We measure the excitation spectrum of a superconducting atomic contact. In addition to the usual continuum above the superconducting gap, the single-particle excitation spectrum contains discrete, spin-degenerate Andreev levels inside the gap. Quasiparticle excitations are induced by a broadband on-chip microwave source and detected by measuring changes in the supercurrent flowing through the atomic contact. Since microwave photons excite quasiparticles in pairs, two types of transitions are observed: Andreev transitions, which consist of putting two quasiparticles in an Andreev level, and transitions to odd states with a single quasiparticle in an Andreev level and the other one in the continuum. In contrast to absorption spectroscopy, supercurrent spectroscopy allows detection of long-lived odd states.

$\mathcal{P}\mathcal{T}$-symmetric scattering systems with balanced gain and loss can undergo a symmetry-breaking transition in which the eigenvalues of the nonunitary scattering matrix change their phase shifts from real to complex values. We relate the $\mathcal{P}\mathcal{T}$-symmetry-breaking points of such an unbounded scattering system to those of the underlying bounded systems. In particular, we show how the $\mathcal{P}\mathcal{T}$ thresholds in the scattering matrix of the unbounded system translate into analogous transitions in the Robin boundary conditions of the corresponding bounded systems. Based on this relation, we argue and then confirm that the $\mathcal{P}\mathcal{T}$ transitions in the scattering matrix are, under very general conditions, entirely insensitive to a variable coupling strength between the bounded region and the unbounded asymptotic region, a result that can be tested experimentally and visualized using the concept of Smith charts.

Highly nonclassical quantum states of light, characterized by Wigner functions with negative values, have been all-optically created so far only in a heralded fashion. In this case, the desired output emerges rarely and randomly from a quantum-state generator. An important example is the heralded production of high-purity single-photon states, typically based on some nonlinear optical interaction. In contrast, on-demand single-photon sources are also reported, exploiting the quantized level structure of matter systems. These sources, however, lead to highly impure output states, composed mostly of vacuum. While such impure states may still exhibit certain single-photon-like features such as antibunching, they are not nonclassical enough for advanced quantum-information processing. On the other hand, the intrinsic randomness of pure, heralded states can be circumvented by first storing and then releasing them on demand. Here, we propose such a controlled release, and we experimentally demonstrate it for heralded single photons. We employ two optical cavities, where the photons are both created and stored inside one cavity and finally released through a dynamical tuning of the other cavity. We demonstrate storage times of up to 300 ns while keeping the single-photon purity around $50\%$ after storage. Our experiment is the first demonstration of a negative Wigner function at the output of an on-demand photon source or a quantum memory. In principle, our storage system is compatible with all kinds of nonclassical states, including those known to be essential for many advanced quantum-information protocols.

Metal silicides formed by means of thermal annealing processes are employed as contact materials in microelectronics. Control of the structure of silicide/silicon interfaces becomes a critical issue when the characteristic size of the device is reduced below a few tens of nanometers. Here, we report on silicide clustering occurring within the channel of PtSi/Si/PtSi Schottky-barrier transistors. This phenomenon is investigated through atomistic simulations and low-temperature resonant-tunneling spectroscopy. Our results provide evidence for the segregation of a PtSi cluster with a diameter of a few nanometers from the silicide contact. The cluster acts as a metallic quantum dot giving rise to distinct signatures of quantum transport through its discrete energy states.

Adiabatic quantum computation (AQC) has been lauded for its inherent robustness to control imperfections and relaxation effects. A considerable body of previous work, however, has shown AQC to be acutely sensitive to noise that causes excitations from the adiabatically evolving ground state. In this paper, we develop techniques to mitigate such noise, and then we point out and analyze some obstacles to further progress. First, we examine two known techniques that leverage quantum error-detecting codes to suppress noise and show that they are intimately related and may be analyzed within the same formalism. Next, we analyze the effectiveness of such error-suppression techniques in AQC, identify critical constraints on their performance, and conclude that large-scale, fault-tolerant AQC will require error correction, not merely suppression. Finally, we study the consequences of encoding AQC in quantum stabilizer codes and discover that generic AQC problem Hamiltonians rapidly convert physical errors into uncorrectable logical errors. We present several techniques to remedy this problem, but all of them require unphysical resources, suggesting that the adiabatic model of quantum computation may be fundamentally incompatible with stabilizer quantum error correction.

