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The functional characterization of different neuronal types has been a longstanding and crucial challenge. With the advent of physical quantum computers, it has become possible to apply quantum machine learning algorithms to translate theoretical research into practical solutions. Previous studies have shown the advantages of quantum algorithms on artificially generated datasets, and initial experiments with small binary classification problems have yielded comparable outcomes to classical algorithms. However, it is essential to investigate the potential quantum advantage using real-world data. To the best of our knowledge, this study is the first to propose the utilization of quantum systems to classify neuron morphologies, thereby enhancing our understanding of the performance of automatic multiclass neuron classification using quantum kernel methods. We examined the influence of feature engineering on classification accuracy and found that quantum kernel methods achieved similar performance to classical methods, with certain advantages observed in various configurations.

An artificial Kerr medium has been engineered using superconducting circuits, enabling the observation of the characteristic collapse and revival of a coherent state; this behaviour could, for example, be used in single-photon generation and quantum logic operations. Single-photon manipulation makes quantum logic Photons are ideal carriers of quantum information and a natural choice for quantum information processing, in part because they interact only weakly with the media through which they travel. But these same weak interactions make it difficult to manipulate the photons' quantum state. To create and manipulate the non-classical states of light needed for quantum information protocols, strong interactions between photons are required. Such photon–photon interactions occur in so-called Kerr media, but it has not been possible to reach a regime in which the interaction strength between individual photons exceeds the loss rate. Now Gerhard Kirchmair et al . have engineered an artificial Kerr medium using superconducting circuits that allow them to reach this regime and observe characteristic collapse and revivals of a coherent state. The authors suggest that this effect could be used in a range of quantum information protocols, such as single-photon generation, delicate measurement of photons and quantum logic operations. To create and manipulate non-classical states of light for quantum information protocols, a strong, nonlinear interaction at the single-photon level is required. One approach to the generation of suitable interactions is to couple photons to atoms, as in the strong coupling regime of cavity quantum electrodynamic systems 1 , 2 . In these systems, however, the quantum state of the light is only indirectly controlled by manipulating the atoms 3 . A direct photon–photon interaction occurs in so-called Kerr media, which typically induce only weak nonlinearity at the cost of significant loss. So far, it has not been possible to reach the single-photon Kerr regime, in which the interaction strength between individual photons exceeds the loss rate. Here, using a three-dimensional circuit quantum electrodynamic architecture 4 , we engineer an artificial Kerr medium that enters this regime and allows the observation of new quantum effects. We realize a gedanken experiment 5 in which the collapse and revival of a coherent state can be observed. This time evolution is a consequence of the quantization of the light field in the cavity and the nonlinear interaction between individual photons. During the evolution, non-classical superpositions of coherent states (that is, multi-component ‘Schrödinger cat’ states) are formed. We visualize this evolution by measuring the Husimi Q function and confirm the non-classical properties of these transient states by cavity state tomography. The ability to create and manipulate superpositions of coherent states in such a high-quality-factor photon mode opens perspectives for combining the physics of continuous variables 6 with superconducting circuits. The single-photon Kerr effect could be used in quantum non-demolition measurement of photons 7 , single-photon generation 8 , autonomous quantum feedback schemes 9 and quantum logic operations 10 .

The resonator-induced phase gate is a two-qubit operation in which driving a bus resonator induces a state-dependent phase shift on the qubits equivalent to an effective \\(ZZ\\) interaction. In principle, the dispersive nature of the gate offers flexibility for qubit parameters. However, the drive can cause resonator and qubit leakage, the physics of which cannot be fully captured using either the existing Jaynes-Cummings or Kerr models. In this paper, we adopt an ab-initio model based on Josephson nonlinearity for transmon qubits. The ab-initio analysis agrees well with the Kerr model in terms of capturing the effective \\(ZZ\\) interaction in the weak-drive dispersive regime. In addition, however, it reveals numerous leakage transitions involving high-excitation qubit states. We analyze the physics behind such novel leakage channels, demonstrate the connection with specific qubits-resonator frequency collisions, and lay out a plan towards device parameter optimization. We show this type of leakage can be substantially suppressed using very weakly anharmonic transmons. In particular, weaker qubit anharmonicity mitigates both collision density and leakage amplitude, while larger qubit frequency moves the collisions to occur only at large anharmonicity not relevant to experiment. Our work is broadly applicable to the physics of weakly anharmonic transmon qubits coupled to linear resonators. In particular, our analysis confirms and generalizes the measurement-induced state transitions noted in Sank et al. (Phys. Rev. Lett. 117, 190503) and lays the groundwork for both strong-drive resonator-induced phase gate implementation and strong-drive dispersive qubit measurement.

