Reversible logic can provide lower switching-energy costs relative to all irreversible logic, including those developed by industry in semiconductor circuits; however, more research is needed to understand what is possible. Superconducting logic, an exemplary platform for both irreversible and reversible logic, uses flux quanta to represent bits and the reversible implementation may switch state with low energy dissipation relative to the energy of a flux quantum. Here, we simulate reversible shift register gates that are ballistic: their operation is powered by the input bits alone. A storage loop is added relative to previous gates as a key innovation, which bestows an asynchronous property to the gate such that input bits can arrive at different times as long as their order is clearly preserved. The shift register represents bit states by flux polarity, both in the stored bit as well as the ballistic input and output bits. Its operation consists of the elastic swapping of flux between the stored and the moving bit. This is related to a famous irreversible shift register, developed prior to the advent of superconducting flux quanta logic (which used irreversible gates). In the base design of our ballistic shift register (BSR), there is one input port and one output port but we find that we can make other asynchronous ballistic gates by extension. For example, we show that two BSRs operate in sequence without power added, giving a 2-bit sequential memory. We also show that a BSR with two input and two output ports allows a bit state to be set from one input port and then conveyed out on either output port. The gate constitutes the first asynchronous reversible 2-input gate. Finally, for a better insight into the dynamics, we introduce a collective-coordinate model. We find that the gate can be described as motion in two coordinates subject to a potential determined by the input bit and initial stored flux quantum. Aside from the favorable asynchronous feature, the gate is considered practical in the context of energy efficiency, parameter margins, logical depth, and speed.

Micromagnet-based electric dipole spin resonance offers an attractive path for the near-term scaling of dense arrays of silicon spin qubits in gate-defined quantum dots while maintaining long coherence times and high control fidelities. However, accurately controlling dense arrays of qubits using a multiplexed drive will require an understanding of the cross-talk mechanisms that may reduce operational fidelity. We identify an unexpected cross-talk mechanism whereby the Rabi frequency of a driven qubit is drastically changed when the drive of an adjacent qubit is turned on. These observations raise important considerations for scaling single-qubit control.

We experimentally demonstrate a programmable parametric mode sorter of high-dimensional signals in a composite spatiotemporal Hilbert space through mode-selective quantum frequency up-conversion. As a concrete example and with quantum communication applications in mind, we consider the Laguerre-Gaussian and Hermite-Gaussian modes as the spatial and temporal state basis for the signals, respectively. By modulating the spatiotemporal profiles of the up-conversion pump, we demonstrate the faithful selection of signal photons in those modes and their superposition modes. Our results show an improvement in the quantum mode-sorting performance by coupling the up-converted light into a single-mode fiber and/or operating the up-conversion at the edge of phase matching. Optimizing pump temporal profiles allows us to achieve more than 12-dB extinction for mutually unbiased basis (MUB) sets of the spatiotemporal modes. This fully programmable and efficient system could be a viable resource for quantum communications, quantum computation, and quantum metrology.

Dielectric loss is known to limit state-of-the-art superconducting qubit lifetimes. Recent experiments imply upper bounds on bulk dielectric loss tangents on the order of 100 parts per billion but because these inferences are drawn from fully fabricated devices with many loss channels, these experiments do not definitely implicate or exonerate the dielectric. To resolve this ambiguity, we devise a measurement method capable of separating and resolving bulk dielectric loss with a sensitivity at the level of $5\times {10}^{-9}$. The method, which we call the dielectric dipper, involves the in situ insertion of a dielectric sample into a high-quality microwave cavity mode. Smoothly varying the participation of the sample in the cavity mode enables a differential measurement of the dielectric loss tangent of the sample. The dielectric dipper can probe the low-power behavior of dielectrics at cryogenic temperatures and does so without the need for any lithographic process, enabling controlled comparisons of substrate materials and processing techniques. We demonstrate the method with measurements of sapphire grown by edge-defined film-fed growth (EFG) in comparison to high-grade sapphire grown by the heat-exchanger method (HEMEX). For EFG sapphire, we infer a bulk loss tangent of $63(8)\times {10}^{-9}$ and a substrate-air interface loss tangent of $15(3)\times {10}^{-4}$ (assuming a sample surface thickness of $3\mathrm{nm}$). For a typical transmon, this bulk loss tangent would limit device quality factors to $Q⪅20\times {10}^6$, suggesting that bulk loss is likely the dominant loss mechanism in the longest-lived transmons on sapphire. We also demonstrate this method on HEMEX sapphire and bound its bulk loss tangent to be less than $19(6)\times {10}^{-9}$. As this bound is about 3 times smaller than the bulk loss tangent of EFG sapphire, the use of HEMEX sapphire as a substrate would lift the bulk dielectric coherence limit of a typical transmon qubit to several milliseconds.

