arXiv subjects

Lijiong Shen

Publications and source records attributed to Lijiong Shen.

12 recordsLinked to original sources

arXiv2026

Superradiant phase transition in cavity magnonics via Floquet engineering

We propose a scheme to engineer the superradiant phase transition (SPT) in cavity magnonics by periodically modulating the frequency of the magnon mode. The studied system is composed of a yttrium iron garnet (YIG) sphere positioned inside a microwave cavity, where magnons in the YIG sphere are strongly coupled to microwave photons. Under the Floquet drive, the effective frequencies of both the cavity and magnon modes can be readily controlled via the frequency and strength of Floquet field. This tunability allows the cavity magnonic system to support a rich steady-state phase diagram, featuring parity-symmetric, parity-symmetry-broken, bistable, and unstable phases. With the increase of Floquet-field strength, the system exhibit a discontinuous phase transition from the parity-symmetric phase to the parity-symmetry-broken phase at a critical threshold, accompanied by an abrupt jump of the magnon occupation from zero to a finite value. Upon further increase of Floquet-field strength, the magnon occupation declines continuously from a nonzero value back to zero, corresponding to a second-order phase transition that restores the parity-symmetric phase. Additionally, fluctuations in magnon number during the SPT process are examined. Our work establishes an alternative route to engineer the cavity-magnon SPT without relying on microwave parametric drive.

quant-ph↗

arXiv2026

Nonreciprocal superradiant quantum phase transition induced by the magnon Kerr effect

Recently, proposals for realizing a nonreciprocal superradiant quantum phase transition (SQPT) have been put forward, based on either nonreciprocal interactions between two spin ensembles or the Sagnac-Fizeau shift in a spinning cavity. However, experimental implementation of such a nonreciprocal SQPT remains challenging. This motivates the search for new mechanisms capable of producing a nonreciprocal SQPT. Here, we propose an alternative approach to realize a nonreciprocal SQPT, induced by the magnon Kerr effect (MKE), in a cavity magnonic system, where magnons in a yttrium iron garnet (YIG) sphere are coupled to cavity photons. The MKE coefficient is positive ($K>0$) when the bias magnetic field is aligned along the crystallographic axis [100], but negative ($K<0$) when aligned along the axis [110]. We show that the steady-state phase diagram for $K > 0$ differs markedly from that for $K < 0$. This contrast is the origin of the nonreciprocal SQPT. By further studying the steady-state magnon occupation and its fluctuations versus the parametric drive strength, we demonstrate that the SQPT becomes nonreciprocal, characterized by distinct critical thresholds for $K > 0$ and $K < 0$. Moreover, we introduce a bidirectional contrast ratio to quantify this nonreciprocal behavior. Our work provides a new mechanism for realizing the nonreciprocal SQPT, with potential applications in designing nonreciprocal quantum devices.

quant-ph↗

arXiv2023

Direct measurement of coherent light proportion from a practical laser source

We present a technique to estimate the proportion of coherent emission in the light emitted by a practical laser source without spectral filtering. The technique is based on measuring interferometric photon correlations between the output ports of an asymmetric Mach-Zehnder interferometer. With this, we characterize the fraction of coherent emission in the light emitted by a laser diode when transiting through the lasing threshold.

quant-ph↗

arXiv2023

Practical Quantum Sensing with Thermal Light

Many quantum sensing suggestions rely on temporal correlations found in photon pairs generated by parametric down-conversion. In this work, we show that the temporal correlations in light with a thermal photon statistics can be equally useful for such applications. Using a sub-threshold laser diode as an ultrabright source of thermal light, we demonstrate optical range finding to a distance of up to 1.8 km.

quant-ph↗

arXiv2022

Countering detector manipulation attacks in quantum communication through detector self-testing

In practical quantum key distribution systems, imperfect physical devices open security loopholes that challenge the core promise of this technology. Apart from various side channels, a vulnerability of single-photon detectors to blinding attacks has been one of the biggest concerns, and has been addressed both by technical means as well as advanced protocols. In this work, we present a countermeasure against such attacks based on self-testing of detectors to confirm their intended operation without relying on specific aspects of their inner working, and to reveal any manipulation attempts. We experimentally demonstrate this countermeasure with a typical InGaAs avalanche photodetector, but the scheme can be easily implemented with any single photon detector.

quant-ph↗

arXiv2022

Distributing Polarization Entangled Photon Pairs with High Rate over Long Distance through Standard Telecommunication Fiber

Entanglement distribution over long distances is essential for many quantum communication schemes like quantum teleportation, some variants of quantum key distribution, or implementations of a quantum internet. Distributing entanglement through standard telecommunication fiber is particularly important for quantum key distribution protocols with low vulnerability over metropolitan distances. However, entanglement distribution over long distance through optical fiber so far could only be accomplished with moderate photon pair rates. In this work, we present entanglement distribution over 50km of standard telecommunication fiber with pair rate more than 10,000 s$^{-1}$ using a bright non-degenerate photon pair source. Signal and idler wavelengths of this source are optimized for low dispersion in optical fiber and high efficiency for single-photon avalanche diode detectors, respectively. The resulting modest hardware requirement and high rate of detected entangled photon pairs could significantly enhance practical entanglement-based quantum key distribution in existing metropolitan fiber networks.