I report measurements of energy relaxation and quantum coherence times in an aluminum dc SQUID phase qubit and a niobium dc SQUID phase qubit at 80 mK. In a dc SQUID phase qubit, the energy levels of one Josephson junction are used as qubit states and the rest of the SQUID forms an inductive network to isolate the qubit junction. Noise current from the SQUID's current bias leads is filtered by the network, with the amount of filtering depending on the ratio of the loop inductance to the Josephson inductance of the isolation junction. The isolation unction inductance can be tuned by adjusting the current, and this allows the isolation to be varied in situ. I quantify the isolation by the isolation factor rI which is the ratio of the current noise power in the qubit junction to the total noise current power on its bias leads. I measured the energy relaxation time T1, the spectroscopic coherence time [special characters omitted] and the decay time constant T' of Rabi oscillations in the Al dc SQUID phase qubit AL1 and the Nb dc SQUID phase qubit NBG, which had a gradiometer loop. In particular, I investigated the dependence of T1 on the isolation rI. T1 from the relaxation measurements did not reveal any dependance on the isolation factor rI. For comparison, I found T1 by fitting to the thermally induced background escape rate and found that it depended on rI. However, further investigation suggests that this apparent dependence may be due to a small-noise induced population in |2⟩ so I cannot draw any firm conclusion. I also measured the spectroscopic coherence time [special characters omitted], Rabi oscillations and the decay constant T' at significantly different isolation factors. Again, I did not observe any dependence of [special characters omitted] and T' on rI, suggesting that the main decoherence source in the qubit AL1 was not the noise from the bias current. Similar results were found previously in our group's Nb devices. I compared T1, [special characters omitted] and T' for the qubit AL1 with those for NBG and a niobium dc SQUID phase qubit NB1 and found significant differences in [special characters omitted] and T' among the devices but similar T 1 values. If flux noise was dominant, NBG which has a gradiometer loop would have the longest Rabi decay time T'. However, T' for NBG was similar to NB1, a Nb dc SQUID phase qubit without a gradiometer. I found that T' = 28 ns for AL1, the Al dc SQUID phase qubit, and this was more than twice as long as in NBG ( T' ≃ 15 ns) or NB1 (T' ≃ 15 ns). This suggests that materials played an important role in determining the coherence times of the different devices. Finally, I discuss the possibility of using a Cooper pair box to produce variable coupling between phase qubits. I calculated the effective capacitance of a Cooper pair box as a function of gate voltage. I also calculated the energy levels of a Josephson phase qubit coupled to a Cooper pair box and showed that the energy levels of the phase qubit can be tuned with the coupled Cooper pair box.

This paper demonstrates the integration of Reinforcement Learning (RL) into quantum transpiling workflows, significantly enhancing the synthesis and routing of quantum circuits. By employing RL, we achieve near-optimal synthesis of Linear Function, Clifford, and Permutation circuits, up to 9, 11 and 65 qubits respectively, while being compatible with native device instruction sets and connectivity constraints, and orders of magnitude faster than optimization methods such as SAT solvers. We also achieve significant reductions in two-qubit gate depth and count for circuit routing up to 133 qubits with respect to other routing heuristics such as SABRE. We find the method to be efficient enough to be useful in practice in typical quantum transpiling pipelines. Our results set the stage for further AI-powered enhancements of quantum computing workflows.

Silicon-Germanium (SiGe) is a material that possesses a multitude of applications ranging from transistors to eletro-optical modulators and quantum dots. The diverse properties of SiGe also make it attractive to implementations involving superconducting quantum computing. Here we demonstrate the fabrication of transmon quantum bits on SiGe layers and investigate the microwave loss properties of SiGe at cryogenic temperatures and single photon microwave powers. We find relaxation times of up to 100 \\(\\mu\\)s, corresponding to a quality factor Q above 4 M for large pad transmons. The high Q values obtained indicate that the SiGe/Si heterostructure is compatible with state of the art performance of superconducting quantum circuits.

We present a method for detecting electromagnetic (EM) modes that couple to a superconducting qubit in a circuit-QED architecture. Based on measurement-induced dephasing, this technique allows the measurement of modes that have a high quality factor (Q) and may be difficult to detect through standard transmission and reflection measurements at the device ports. In this scheme the qubit itself acts as a sensitive phase meter, revealing modes that couple to it through measurements of its coherence time. Such modes are indistinguishable from EM modes that do not couple to the qubit using a vector network analyzer. Moreover, this technique provides useful characterization parameters including the quality factor and the coupling strength of the unwanted resonances. We demonstrate the method for detecting both high-Q coupling resonators in planar devices as well as spurious modes produced by a 3D cavity.

Defining the right metrics to properly represent the performance of a quantum computer is critical to both users and developers of a computing system. In this white paper, we identify three key attributes for quantum computing performance: quality, speed, and scale. Quality and scale are measured by quantum volume and number of qubits, respectively. We propose a speed benchmark, using an update to the quantum volume experiments that allows the measurement of Circuit Layer Operations Per Second (CLOPS) and identify how both classical and quantum components play a role in improving performance. We prescribe a procedure for measuring CLOPS and use it to characterize the performance of some IBM Quantum systems.

The loss of amorphous hydrogenated silicon nitride (a-SiN\\(_{x}\\):H) is measured at 30 mK and 5 GHz using a superconducting LC resonator down to energies where a single-photon is stored, and analyzed with an independent two-level system (TLS) defect model. Each a-SiN\\(_{x}\\):H film was deposited with different concentrations of hydrogen impurities. We find that quantum-regime dielectric loss tangent \\(\\tan\\delta_{0}\\) in a-SiN\\(_{x}\\):H is strongly correlated with N-H impurities, including NH\\(_{2}\\). By slightly reducing \\(x\\) we are able to reduce \\(\\tan\\delta_0\\) by approximately a factor of 50, where the best films show \\(\\tan\\delta_0\\) \\(\\simeq\\) 3 \\(\\times\\) 10\\(^{-5}\\).

We demonstrate fully crystalline, single-mode ultrahigh quality factor integrated microresonators comprising epitaxially grown Si\\(_{0.86}\\)Ge\\(_{0.14}\\) waveguide cores with silicon claddings. These waveguides support resonances with internal \\(Q >10^8\\) for both polarization modes, a nearly order-of-magnitude improvement over that seen in prior integrated Si photonics platforms. The maximum \\(Q\\) is \\(1.71\\pm0.06 \\times 10^8\\) for the transverse magnetic (TM) polarization mode, corresponding to a loss of \\(0.39\\pm0.02\\) dB/m. Together with silicon's strong Kerr nonlinearity and low losses in the optical, microwave and acoustic regimes, our results could lead to the Si\\(_{1-x}\\)Ge\\(_x\\)/Si architecture unlocking important new avenues for Kerr frequency combs, optomechanics, and quantum transduction.