QuSoft

We demonstrate a continuous spectroscopy technique with frequency sensitivity well below the natural transition linewidth, while maintaining a locking range hundreds of times larger. The method exploits the position dependence of a continuous, broadband magneto-optical trap (MOT) operating on the 7.5-kHz-wide intercombination line of strontium. We show that the frequency sensitivity is fundamentally insensitive to the effective MOT laser linewidth. By applying active feedback on the MOT position to a dispersion-optimized frequency comb, which serves as the reference for stabilizing the MOT laser (Boughdachi , arXiv:2507.23617), we achieve a frequency instability below <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mn>4.4</a:mn> <a:mo>×</a:mo> <a:msup> <a:mn>10</a:mn> <a:mrow> <a:mo>−</a:mo> <a:mn>13</a:mn> </a:mrow> </a:msup> </a:math> after 400 s of averaging in both the optical and radio-frequency domains, surpassing the performance of conventional hot-vapor modulation transfer spectroscopy. Our method is a broadly applicable alternative route to frequency references in the high <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:msup> <c:mn>10</c:mn> <c:mrow> <c:mo>−</c:mo> <c:mn>14</c:mn> </c:mrow> </c:msup> </c:math> range around 100 s.

We demonstrate nuclear magnetic resonance of optically trapped ground-state ultracold ^{87}Sr atoms. Using a scheme in which a cloud of ultracold ^{87}Rb is co-trapped nearby, we improve the determination of the nuclear g factor, g_{I}, of atomic ^{87}Sr by more than two orders of magnitude, reaching accuracy at the parts-per-million level. We achieve similar accuracy in the ratio of relevant g factors between Rb and Sr. This establishes ultracold ^{87}Sr as an excellent linear in-vacuum magnetometer. More generally, our Letter demonstrates how ultracold neutral atoms can be used for the precise determination of highly relevant atomic and nuclear magnetic properties. These results are important for ongoing efforts toward quantum simulation, quantum computation and optical atomic clocks employing ^{87}Sr, and other neutral alkaline-earth and alkaline-earth-like atoms.

We show our progress towards experimental demonstration of quantum position verification, a future quantum network application where the geographic location enables remote authentication of a party. We implement a loss-tolerant protocol with photons produced in a semiconductor quantum dot - cavity system that undergo Hong-Ou-Mandel quantum interference at the location to be verified.

We present spectroscopic data for four metastable state clear-out transitions between 399 nm and 412 nm for all even long-lived isotopes of ${\mathrm{Yb}}^{+}$ as well as their hyperfine structure in $^{171}\mathrm{Yb}^{+}$. The strong ${}^{2}{D}_{3/2}\ensuremath{\rightarrow}{}^{1}{[1/2]}_{1/2}$ transition at 412 nm represents an attractive alternative for the standard 935 nm repumper used in cooling and trapping experiments, while the transition to the ${}^{3}{[3/2]}_{3/2}$ state at 411 nm clears out the $^{2}F_{7/2}$ state, for which typically 638 nm or 760 nm is used. These two alternative transitions simplify the experimental setup by removing the need for infrared optics to cool and manipulate ${\mathrm{Yb}}^{+}$ and may be of particular interest when considering integrated photonics solutions. We also present data for the transitions $^{2}D_{3/2}\ensuremath{\rightarrow}{}^{3}{[1/2]}_{3/2}$ at 399 nm and $^{2}D_{3/2}\ensuremath{\rightarrow}{}^{1}{[5/2]}_{5/2}$ at 410 nm including decay branching ratios of the excited states.

Using recent developments on the theory of locally decodable codes, we prove that the critical size for Szemerédi's theorem with random differences is bounded from above by $N^{1-\frac{2}{k} + o(1)}$ for length-$k$ progressions. This improves the previous best bounds of $N^{1-\frac{1}{\lceil k/2 \rceil} + o(1)}$ for all odd $k$.

As quantum network hardware has made significant progress in recent years, there is an increasing focus on enhancing both the infrastructure and the corresponding software of quantum networks. To establish a comprehensive quantum network, two key stacks are employed: the quantum node stack and the quantum control stack. In this survey, we first clearly distinguish between the quantum network and the quantum internet. Then we provide a comprehensive survey of the state-of-the-art and use cases for quantum network stacks. Regarding the quantum control stack, we categorize it into traditional and programmable approaches. We further explore the current research challenges associated with each method to inspire ongoing investigation.

Noisy hardware forms one of the main hurdles to the realization of a near-term quantum internet. Distillation protocols allows one to overcome this noise at the cost of an increased overhead. We consider here an experimentally relevant class of distillation protocols, which distill n to k endto-end entangled pairs using bilocal Clifford operations, a single round of communication and a possible final local operation depending on the observed measurement outcomes. In the case of permutationally invariant depolarizing noise on the input states, we find a correspondence between these distillation protocols and graph codes. We leverage this correspondence to find provably optimal distillation protocols in this class for several tasks important for the quantum internet. This correspondence allows us to investigate use cases for so-called non-trivial measurement syndromes. Furthermore, we detail a recipe to construct the circuit used for the distillation protocol given a graph code. We use this to find circuits of short depth and small number of two-qubit gates. Additionally, we develop a black-box circuit optimization algorithm, and find that both approaches yield comparable circuits. Finally, we investigate the teleportation of encoded states and find protocols which jointly improve the rate and fidelities with respect to prior art.

Transitions out of the ground space limit the performance of quantum adiabatic algorithms, while hardware imperfections impose stringent limitations on the circuit depth. We propose an adiabatic echo verification protocol which mitigates both coherent and incoherent errors, arising from non-adiabatic transitions and hardware noise, respectively. Quasi-adiabatically evolving forward and backward allows for an echo-verified measurement of any observable. In addition to mitigating hardware noise, our method uses positive-time dynamics only. Crucially, the estimator bias of the observable is reduced when compared to standard adiabatic preparation, achieving up to a quadratic improvement.

We show how to find allkmarked elements in a list of sizeNusing the optimal numberO(Nk)of quantum queries and only a polylogarithmic overhead in the gate complexity, in the setting where one has a small quantum memory. Previous algorithms either incurred a factorkoverhead in the gate complexity, or had an extra factorlog&amp;#x2061;(k)in the query complexity.We then consider the problem of finding a multiplicative&amp;#x03B4;-approximation ofs=&amp;#x2211;i=1Nviwherev=(vi)&amp;#x2208;[0,1]N, given quantum query access to a binary description ofv. We give an algorithm that does so, with probability at least1&amp;#x2212;&amp;#x03C1;, usingO(Nlog&amp;#x2061;(1/&amp;#x03C1;)/&amp;#x03B4;)quantum queries (under mild assumptions on&amp;#x03C1;). This quadratically improves the dependence on1/&amp;#x03B4;andlog&amp;#x2061;(1/&amp;#x03C1;)compared to a straightforward application of amplitude estimation. To obtain the improvedlog&amp;#x2061;(1/&amp;#x03C1;)dependence we use the first result.