We investigate lasing action in aligned nitrogen molecular ions ($\mathrm{N}_2{}^{+}$) produced in an intense laser field. We find that, besides the population inversion between the ${\mathrm{B}}^2\Sigma _u{}^{+}-{\mathrm{X}}^2\Sigma _g{}^{+}$ states, which is responsible for the observed simulated amplification of a seed pulse, a rotational wave packet in the ground vibrational state ($v=0$) of the excited electronic ${\mathrm{B}}^2\Sigma _u{}^{+}$ state has been created in $\mathrm{N}_2{}^{+}$. The rotational coherence can faithfully encode its characteristics into the amplified seed pulses, enabling reconstruction of rotational wave packets of molecules in a single-shot detection manner from the frequency-resolved laser spectrum. Our results suggest that the air laser can potentially provide a promising tool for remote characterization of coherent molecular rotational wave packets.

In this theoretical study, we analyze quantum walks on complex networks, which model network-based processes ranging from quantum computing to biology and even sociology. Specifically, we analytically relate the average long-time probability distribution for the location of a unitary quantum walker to that of a corresponding classical walker. The distribution of the classical walker is proportional to the distribution of degrees, which measures the connectivity of the network nodes and underlies many methods for analyzing classical networks, including website ranking. The quantum distribution becomes exactly equal to the classical distribution when the walk has zero energy, and at higher energies, the difference, the so-called quantumness, is bounded by the energy of the initial state. We give an example for which the quantumness equals a Rényi entropy of the normalized weighted degrees, guiding us to regimes for which the classical degree-dependent result is recovered and others for which quantum effects dominate.

Single quantum dots are solid-state emitters that mimic two-level atoms but with a highly enhanced spontaneous emission rate. A single quantum dot is the basis for a potentially excellent single-photon source. One outstanding problem is that there is considerable noise in the emission frequency, making it very difficult to couple the quantum dot to another quantum system. We solve this problem here with a dynamic feedback technique that locks the quantum-dot emission frequency to a reference. The incoherent scattering (resonance fluorescence) represents the single-photon output, whereas the coherent scattering (Rayleigh scattering) is used for the feedback control. The fluctuations in emission frequency are reduced to 20 MHz, just approximately $5\%$ of the quantum-dot optical linewidth, even over several hours. By eliminating the $1/f$-like noise, the relative fluctuations in quantum-dot noise power are reduced to approximately ${10}^{-5}$ at low frequency. Under these conditions, the antibunching dip in the resonance fluorescence is described extremely well by the two-level atom result. The technique represents a way of removing charge noise from a quantum device.

A laser generates light through stimulated emission of radiation and requires population inversion. Quantum interference can yield lasing without inversion. However, such phase-sensitive quantum amplification still requires some atomic population in the excited state. Here, we present a new kind of quantum amplifier based on collective superradiant emission which does not need any population in the excited state. We show that parametric resonance between the driving (e.g., infrared) field and collective superradiant oscillations of the atomic polarization can yield light amplification at high (e.g., XUV) frequencies. To achieve gain, one must suppress a time-dependent Stark shift caused by the driving field. The resulting superradiant amplifier is many orders of magnitude more efficient than the usual nonlinear multiphoton excitation and holds promise for a new kind of generator of high-frequency coherent radiation. In addition to a detailed analytical analysis, confirmed by numerical simulations, we provide a physically appealing explanation of the quantum amplification by superradiant emission of radiation (QASER) operation in terms of coupled classical oscillators. We also present an experiment that demonstrates the QASER amplification mechanism in an electronic circuit, which, to the best of our knowledge, is the first experimental demonstration of the difference combination resonance.

