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<jats:title>Abstract</jats:title> <jats:p>We designed and implemented a novel combination of a Sagnac-interferometer with a Mach–Zehnder interferometer for a source of polarization-entangled photons. The new versatile configuration does not require multi-wavelength polarization optics, yet it performs with a good polarization quality and phase-stability over a wide wavelength range. We demonstrate the interferometer using only standard commercial optics to experimentally realize the pulsed generation of polarization-entangled photon-pairs at wavelengths of 764 nm and 1221 nm via type-I spontaneous four-wave mixing in a polarization-maintaining fiber. Polarization entanglement was verified by a polarization-correlation measurement with a visibility of 95.5% from raw coincidence counts and the violation of the Clauser–Horne–Shimony–Holt (CHSH) inequality with <jats:italic>S</jats:italic> = 2.70 ± 0.04. The long-term phase-stability was characterized by an Allan deviation of 8° over an integration time of about 1 h with no active phase-stabilization.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We study the dynamics of correlations in a paradigmatic setup to observe <jats:inline-formula> <jats:tex-math><?CDATA $\mathcal{PT}$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi mathvariant="script">PT</mml:mi> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="qstabcfccieqn2.gif" xlink:type="simple" /> </jats:inline-formula>-symmetric physics: a pair of coupled oscillators, one subject to a gain one to a loss. Starting from a coherent state, quantum correlations (QCs) are created, despite the system being driven only incoherently, and can survive indefinitely. Both total and QCs exhibit different scalings of their long-time behavior in the <jats:inline-formula> <jats:tex-math><?CDATA $\mathcal{PT}$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi mathvariant="script">PT</mml:mi> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="qstabcfccieqn3.gif" xlink:type="simple" /> </jats:inline-formula>-broken/unbroken phase and at the exceptional point (EP). In particular, <jats:inline-formula> <jats:tex-math><?CDATA $\mathcal{PT}$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi mathvariant="script">PT</mml:mi> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="qstabcfccieqn4.gif" xlink:type="simple" /> </jats:inline-formula> symmetry breaking is accompanied by non-zero stationary QCs. This is analytically shown and quantitatively explained in terms of entropy balance. The EP in particular stands out as the most classical configuration, as classical correlations diverge while QCs vanish.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In traditional quantum metrology protocols, the initial multipartite entangled pure quantum probes are considered to be isolated, i.e., free of quantum many-body effects. Here, we study the impact of inherent many-body effects such as interaction with noisy environment and nonlocal interactions among particles on metrologically resourceful multipartite entanglement of initially mixed quantum probes. In this regard, we employ an information-theoretic multipartite entanglement measure as a figure-of-merit. The inevitable interaction with the noisy environment leads to disentanglement in multipartite quantum probes which restricts its metrological advantage. For this, we use entanglement dissociation to derive bounds on the multipartite entanglement measure that can identify the relevant entanglement structure under global as well as local noisy evolution. Furthermore, we investigate nonlocal interactions in terms of their entangling capability in a multipartite quantum probe. We show that such nonlocal interactions can be exploited as a valuable resource that exhibits better precision scaling in mixed-state quantum metrology. Moreover, we numerically observe these results for GHZ-W class states.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Scaling of variational quantum algorithms to large problem sizes requires efficient optimization of random parameterized quantum circuits. For such circuits with uncorrelated parameters, the presence of exponentially vanishing gradients in cost function landscapes is an obstacle to optimization by gradient descent methods. In this work, we prove that reducing the dimensionality of the parameter space by utilizing circuit modules containing spatially or temporally correlated gate layers can allow one to circumvent the vanishing gradient phenomenon. Examples are drawn from random separable circuits and asymptotically optimal variational versions of Grover’s algorithm based on the quantum alternating operator ansatz. In the latter scenario, our bounds on cost function variation imply a transition between vanishing gradients and efficient trainability as the number of layers is increased toward <jats:inline-formula> <jats:tex-math><?CDATA $\mathcal{O}\left({2}^{n/2}\right)$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi mathvariant="script">O</mml:mi> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mrow> <mml:msup> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mi>n</mml:mi> <mml:mo>/</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="qstabd891ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, the optimal oracle complexity of quantum unstructured search.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Measurement errors limit the performance of near-term quantum computers and their potential for practical application. However they are partly correctable after a calibration step that requires, for a complete implementation on a register of <jats:italic>n</jats:italic> qubits, 2<jats:sup> <jats:italic>n</jats:italic> </jats:sup> additional measurements. Here we introduce an approximate but efficient method for multiqubit measurement error characterization and mitigation requiring the classical processing of 2<jats:sup> <jats:italic>n</jats:italic> </jats:sup> × 2<jats:sup> <jats:italic>n</jats:italic> </jats:sup> matrices, but only <jats:italic>O</jats:italic>(4<jats:sup> <jats:italic>k</jats:italic> </jats:sup> <jats:italic>n</jats:italic> <jats:sup>2</jats:sup>) measurements, where <jats:italic>k</jats:italic> = <jats:italic>O</jats:italic>(1) is the number of qubits in a correlation volume. We demonstrate and validate the method on an IBM Q processor with registers of 4 and 8 superconducting qubits.