Average Quantum Research Articles

Nonclassical trajectories in head-on collisions

Rutherford scattering is usually described by treating the projectile either classically or as quantum mechanical plane waves. Here we treat them as wave packets and study their head-on collisions with the stationary target nuclei. We simulate the quantum dynamics of this one-dimensional system and study deviations of the average quantum solution from the classical one. These deviations are traced back to the convexity properties of Coulomb potential. Finally, we sketch how these theoretical findings could be tested in experiments looking for the onset of nuclear reactions.

Rigorous factorization method of the vibrational and rotational contributions to the ro-vibrational partition function

The study reports an accurate and new closed-form representation of the ro-vibrational partition function, using a small amount of spectroscopic constants of the ground electronic state. A rigorous factorization method is proposed for the vibrational and rotational contributions to the partition function using an average vibrational quantum number.

All‐Inorganic Rare‐Earth Halide Double Perovskite Single Crystals with Highly Efficient Photoluminescence

AbstractHalide double perovskites have been regarded as promising alternatives to famous lead halide perovskites in the field of luminescent materials. However, the low‐emission efficiency of pristine ones and the lack of efficient blue emitters remain considerable obstacles for their applications in light‐emitting devices. Here, an air‐stable, all‐inorganic Sc‐based double perovskite Cs2NaScCl6 is reported, which exhibits strong blue emission peaking at 445 nm with an averaged quantum yield of 29.05% (the highest reported value for pristine halide double perovskites). The rare‐earth double perovskite also shows excellent stability against heat and irradiation. Bulk perovskite Cs2NaScCl6 can be an effective host to incorporate Mn2+ ions, yielding interesting tunable dual broadband emission, slightly redshifted blue emission (450 nm) from the host and red emission (635 nm) from Mn2+ d–d transitions. These findings present a new direction toward the design of high‐efficiency, stable, and environmentally friendly perovskite light emitters.

Effect of anti-reflection coating on the performance of silicon solar cells with nanocrystalline quantum dots downshifting film

This experiment explores the effect of lithium fluoride (LiF) antireflection coating (ARC) on the performance of commercial silicon solar cells with mist deposited nanocrystalline quantum dots (NQD) downshifting film. The effect of blanket deposited and patterned NQD and ARC films is investigated. Significant improvement of the performance of cells with ARC is observed. Specifically, 67% increase in the average external quantum efficiency of silicon solar cells covered with AR coating in the UV range of 300–400 nm is noted. The micropatterning of NQD/LiF films improves light trapping inside the cell and enhances power conversion efficiency (PCE) of the cell by 19.5%. Overall, this experiment demonstrates that the ARC formation on top of the mist deposited NQD downshifting film results in measurable improvement in the performance of the commercial silicon solar cells.

Efficient sympathetic cooling in mixed barium and ytterbium ion chains

We study sympathetic cooling of the radial ion motion in a linear RF trap in mixed barium-ytterbium chains. Barium ions are Doppler-cooled, while ytterbium ions are cooled through their interaction with cold barium ions. We estimate the efficiency of sympathetic cooling by measuring the average occupation quantum numbers, and thus the temperature, of all radial normal modes of motion in the ion chain. The full set of orderings in a chain of two barium and two ytterbium ions have been probed, and we show that the average thermal occupation numbers for all chain configurations strongly depend on the trap aspect ratio. We demonstrate efficient sympathetic cooling of all radial normal modes for the trap aspect ratio of approximately 2.9.

Average quantum coherence of pure-state decomposition

We study the average quantum coherence over the pure state decompositions of a mixed quantum state. An upper bound of the average quantum coherence is provided and sufficient conditions for the saturation of the upper bound are shown. These sufficient conditions always hold for two and three dimensional systems. This provides a tool to estimate the average coherence experimentally by measuring only the diagonal elements, which remarkably requires less measurements compared with state tomography. We then describe the pure state decompositions of qubit state in Bloch sphere geometrically. For any given qubit state, the optimal pure state decomposition achieving the maximal average quantum coherence as well as three other pure state decompositions are shown in the Bloch sphere. The order relations among their average quantum coherence are invariant for any coherence measure. The results presented in this paper are universal and suitable for all coherence measures.

Simple quantum key distribution using a stable transmitter-receiver scheme.