Defect-free atom arrays are a precursor for quantum information processing and quantum simulation with neutral atoms. Yet, large-scale defect-free atom arrays can be challenging to realize, due to the losses encountered when rearranging stochastically loaded atoms to achieve a desired target array. Here, we demonstrate a parallel rearrangement algorithm that uses multiple mobile tweezers to independently sort and compress atom arrays in a way that naturally avoids atom collisions. With a high degree of parallelism, our algorithm offers a reduced move complexity compared to both single-tweezer algorithms and existing multitweezer algorithms. We further determine the optimal degree of parallelism to be a balance between an algorithmic speedup and multitweezer inhomogeneity effects. The defect-free probability for a 225-atom array is demonstrated to be as high as 33(1)% in a room-temperature setup after multiple cycles of rearrangement. The algorithm presented here can be implemented for any target array geometry with an underlying periodic structure.

Recent theoretical work has highlighted that quantizing a superconducting circuit in the presence of time-dependent flux $\mathrm{\Phi }(t)$ generally produces Hamiltonian terms proportional to $d\mathrm{\Phi }/dt$ unless a special allocation of the flux across inductive terms is chosen. Here, we present an experiment probing the effects of a fast flux ramp applied to a heavy-fluxonium circuit. The experiment confirms that naïve omission of the $d\mathrm{\Phi }/dt$ term leads to theoretical predictions inconsistent with experimental data. Experimental data are fully consistent with recent theory that includes the derivative term or equivalently uses “irrotational variables” that uniquely allocate the flux to properly eliminate the $d\mathrm{\Phi }/dt$ term.

A distributed quantum network would require quantum nodes capable of performing arbitrary quantum information protocols with high fidelity. So far the challenge has been in realizing such quantum nodes with features for scalable quantum computing. We show here that using the solid-state spins in 4H silicon carbide (4H $\mathrm{Si}\mathrm{C}$) such a goal could be realized, wherein a controlled generation of highly coherent qubit registers using nuclear spins is possible. Using a controlled isotope concentration and coherent control we perform here atomistic modeling of the central spin system formed by the electron spin of a silicon-vacancy color center ($V_{\mathrm{Si}}^{-}$ center) and the noninteracting nuclear spins. From this we lay out conditions for realizing a scalable nuclear-spin (${}^{13}\mathrm{C}$ or ${}^{29}\mathrm{Si}$) register, wherein independent control of the qubits alongside their mutual controlled operations using the central electron spin associated to the $V_{\mathrm{Si}}^{-}$ center in 4H $\mathrm{Si}\mathrm{C}$ are achieved. Further, the decoherence and entanglement analysis provided here could be used to evaluate the quantum volume of these nodes. Our results mark a clear route towards realizing scalable quantum memory nodes for applications in distributed quantum computing networks and further for quantum information protocols.

A single qutrit with transitions selectively driven by weakly coupled reservoirs can implement one of the world’s smallest refrigerators. We analyze the performance of $N$ such fridges that are collectively coupled to the reservoirs. We observe a quantum boost, made manifest in a quadratic scaling of the steady-state cooling current with $N$. As $N$ grows further, the scaling reduces to linear, since the transitions responsible for the quantum boost become energetically unfavorable. Fine-tuned interqutrit interactions may be used to maintain the quantum boost for all $N$ and also for not perfectly collective scenarios.

