quantum enhanced phase estimation.

Quantum metrology can approach measurement precision of Heisenberg Limit using an ideal quantum source, which has attracted a great interest in fundamental physical studies. However, the quantum metrology precision is impressionable to the system noise in experiments. In this paper, we analyze the influence of multiphoton events on the phase estimation precision when using a nondeterministic single photon source. Our results show there are an extra bias and quantum enhanced region restriction due to multiphoton events, which declines the quantum phase estimation precision. A limitation of multiphoton probability is obtained for quantum enhanced phase estimation accuracy under different experimental model. Our results provide beneficial suggestions for improving quantum metrology precision in future experiments.

Room‐Temperature Generation of Heralded Single Photons on Silicon Chip with Switchable Orbital Angular Momentum

AbstractIn quantum optics, orbital angular momentum (OAM) is very promising to achieve high‐dimensional quantum states due to the nature of infinite and discrete eigenvalues, which is quantized by the topological charge of l. Here, a heralded single‐photon source with switchable OAM modes is proposed and demonstrated on silicon chip. At room temperature, the heralded single photons with 11 OAM modes (l = 2–6, −6 to −1) are successfully generated and switched through thermo‐optical effect. It is believed that such an integrated quantum source with multiple OAM modes and operating at room temperature can provide a practical platform for high‐dimensional quantum information processing. Moreover, this proposed architecture can also be extended to other material systems to further improve the performance of OAM quantum source.

Optimal design and performance evaluation of free-space quantum key distribution systems

Free-space ground-to-ground links will be an integral part of future quantum communication networks. The implementation of free-space and fiber links in daylight inter-modal configurations is however still hard to achieve, due to the impact of atmospheric turbulence, which strongly decreases the coupling efficiency into the fiber. In this work, we present a comprehensive model of the performance of a free-space ground-to-ground quantum key distribution (QKD) system based on the efficient-BB84 protocol with active decoy states. Our model takes into account the atmospheric channel contribution, the transmitter and receiver telescope design constraints, the parameters of the quantum source and detectors, and the finite-key analysis to produce a set of requirements and optimal design choices for a QKD system operating under specific channel conditions. The channel attenuation is calculated considering all effects deriving from the atmospheric propagation (absorption, beam broadening, beam wandering, scintillation, and wavefront distortions), as well as the effect of fiber-coupling in the presence of a partial adaptive optics correction with finite control bandwidth. We find that the channel fluctuation statistics must be considered to correctly estimate the effect of the saturation rate of the single-photon detectors, which may otherwise lead to an overestimation of the secret key rate. We further present strategies to minimize the impact of diffuse atmospheric background in daylight operation by means of spectral and temporal filtering.

Free-Fermionic Topological Quantum Sensors.

Second order quantum phase transitions, with well-known features such as long-range entanglement, symmetry breaking, and gap closing, exhibit quantum enhancement for sensing at criticality. However, it is unclear which of these features are responsible for this enhancement. To address this issue, we investigate phase transitions in free-fermionic topological systems that exhibit neither symmetry-breaking nor long-range entanglement. We analytically demonstrate that quantum enhanced sensing is possible using topological edge states near the phase boundary. Remarkably, such enhancement also endures for ground states of such models that are accessible in solid state experiments. We illustrate the results with 1D Su-Schrieffer-Heeger chain and a 2D Chern insulator which are both experimentally accessible. While neither symmetry-breaking nor long-range entanglement are essential, gap closing remains as the major candidate for the ultimate source of quantum enhanced sensing. In addition, we also provide a fixed and simple measurement strategy that achieves near-optimal precision for sensing using generic edge states irrespective of the parameter value. This paves the way for development of topological quantum sensors which are expected to also be robust against local perturbations.

Design and simulation of a binary full adder based on quantum rings

In this paper, a binary full adder circuit is designed with eight quantum rings where each ring is threaded by a magnetic flux which is an odd multiple of half of the magnetic flux quantum. The quantum rings are divided into two categories with four rings connected to each other in parallel. They are attached symmetrically to two semi-infinite one-dimensional metallic electrodes , and three gate voltages are applied as three inputs of the full adder. The Hamiltonian of the system is approximated by the tight-binding method, and the calculations are performed by using Green's function formalism. The current-voltage characteristics are obtained for different values of applied gate voltages in the strong and weak-coupling strengths between the quantum rings and source and drain electrodes. The results are in accordance with the truth table of a binary full adder and show this quantum circuit behaves as a binary full adder. • This work proposes a binary full adder consisting of eight quantum rings where a constant magnetic flux is passing through each of them. • The designed full adder with eight quantum rings is divided into two separate parts (Sum and Carry-out) which are connected symmetrically to the source and drain electrodes. • The values of the applied gate voltages are considered as the full adder inputs and the electric current is the output of the system. • The output current of the proposed circuit based on the system input voltages is in accordance with the truth table of a binary full adder.

Bright single-photon sources in the telecom band by deterministically coupling single quantum dots to a hybrid circular Bragg resonator

High-performance solid-state quantum sources in the telecom band are of paramount importance for long-distance quantum communications and the quantum Internet by taking advantage of a low-loss optical fiber network. Here, we demonstrate bright telecom-wavelength single-photon sources based on In(Ga)As/GaAs quantum dots (QDs) deterministically coupled to hybrid circular Bragg resonators (h-CBRs) by using a wide-field fluorescence imaging technique. The QD emissions are redshifted toward the telecom O-band by using an ultra-low InAs growth rate and an InGaAs strain reducing layer. Single-photon emissions under both continuous wave (CW) and pulsed operations are demonstrated, showing high brightness with count rates of 1.14 MHz and 0.34 MHz under saturation powers and single-photon purities of g ( 2 ) ( 0 ) = 0.11 ± 0.02 (CW) and g ( 2 ) ( 0 ) = 0.087 ± 0.003 (pulsed) at low excitation powers. A Purcell factor of 4.2 with a collection efficiency of 11.2 % ± 1 % at the first lens is extracted, suggesting efficient coupling between the QD and h-CBR. Our work contributes to the development of highly efficient single-photon sources in the telecom band for fiber-based quantum communication and future distributed quantum networks.