Quantum key distribution (QKD) is a technology that allows secure key exchange between two distant users. A widespread adoption of QKD requires the development of simple, low-cost, and stable systems. However, implementation of the current QKD requires a complex self-alignment process during the initial stage and an additional hardware to compensate the environmental disturbances. In this study, we present the implementation of a simple QKD with the help of a stable transmitter-receiver scheme, which simplifies the self-alignment and is robust enough to withstand environmental disturbances. In case of the stability test, the implementation system is able to remain stable for 48 h and exhibits an average quantum bit error rate of less than 1% without any feedback control. The scheme is also tested over a fiber spool, obtaining a stable and secure finite key rate of 7.32k bits per second over a fiber spool extending up to 75 km. The demonstrated long-term stability and obtained secure key rate prove that our method of implementation is a promising alternative for practical QKD systems, in particular, for CubeSat platform and satellite applications.

Quantum cryptography with highly entangled photons from semiconductor quantum dots.

Semiconductor quantum dots are capable of emitting polarization entangled photon pairs with ultralow multipair emission probability even at maximum brightness. Using a quantum dot source with a fidelity as high as 0.987(8), we implement here quantum key distribution with an average quantum bit error rate as low as 1.9% over a time span of 13 hours. For a proof of principle, the key generation is performed with the BBM92 protocol between two buildings, connected by a 350-m-long fiber, resulting in an average raw (secure) key rate of 135 bits/s (86 bits/s) for a pumping rate of 80 MHz, without resorting to time- or frequency-filtering techniques. Our work demonstrates the viability of quantum dots as light sources for entanglement-based quantum key distribution and quantum networks. By increasing the excitation rate and embedding the dots in state-of-the-art photonic structures, key generation rates in the gigabits per second range are in principle at reach.

Heralded Nondestructive Quantum Entangling Gate with Single-Photon Sources

Heralded entangling quantum gates are an essential element for the implementation of large-scale optical quantum computation. Yet, the experimental demonstration of genuine heralded entangling gates with free-flying output photons in linear optical system, was hindered by the intrinsically probabilistic source and double-pair emission in parametric down-conversion. Here, by using an on-demand single-photon source based on a semiconductor quantum dot embedded in a micropillar cavity, we demonstrate a heralded controlled-NOT (CNOT) operation between two single photons for the first time. To characterize the performance of the CNOT gate, we estimate its average quantum gate fidelity of (87.8±1.2)%. As an application, we generated event-ready Bell states with a fidelity of (83.4±2.4)%. Our results are an important step towards the development of photon-photon quantum logic gates.

Excess-noise suppression for a squeezed state propagating through random amplifying media via wave-front shaping

After propagating through a random amplifying medium, a squeezed state commonly shows excess noise above the shot-noise level. Since large noise can significantly reduce the signal-to-noise ratio, it is detrimental for precision measurement. To circumvent this problem, we propose a noise-reduction scheme using wavefront shaping. It is demonstrated that the average output quantum noise can be effectively suppressed even beyond the shot-noise limit. Both the decrease on amplification strength and the increase on input squeezing strength can give rise to a decrease in the suppressed average quantum noise. Our results not only show the feasibility of manipulating the output quantum noise of random amplifying media, but also indicate potential applications in quantum information processing in complex environments, such as, quantum imaging, quantum communication, and quantum key distribution.

Effects of halogen ligands of complexes supported by bis(methylthioether)pyridine on catalytic activities for electrochemical and photochemical driven hydrogen evolution

Reactions of bis(methylthioether)pyridine (btep) with CuX2 (X = Br and Cl) form two new complexes, [Cu(btep)Br2] and [Cu(btep)Cl2], respectively, which have been determined by X‐ray crystallography. Both of them can serve as catalysts for electrochemical and photochemical driven hydrogen evolution. Under an overpotential (OP) of 837.6 mV, [Cu(btep)Br2] or [Cu(btep)Cl2] can electrocatalyze hydrogen evolution from a neutral water with a turnover frequency (TOF) of 373 and 120 mol of hydrogen per mole of catalyst per hour (mol H2/mol catalyst/h), respectively. Under blue light, mixing with CdS nanorods (CdS NRs) as a photosensitizer, and ascorbic acid (H2A) as a sacrificial electron donor, the photolysis of an aqueous solution (pH 4.5) with [Cu(btep)Br2] or [Cu(btep)Cl2] can provide 6180 and 5120 mol of H2 per mole of catalyst (mol of H2 (mol of cat)−1) during 48‐h irradiation with an average apparent quantum yield of 16.7% and 11.0%, respectively. The results show that [Cu(btep)Br2] shows a more efficient activity for H2 generation than [Cu(btep)Cl2]. Several electrochemical and photochemical measurements and analysis are carried out to study catalytic mechanism for H2 production.