Donor dots realized with phosphorus atoms in silicon have proven to be excellent hosts for electron spin qubits as they provide a strong confining potential that results in small wave functions and well-isolated ground states. As a promising candidate for large-scale quantum computers, such qubits have demonstrated fast, high-fidelity single-shot readout (99.8%) and extremely long coherence times (seconds) with single-qubit gates exceeding 99.94% fidelity. However, high-fidelity two-qubit gates in this platform have been elusive, with charge noise being one of the key limiting factors. Charge noise causes unwanted fluctuations in the exchange coupling between electron spins resulting in logic gate errors, a process that could be minimized if we could engineer a large enough magnetic field difference between the qubits. In this work, we show that using the donor nuclear spins as nanomagnets we can engineer a large magnetic field gradient ($>800\mathrm{MHz}$) between the qubits thereby minimizing sensitivity to charge noise and reducing errors during two-qubit controlled-NOT (cnot) gate operation. We develop a comprehensive theoretical framework with realistic noise sources for performing cnot gates via controlled rotation (crot) using multidonor dot qubits. We show that by engineering the number and location of donors within the dots we can control the hyperfine couplings to maximize the energy difference between electron spin qubits. As a result, we show that the cnot gate error rates can be reduced by a factor of 4 when using multidonor dots as compared to single donors. Our results provide a theoretical roadmap to show how to achieve cnot fidelities as high as 99.98% by optimizing both the local magnetic environment and the operating parameters of multidonor dot qubits.

We report the narrow inhomogeneous distribution of the zero-phonon line from tin-vacancy ($\mathrm{Sn}$-V) centers in diamond and the overlap of spectra from multiple separated $\mathrm{Sn}$-V centers. Photoluminescence excitation spectroscopy measurements at a cryogenic temperature show that $\mathrm{Sn}$-V centers exhibit stable fluorescence and linewidths close to the Fourier-transform-limited linewidth. The inhomogeneous distribution is as low as about 4 GHz, which enables the observation of $\mathrm{Sn}$-isotope-dependent resonant frequencies. Owing to the narrow inhomogeneous distribution, we observe multiple $\mathrm{Sn}$-V centers showing almost identical photon frequencies and linewidths. Identical $\mathrm{Sn}$-V centers are also observed between two diamond samples, confirming the reliable fabrication of high-quality $\mathrm{Sn}$-V centers.

The coherent one-way (COW) quantum key distribution (QKD) is a highly practical quantum communication protocol that is currently deployed in off-the-shelve products. However, despite its simplicity and widespread use, the security of COW QKD is still an open problem. This is largely due to its unique security feature based on the intersignal phase distribution, which makes it very difficult to analyze using standard security proof techniques. Here, to overcome this problem, we present a simple variant of COW QKD and prove its security in the infinite-key limit. The proposed modifications only involve an additional vacuum tail signal following every encoded signal and a balanced beam splitter for passive measurement basis choice. Remarkably, the resulting key rate of our protocol is comparable with both the existing upper bound on the COW-QKD key rate and the secure key rate of the coherent-state BB84 protocol. Our findings therefore suggest that the secured deployment of COW-QKD systems in high-loss optical networks is indeed feasible with minimal adaptations applied to its hardware and software.

Quantum dot arrays possess ground states governed by Coulomb energies, utilized prominently by singly occupied quantum dots, each implementing a spin qubit. For such quantum processors, the controlled transitions between ground states are of operational significance, as these allow the control of quantum information within the array such as qubit initialization and entangling gates. For few-dot arrays, ground states are traditionally mapped out by performing dense raster-scan measurements in control-voltage space. These become impractical for larger arrays due to the large number of measurements needed to sample the high-dimensional gate-voltage hypercube and the comparatively little information extracted. We develop a hardware-triggered detection method based on reflectometry, to acquire measurements directly corresponding to transitions between ground states. These measurements are distributed sparsely within the high-dimensional voltage space by executing line searches proposed by a learning algorithm. Our autonomous software-hardware algorithm accurately estimates the polytope of Coulomb blockade boundaries, experimentally demonstrated in a $2\times 2$ array of silicon quantum dots.