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present an exact analytical solution of the anisotropic Hopfield model, and we use it to investigate in detail the spectral and thermometric response of two ultrastrongly coupled quantum systems. Interestingly, we show that depending on the initial state of the coupled system, the vacuum Rabi splitting manifests significant asymmetries that may be considered spectral signatures of the counterintuitive decoupling effect. Using the coupled system as a thermometer for quantum thermodynamics applications, we obtain the ultimate bounds on the estimation of temperature that remain valid in the ultrastrong coupling regime. Remarkably, if the system performs a quantum phase transition, the quantum Fisher information exhibits periodic divergences, suggesting that one can have several points of arbitrarily high thermometric precision for such a critical quantum sensor.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Machine learning techniques enhanced by noisy intermediate-scale quantum (NISQ) devices and especially variational quantum circuits (VQC) have recently attracted much interest and have already been benchmarked for certain problems. Inspired by classical deep learning, VQCs are trained by gradient descent methods which allow for efficient training over big parameter spaces. For NISQ sized circuits, such methods show good convergence. There are however still many open questions related to the convergence of the loss function and to the trainability of these circuits in situations of vanishing gradients. Furthermore, it is not clear how ‘good’ the minima are in terms of generalization and stability against perturbations of the data and there is, therefore, a need for tools to quantitatively study the convergence of the VQCs. In this work, we introduce a way to compute the Hessian of the loss function of VQCs and show how to characterize the loss landscape with it. The eigenvalues of the Hessian give information on the local curvature and we discuss how this information can be interpreted and compared to classical neural networks. We benchmark our results on several examples, starting with a simple analytic toy model to provide some intuition about the behaviour of the Hessian, then going to bigger circuits, and also train VQCs on data. Finally, we show how the Hessian can be used to adjust the learning rate for faster convergence during the training of variational circuits.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Source monitoring is a matter of vital importance for continuous-variable quantum key distribution (QKD), since it is the first barrier to ensure the practical security. In a practical scenario, the difficulty arises with the existence of the laser intensity fluctuation, which not only contributes to the source noise but also causes the deviation of source monitoring result. We first illustrate the effect of the relative intensity noise (RIN) on the ultimate security by explicitly showing the parameter estimation procedure, and find that the secret key rate is overestimated in this situation. To defend the continuous-variable QKD system against potential attacks utilizing this impairment to conceal itself, a source monitoring scheme is devised by adopting the seminal one-time shot-noise unit calibration approach into the monitor module. It is shown that by using the practical source monitoring scheme the impact of the RIN can be modeled and monitored in real time to circumvent an overestimated secret key rate. This scheme also lowers the system complexity thus is suitable for widespread application.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Quantum annealing is a powerful alternative model of quantum computing, which can succeed in the presence of environmental noise even without error correction. However, despite great effort, no conclusive demonstration of a quantum speedup (relative to state of the art classical algorithms) has been shown for these systems, and rigorous theoretical proofs of a quantum advantage (such as the adiabatic formulation of Grover’s search problem) generally rely on exponential precision in at least some aspects of the system, an unphysical resource guaranteed to be scrambled by experimental uncertainties and random noise. In this work, we propose a new variant of quantum annealing, called RFQA, which can maintain a scalable quantum speedup in the face of noise and modest control precision. Specifically, we consider a modification of flux qubit-based quantum annealing which includes low-frequency oscillations in the directions of the transverse field terms as the system evolves. We show that this method produces a quantum speedup for finding ground states in the Grover problem and quantum random energy model, and thus should be widely applicable to other hard optimization problems which can be formulated as quantum spin glasses. Further, we explore three realistic noise channels and show that the speedup from RFQA is resilient to 1/<jats:italic>f</jats:italic>-like local potential fluctuations and local heating from interaction with a sufficiently low temperature bath. Another noise channel, bath-assisted quantum cooling transitions, actually accelerates the algorithm and may outweigh the negative effects of the others. We also detail how RFQA may be implemented experimentally with current technology.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Quantum computing for real world applications requires large arrays of qubits. This requires advanced integration technologies. In this work, we propose a simple integration method with fine interconnect pitch (10 <jats:italic>μ</jats:italic>m) and close spacing that can overcome the large input and output (I/O) and high wiring level requirements of very large scale of quantum circuits. A system-on-wafer (SoW) packaging concept called superconducting silicon interconnect fabric (superconducting-IF), based on silicon interconnect fabric (Si-IF), is proposed, with the help of a Au interlayer technology onto the superconducting-IF. The fine-pitch and die-to-wafer-scale Au interlayer is the first demonstration of direct metal–metal thermocompression bonding (TCB) that is optimized for superconducting applications without the use of solders. The developed Au interlayer integration technology is demonstrated to be Josephson-junction-compatible (&lt;150 °C), mechanically robust (&gt;30 MPa), and electrically reliable down to 2 K. The mechanical strength of the Au interlayer integration method is optimized through shear tests with a shear force around 150 N on 2 × 2 mm<jats:sup>2</jats:sup> dies. The transition temperature (<jats:italic>T</jats:italic>c) of Nb, which is at 9 K, is experimentally verified to be unchanged after each fabrication process. Electrical and temperature cycling measurements on over 20 bonded dies of large-pitch Kelvin structures as well as fine-pitch daisy chain structures reveal reliable connections in the low-temperature regime. This work pushes quantum computing a step closer to realize its potential through 3D integration of a very large scale of quantum circuits with a high density of I/O (&gt;10 000 per mm<jats:sup>2</jats:sup>) as well as high wiring capability and without introducing lossy amorphous dielectrics.</jats:p>