Towards efficient near-infrared fluorescent organic light-emitting diodes

The energy gap law (EG-law) and aggregation quenching are the main limitations to overcome in the design of near-infrared (NIR) organic emitters. Here, we achieve unprecedented results by synergistically addressing both of these limitations. First, we propose porphyrin oligomers with increasing length to attenuate the effects of the EG -law by suppressing the non-radiative rate growth, and to increase the radiative rate via enhancement of the oscillator strength. Second, we design side chains to suppress aggregation quenching. We find that the logarithmic rate of variation in the non-radiative rate vs. EG is suppressed by an order of magnitude with respect to previous studies, and we complement this breakthrough by demonstrating organic light-emitting diodes with an average external quantum efficiency of ~1.1%, which is very promising for a heavy-metal-free 850 nm emitter. We also present a novel quantitative model of the internal quantum efficiency for active layers supporting triplet-to-singlet conversion. These results provide a general strategy for designing high-luminance NIR emitters.

Experimental Demonstration of a Local Active Phase Compensation Method for the Phase Encoding Quantum Key Distribution System

Phase drifting is an intrinsic problem in the various phase-encoding quantum key distribution (QKD) system. This letter presents a novel local active phase compensation (LAPC) method, in which the phase drift of the interferometer for each node is tracked and compensated separately. The proof-of-principle demonstration is implemented based on the two common BB84 prepare-and-measure QKD systems, and the experiment result shows that our LAPC setup can run steadily for above 10 hours. A test quantum key distribution based on the Mach–Zehnder interferometer QKD system and the Faraday–Michelson interferometer QKD system with an average quantum bit error rate of 3.8% and 1.9% was achieved, respectively. This study demonstrates the validity of the LAPC method for the BB84 phase-encoding prepare-and-measure QKD system, and then the potential for its use in large-scale QKD networks, while offering a simpler scheme for the practical operation of the MDI-QKD system and the star-type MDI QKD network.

Euler-Milstein and relevant approaches for high-precision stochastic simulation of quantum trajectories

For quantum diffusive measurements, the system’s dynamics can be described by the Itô stochastic master equation, which works well for an infinitesimal time resolution. However, in practical quantum experiments, one cannot make a time step to be infinitesimal, as it can introduce correlation of noise in time, making the Markovian assumption invalid. On the other hand, increasing a time step can cause errors from non-commuting operations describing the system’s dynamics. We therefore consider implementing the Euler-Milstein and relevant approaches, namely the Itô map, the high-order completely positive map, and the quantum Bayesian, to simulate quajntum trajectory for a quantum system under diffusive continuous measurements. In particular, we numerically simulate trajectories for a qubit measurement in z basis. We show the comparison of individual trajectories and their averaged trajectories among these approaches. We find that the high-order completely positive map approach yields the most accurate averaged quantum trajectory. Furthermore, we also investigate the trace distance from true stochastic quantum trajectories, comparing the four approaches using the numerical simulation. We show that, for a realistic time resolution (as in a superconducting qubit, experiment), the high-order map does give the most accurate estimate of the qubit trajectories.

Optical Sideband Cooling of a Radial Motional Mode of a Trapped 138Ba+ Ion

The trapped ion system is one of the most promising and reliable platforms to control and study quantum mechanical phenomena and their applications. For efficient coherent manipulation of both external and internal motional states of an ion, the ion has to be in the ground state of the trapping potential. This can be achieved by resolved sideband cooling, so cooling of the motional mode is a pre-requisite. In a radially symmetric ideal linear trap, the two radial modes are often degenerate, leading to strong coupling between them and hence inefficient cooling by a single laser beam. Here, we show that by breaking the degeneracy of theradial motional modes, sideband cooling can be performed efficiently for a selected radial motional mode. We have achieved an averaged vibrational quantum number = 0.03(8) in a selected radial direction after resolved sideband cooling for a single barium ion.

Facile Synthesis of Defect-Modified Thin-Layered and Porous g-C3N4 with Synergetic Improvement for Photocatalytic H2 Production.

Modulating and optimizing the diverse parameters of photocatalysts synergistically as well as exerting these advantages fully in photocatalytic reactions are crucial for the sufficient utilization of solar energy but still face various challenges. Herein, a novel and facile urea- and KOH-assisted thermal polymerization (UKATP) strategy is first developed for the preparation of defect-modified thin-layered and porous g-C3N4 (DTLP-CN), wherein the thickness of g-C3N4 was dramatically decreased, and cyano groups, nitrogen vacancies, and mesopores were simultaneously introduced into g-C3N4. Importantly, the roles of thickness, pores, and defects can be targetedly modulated and optimized by changing the mass ratio of urea, KOH, and melamine. This can remarkably increase the specific area, improve the light-harvesting capability, and enhance separation efficiency of photoexcited charge carriers, strengthening the mass transfer in g-C3N4. Consequently, the photocatalytic hydrogen evolution efficiency of the DTLP-CN (1.557 mmol h-1 g-1, λ > 420 nm) was significantly improved more than 48.5 times with the highest average apparent quantum yield (AQY) of 18.5% and reached as high as 0.82% at 500 nm. This work provides an effective strategy for synergistically regulating the properties of g-C3N4, and opens a new horizon to design g-C3N4-based catalysts for highly efficient solar-energy conversion.