Boosting Quantum Battery-Based IoT Gadgets via RF-Enabled Energy Harvesting.

The search for a highly portable and efficient supply of energy to run small-scale wireless gadgets has captivated the human race for the past few years. As a part of this quest, the idea of realizing a Quantum battery (QB) seems promising. Like any other practically tractable system, the design of QBs also involve several critical challenges. The main problem in this context is to ensure a lossless environment pertaining to the closed-system design of the QB, which is extremely difficult to realize in practice. Herein, we model and optimize various aspects of a Radio-Frequency (RF) Energy Harvesting (EH)-assisted, QB-enabled Internet-of-Things (IoT) system. Several RF-EH modules (in the form of micro- or nano-meter-sized integrated circuits (ICs)) are placed in parallel at the IoT receiver device, and the overall correspondingly harvested energy helps the involved Quantum sources achieve the so-called quasi-stable state. Concretely, the Quantum sources absorb the energy of photons that are emitted by a photon-emitting device controlled by a micro-controller, which also manages the overall harvested energy from the RF-EH ICs. To investigate the considered framework, we first minimize the total transmit power under the constraints on overall harvested energy and the number of RF-EH ICs at the QB-enabled wireless IoT device. Next, we optimize the number of RF-EH ICs, subject to the constraints on total transmit power and overall harvested energy. Correspondingly, we obtain suitable analytical solutions to the above-mentioned problems, respectively, and also cross-validate them using a non-linear program solver. The effectiveness of the proposed technique is reported in the form of numerical results, which are both theoretical and simulations based, by taking a range of operating system parameters into account.

Raman gain control in optical fibers with orbital-angular-momentum-induced chirality of light.

Stimulated Raman scattering is a particularly robust nonlinearity, occurring in virtually every material because its spectral linewidth and associated frequency shift do not typically depend on phases or directions (i.e. wavevectors) of the interacting light beams. In amorphous materials such as glass fibers, Raman bandwidths are large, enabling its use as a broadband gain element. This ubiquity makes it a versatile means for achieving optical amplification or realizing lasers over a large range of pulsewidths at user-defined colors. However, this ease of deploying the effect also presents itself as a stubborn source of noise in fiber-based quantum sources or parasitic emission in fiber lasers. Here, we show that orbital angular momentum carrying light beams experiencing spin-orbit interactions yield novel phase-matching criteria for Raman scattering. This enables tailoring its spectral shape (by over half the Raman shift in a given material) as well as strength (by ∼ 100×) simply by controlling light's topological charge - a capability of utility across the multitude of applications where modulating Raman scattering is desired.

(Invited) Cavity Coupled Multi-Emitters in Carbon Nanotubes

The development of covalent diazonium functionalization of carbon nanotubes brings unique oportunities in terms of original quantum sources of light. In particular, it should be possible to create a linear ensemble of closely packed single-photon sources along the carbon nanotube backbone. The physical proximity of several dopants would make it possible to couple them simultaneous to a resonant cavity mode and to induce original dynamics and light emission properties. Here, we investigate experimentally the case of a series of four quantum emitters attached to the same carbon nanotube coupled to a high finesse fiber cavity. We explore their properties through PLE, polarization and superlocalization measurements and their light emission properties using time-correlation techniques, photo-luminescence decay measurements.

Coupling Perovskite Quantum Dot Pairs in Solution using a Nanoplasmonic Assembly.

Perovskite quantum dots (PQDs) provide a robust solution-based approach to efficient solar cells, bright light emitting devices, and quantum sources of light. Quantifying heterogeneity and understanding coupling between dots is critical for these applications. We use double-nanohole optical trapping to size individual dots and correlate to emission energy shifts from quantum confinement. We were able to assemble a second dot in the trap, which allows us to observe the coupling between dots. We observe a systematic red-shift of 1.1 ± 0.6 meV in the emission wavelength. Theoretical analysis shows that the observed shift is consistent with resonant energy transfer and is unusually large due to moderate-to-large quantum confinement in PQDs. This demonstrates the promise of PQDs for entanglement in quantum information applications. This work enables future in situ control of PQD growth as well as studies of the coupling between small PQD assemblies with quantum information applications in mind.

Photon-pair generation from a chip-scale Cs atomic vapor cell.

The realization of a narrowband photonic quantum source based on an atomic device is considered essential in the practical development of photonic quantum information science and technology. In this study, we present the first step toward the development of a photon-pair source based on a microfabricated Cs atomic vapor cell. Time-correlated photon pairs from the millimeter-scale Cs vapor cell are emitted via the spontaneous four-wave mixing process of the cascade-type 6S1/2-6P3/2-8S1/2 transition of 133Cs. The maximum normalized cross-correlation value between the signal and idler photons is measured as 622(8) under a weak pump power of 10 µ;W. Our photon source violates the Cauchy-Schwartz inequality by a factor of >105. We believe that our approach has very important applications in the context of realizing practical scalable quantum networks based on atom-photon interactions.