Two-dimensional (2D) materials are becoming increasingly popular as a platform for studies of quantum phenomena and for the production of prototype quantum technologies. Quantum emitters in 2D materials can host two-level systems that can act as qubits for quantum information processing. Here, we characterize the behavior of position-controlled quantum emitters in hexagonal boron nitride at cryogenic temperatures. Over two dozen sites, we observe an ultranarrow distribution of the zero phonon line at approximately 436 nm, together with strong linearly polarized emission. We employ resonant excitation to characterize the emission lineshape and find spectral diffusion and phonon broadening contribute to linewidths in the range 1–2 GHz. Rabi oscillations are observed at a range of resonant excitation powers, and under 1-µW excitation a coherent superposition is maintained up to 0.90 ns. Our results are promising for future employment of quantum emitters in h-BN for scalable quantum technologies.

High-performance quantum memories are an essential component for regulating temporal events in quantum networks. As a component in quantum repeaters, they have the potential to support the distribution of entanglement beyond the physical limitations of fiber loss. This will enable key applications such as quantum key distribution, network-enhanced quantum sensing, and distributed quantum computing. Here, we present a quantum memory engineered to meet real-world deployment and scaling challenges. The memory technology utilizes a warm rubidium vapor as the storage medium, and operates at room temperature, without the need for vacuum and/or cryogenic support. We demonstrate performance specifications of high-fidelity retrieval (95%) and low operation error $({10}^{-2})$ at a storage time of $160\mu \mathrm{s}$ for single-photon level quantum memory operations. We further show a substantially improved storage time (with classical-level light) of up to 1 ms by suppressing atomic diffusions. The device is housed in an enclosure with a standard 2U rackmount form factor, and can robustly operate on a day scale in a noisy environment. This result marks a step toward implementing quantum networks in the field.

We present designs for silicon-on-insulator nanowaveguide sources capable of generating polarization-entangled photon pairs with high concurrence (approximately $0.98$) across the S, C, and L bands (approximately $1475–1625$ nm) in the telecommunications spectrum via degenerate-pump spontaneous four-wave mixing. Silica-clad waveguides with $\mathrm{Si}$ film thickness 280 nm, width in the range 700–800 nm, and length up to 1 cm are shown to be optimum, generating near-perfect entangled pairs over a bandwidth >100 nm around 1550 nm. Further, the use of relatively narrow pump bandwidths (<10 GHz) can still yield a concurrence $>0.95$ in waveguides up to 7 mm long despite the presence of large polarization-mode dispersion in these high-index-contrast structures. Such broadband integrated sources can be employed in multichannel wavelength-division multiplexed entanglement-based quantum key distribution protocols over standard optical-fiber networks.

Flux-tunable qubits are a useful resource for superconducting quantum processors. They can be used to perform cphase gates, facilitate fast reset protocols, avoid qubit-frequency collisions in large processors, and enable certain fast readout schemes. However, flux-tunable qubits suffer from a trade-off between their tunability range and sensitivity to flux noise. Optimizing this trade-off is particularly relevant for enabling fast, high-fidelity, all-microwave cross-resonance gates in large, high-coherence processors. This is mainly because cross-resonance gates set stringent conditions on the frequency landscape of neighboring qubits, which are difficult to satisfy with nontunable transmons due to their relatively large fabrication imprecision. To solve this problem, we realize a coherent, flux-tunable, transmonlike qubit, which exhibits a frequency tunability range as small as $43\mathrm{MHz}$, and whose frequency, anharmonicity and tunability range are set by a few experimentally achievable design parameters. Such a weakly tunable qubit may be used to avoid frequency collisions in a large lattice while exhibiting minimal susceptibility to flux noise.