Stable polarization entanglement based quantum key distribution over a deployed metropolitan fiber

We demonstrate a quantum key distribution (QKD) implementation over deployed dark telecom fibers with polarization-entangled photons generated at the O-band. One of the photons in the pairs is propagated through 10 km of deployed fiber, while the others are detected locally. Polarization drifts experienced by the photons propagating through the fibers are compensated with liquid crystal variable retarders. This ensures continuous and stable QKD operation with an average quantum bit error rate of 6.4% and a final key rate of 109 bits/s.

Determination of the contributions of degenerate and non-degenerate two-photon absorption response in silicon avalanche photodiode for infrared photon detection

The study of non-linear interactions in semiconductor photo-electronic devices to achieve effective and fast photon identification is an ongoing and important task. We investigated the specific contribution of degenerate and non-degenerate two-photon absorption (D-TPA and ND-TPA) response in silicon avalanche photodiode (Si-APD) for infrared photon detection at room temperature. We experimentally demonstrated that when the two laser pulses overlapped, the average D-TPA quantum detection efficiencies at 1800 nm, and that at 1550 nm were measured as 3.3 × 10−16 counts•pulse/photon2, 4.3 × 10−15 counts•pulse/photon2, respectively. And the ND-TPA quantum detection efficiency of 1800 nm and 1550 nm was measured to be 7.9 × 10−16 counts•pulse/photon2. This study provides a solution for the practical infrared photon detection devices based on TPA effect in Si-APDs.

Deep-blue organic light-emitting diodes based on a doublet d\u2013f transition cerium(III) complex with 100% exciton utilization efficiency

Compared to red and green organic light-emitting diodes (OLEDs), blue OLEDs are still the bottleneck due to the lack of efficient emitters with simultaneous high exciton utilization efficiency (EUE) and short excited-state lifetime. Different from the fluorescence, phosphorescence, thermally activated delayed fluorescence (TADF), and organic radical materials traditionally used in OLEDs, we demonstrate herein a new type of emitter, cerium(III) complex Ce-1 with spin-allowed and parity-allowed d–f transition of the centre Ce3+ ion. The compound exhibits a high EUE up to 100% in OLEDs and a short excited-state lifetime of 42 ns, which is considerably faster than that achieved in efficient phosphorescence and TADF emitters. The optimized OLEDs show an average maximum external quantum efficiency (EQE) of 12.4% and Commission Internationale de L’Eclairage (CIE) coordinates of (0.146, 0.078).

Quantum algorithm for the multicollision problem

The current paper presents a new quantum algorithm for finding multicollisions, often denoted by ℓ-collisions, where an ℓ-collision for a function is a set of ℓ distinct inputs that are mapped by the function to the same value. In cryptology, it is important to study how many queries are required to find an ℓ-collision for a random function of which domain is larger than its range. However, the problem of finding ℓ-collisions for random functions has not received much attention in the quantum setting. The tight bound of quantum query complexity for finding a 2-collisions of a random function has been revealed to be Θ(N1/3), where N is the size of the range of the function, but neither the lower nor upper bounds are known for general ℓ-collisions. The paper first integrates the results from existing research to derive several new observations, e.g., ℓ-collisions can be generated only with O(N1/2) quantum queries for any integer constant ℓ. It then provides a quantum algorithm that finds an ℓ-collision for a random function with the average quantum query complexity of O(N(2ℓ−1−1)/(2ℓ−1)), which matches the tight bound of Θ(N1/3) for ℓ=2 and improves upon the known bounds, including the above simple bound of O(N1/2). More generally, the algorithm achieves the average quantum query complexity of O(cN⋅N(2ℓ−1−1)/(2ℓ−1)), and runs over O˜(cN⋅N(2ℓ−1−1)/(2ℓ−1)) qubits in O˜(cN⋅N(2ℓ−1−1)/(2ℓ−1)) expected time for a random function F:X→Y such that |X|≥ℓ⋅|Y|/cN for any 1≤cN∈o(N1/(2ℓ−1)), where it is assumed that QRAM is available. With the same query complexity, it is actually able to find a multiclaw for random functions, which is harder to find than a multicollision.