quant-ph↗

arXiv2020

Absolute clock synchronization with a single time-correlated photon pair source over 10km

We demonstrate a point-to-point clock synchronization protocol based on bidirectionally propagating photons generated in a single spontaneous parametric down-conversion (SPDC) source. Tight timing correlations between photon pairs are used to determine the single and round-trip times measured by two separate clocks, providing sufficient information for distance-independent absolute synchronization secure against symmetric delay attacks. We show that the coincidence signature useful for determining the round-trip time of a synchronization channel, established using a 10\,km telecommunications fiber, can be derived from photons reflected off the end face of the fiber without additional optics. Our technique allows the synchronization of multiple clocks with a single reference clock co-located with the source, without requiring additional pair sources, in a client-server configuration suitable for synchronizing a network of clocks.

quant-ph↗

arXiv2020

A wide-range wavelength-tunable photon-pair source for characterizing single-photon detectors

The temporal response of single-photon detectors is usually obtained by measuring their impulse response to short-pulsed laser sources. In this work, we present an alternative approach using time-correlated photon pairs generated in spontaneous parametric down-conversion (SPDC). By measuring the cross-correlation between the detection times recorded with an unknown and a reference photodetector, the temporal response function of the unknown detector can be extracted. Changing the critical phase-matching conditions of the SPDC process provides a wavelength-tunable source of photon pairs. We demonstrate a continuous wavelength-tunability from 526 nm to 661 nm for one photon of the pair, and 1050 nm to 1760 nm for the other photon. The source allows, in principle, to access an even wider wavelength range by simply changing the pump laser of the SPDC-based source. As an initial demonstration, we characterize single photon avalance detectors sensitive to the two distinct wavelength bands, one based on Silicon, the other based on Indim Gallium Arsenide.

quant-ph↗

arXiv2019

Asymmetric delay attack on an entanglement-based bidirectional clock synchronization protocol

We demonstrate an attack on a clock synchronization protocol that attempts to detect tampering of the synchronization channel using polarization-entangled photon pairs. The protocol relies on a symmetrical channel, where propagation delays do not depend on propagation direction, for correctly deducing the offset between clocks -- a condition that could be manipulated with optical circulators, which rely on static magnetic fields to break the reciprocity of propagating electromagnetic fields. Despite the polarization transformation induced within a set of circulators, our attack creates an error in time synchronization while successfully evading detection.

quant-ph↗

arXiv2018

Symmetrical clock synchronization with time-correlated photon pairs

We demonstrate a point-to-point clock synchronization protocol based on bidirectionally exchanging photons produced in spontaneous parametric down conversion (SPDC). The technique exploits tight timing correlations between photon pairs to achieve a precision of 51ps in 100s with count rates of order 200s$^{-1}$. The protocol is distance independent, secure against symmetric delay attacks and provides a natural complement to techniques based on Global Navigation Satellite Systems (GNSS). The protocol works with mobile parties and can be augmented to provide authentication of the timing signal via a Bell inequality check.

quant-ph↗

arXiv2018

Multi-pulse fitting of Transition Edge Sensor signals from a near-infrared continuous-wave source

Transition-edge sensors (TES) are photon-number resolving calorimetric spectrometers with near unit efficiency. Their recovery time, which is on the order of microseconds, limits the number resolving ability and timing accuracy in high photon-flux conditions. This is usually addressed by pulsing the light source or discarding overlapping signals, thereby limiting its applicability. We present an approach to assign detection times to overlapping detection events in the regime of low signal-to-noise ratio, as in the case of TES detection of near-infrared radiation. We use a two-level discriminator, inherently robust against noise, to coarsely locate pulses in time, and timestamp individual photoevents by fitting to a heuristic model. As an example, we measure the second-order time correlation of a coherent source in a single spatial mode using a single TES detector.

physics.ins-det↗

arXiv2018

Randomness extraction from Bell violation with continuous parametric down conversion

We present a violation of the CHSH inequality without the fair sampling assumption with a continuously pumped photon pair source combined with two high efficiency superconducting detectors. Due to the continuous nature of the source, the choice of the duration of each measurement round effectively controls the average number of photon pairs participating in the Bell test. We observe a maximum violation of S= 2.01602(32) with average number of pairs per round of ~0.32, compatible with our system overall detection efficiencies. Systems that violate a Bell inequality are guaranteed to generate private randomness, with the randomness extraction rate depending on the observed violation and on the repetition rate of the Bell test. For our realization, the optimal rate of randomness generation is a compromise between the observed violation and the duration of each measurement round, with the latter realistically limited by the detection time jitter. Using an extractor composably secure against quantum adversary with quantum side information, we calculate an asymptotic rate of ~1300 random bits/s. With an experimental run of 43 minutes, we generated 617,920 random bits, corresponding to ~240 random bits/s.

quant-ph↗