Although two-qubit entangling gates are necessary for universal quantum computing, they are notoriously difficult to implement with high fidelity. Recently, tunable couplers have become a key component for realizing high-fidelity two-qubit gates in superconducting quantum computers. However, it is still difficult to achieve tunable coupling free of unwanted residual coupling—in particular, for highly detuned qubits, which are desirable for mitigating qubit-frequency crowding or errors due to crosstalk between qubits. We thus propose a kind of tunable coupler that we call a double-transmon coupler, because it is composed of two transmon qubits coupled through a common loop with an additional Josephson junction. By controlling the magnetic flux in the loop, we can achieve not only fast high-fidelity two-qubit gates but also no residual coupling during the idle time, where computational qubits are highly detuned fixed-frequency transmons. The proposed coupler is expected to offer an alternative approach to higher-performance superconducting quantum computers.

The demonstration of universal quantum logic operations near the fault-tolerance threshold has established ion-implanted near-surface donor atoms as a plausible platform for scalable quantum computing in silicon. The next technological step forward requires a deterministic fabrication method to create large-scale arrays of donors, featuring a few-hundred-nanometer interdonor spacing. Here, we explore the feasibility of this approach by implanting low-energy ions into silicon devices featuring a $60\times 60\mu {\mathrm{m}}^2$ sensitive area and an ultrathin 3.2-nm gate oxide—capable of hosting large-scale donor arrays. We employ a characterization system consisting of a modified focused-ion-beam machine and ultralow-noise ion-detection electronics to demonstrate a method for evaluating the device response characteristics to shallowly implanted 12-keV ${}^1$$\mathrm{H}$${}_2^{+}$ ions. Despite the weak internal electric field, near-unity charge-collection efficiency is obtained from the entire sensitive area. This can be explained by the critical role that the thermal gate oxide plays in the ion-detection response, allowing an initial rapid diffusion of ion-induced charge away from the implant site. Next, we adapt our approach to perform deterministic implantation of a few thousand 24-keV ${}^{40}$$\mathrm{Ar}$${}^{2+}$ ions into a predefined microvolume, without any additional collimation. Despite the reduced ionization from the heavier ion species, a fluence-independent detection confidence of $\ge 99.99$% is obtained. Our system thus represents not only a method for mapping the near-surface electrical landscape of electronic devices but also a framework toward mask-free prototyping of large-scale donor arrays in silicon.

Qubit measurement and control in circuit quantum electrodynamics (QED) rely on microwave drives, with higher drive amplitudes ideally leading to faster processes. However, degradation in qubit coherence time and readout fidelity has been observed even under moderate drive amplitudes corresponding to a few photons populating the measurement resonator. Here, we numerically explore the dynamics of a driven transmon-resonator system under strong and nearly resonant measurement drives and find clear signatures of transmon ionization where the qubit escapes out of its cosine potential. Using a semiclassical model, we interpret this ionization as resulting from resonances occurring at specific resonator-photon populations. We find that the photon populations at which these spurious transitions occur are strongly parameter dependent and that they can occur at low resonator-photon population, something that may explain the experimentally observed degradation in measurement fidelity.

In this paper, we present a direct loss-tangent measurement of a high-resistivity intrinsic (100) silicon wafer in a temperature range from approximately $70\mathrm{mK}$ to 1 K, approaching the quantum regime. The measurement is performed using a technique that takes advantage of a high-quality-factor superconducting niobium resonator and allows us to directly measure the loss tangent of insulating materials with a high level of accuracy and precision. We report silicon-loss-tangent values at the lowest temperature and for electric field amplitudes comparable to those found in planar transmon devices, and these are one order of magnitude larger than what was previously estimated. In addition, we discover a nonmonotonic trend in the loss tangent as a function of temperature, which we describe by means of a phenomenological model based on variable-range hopping conduction between localized states around the Fermi energy. We also observe an increasing dependence of the dissipation on the electric field, which can be qualitatively described by the variable-range hopping-conduction mechanism as well. These findings are important for the optimization of quantum devices and for advancing the understanding of their decoherence channels.