Studying entanglement growth in quantum dynamics provides both insight into the underlying microscopic processes and information about the complexity of the quantum states, which is related to the efficiency of simulations on classical computers. Recently, experiments with trapped ions, polar molecules, and Rydberg excitations have provided new opportunities to observe dynamics with long-range interactions. We explore nonequilibrium coherent dynamics after a quantum quench in such systems, identifying qualitatively different behavior as the exponent of algebraically decaying spin-spin interactions in a transverse Ising chain is varied. Computing the buildup of bipartite entanglement as well as mutual information between distant spins, we identify linear growth of entanglement entropy corresponding to propagation of quasiparticles for shorter-range interactions, with the maximum rate of growth occurring when the Hamiltonian parameters match those for the quantum phase transition. Counterintuitively, the growth of bipartite entanglement for long-range interactions is only logarithmic for most regimes, i.e., substantially slower than for shorter-range interactions. Experiments with trapped ions allow for the realization of this system with a tunable interaction range, and we show that the different phenomena are robust for finite system sizes and in the presence of noise. These results can act as a direct guide for the generation of large-scale entanglement in such experiments, towards a regime where the entanglement growth can render existing classical simulations inefficient.

In this work, we describe a scheme to execute all-optical control of the routing or switching of photonic information where, by optically controlling the internal quantum state of a individual scatterer coupled to two independent cavity modes, one can dynamically and rapidly modulate the intermode coupling. This allows all-optical modulation of intercavity couplings via ac Stark or shuffle (stimulated Raman adiabatic passage) control of the scatterer’s internal states, and from this modulation, we show that we can perform all-optical switching and all-optical routing with near-unit switching contrast and with high bandwidth.

We create squeezed light by exploiting the quantum nature of the mechanical interaction between laser light and a membrane mechanical resonator embedded in an optical cavity. The radiation-pressure shot noise (fluctuating optical force from quantum laser amplitude noise) induces resonator motion well above that of thermally driven motion. This motion imprints a phase shift on the laser light, hence correlating the amplitude and phase noise, a consequence of which is optical squeezing. We experimentally demonstrate strong and continuous optomechanical squeezing of $1.7\pm 0.2\mathrm{dB}$ below the shot-noise level. The peak level of squeezing measured near the mechanical resonance is well described by a model whose parameters are independently calibrated and that includes thermal motion of the membrane with no other classical noise sources.

Multipartite entanglement of large numbers of physically distinct linear resonators is of both fundamental and applied interest, but there have been no feasible proposals to date for achieving it. At the same time, the Bose-Hubbard model with attractive interactions (ABH) is theoretically known to have a phase transition from the superfluid phase to a highly entangled nonlocal superposition, but observation of this phase transition has remained out of experimental reach. In this theoretical work, we jointly address these two problems by (1) proposing an experimentally accessible quantum simulation of the ABH phase transition in an array of tunably coupled superconducting circuit microwave resonators and (2) incorporating the simulation into a highly scalable protocol that takes as input any microwave-resonator state with negligible occupation of number states $|0⟩$ and $|1⟩$ and nonlocally superposes it across the whole array of resonators. The large-scale multipartite entanglement produced by the protocol is of the $W$ type, which is well known for its robustness. The protocol utilizes the ABH phase transition to generate the multipartite entanglement of all of the resonators in parallel, and is therefore deterministic and permits an increase in resonator number without any increase in protocol complexity; the number of resonators is limited instead by system characteristics such as resonator-frequency disorder and inter-resonator coupling strength. Only one local and two global controls are required for the protocol. We numerically demonstrate the protocol with realistic system parameters and estimate that current experimental capabilities can realize the protocol with high fidelity for greater than 40 resonators. Because superconducting-circuit microwave resonators are capable of interfacing with other devices and platforms such as mechanical resonators and (potentially) optical fields, this proposal provides a route toward large-scale $W$-type entanglement in those systems as well.

Device-independent quantum key distribution (DIQKD) is a formalism that supersedes traditional quantum key distribution, as its security does not rely on any detailed modeling of the internal working of the devices. This strong form of security is only possible using devices producing correlations that violate a Bell inequality. Full security proofs of DIQKD have recently been reported, but they tolerate zero or small amounts of noise and are restricted to protocols based on specific Bell inequalities. Here, we provide a security proof of DIQKD that is both more efficient and noise resistant, and also more general, as it applies to protocols based on arbitrary Bell inequalities and can be adapted to cover supraquantum eavesdroppers limited by the no-signaling principle only. It is formulated, however, in the bounded-quantum-storage model, where an upper bound on the adversary’s quantum memory is a priori known. This condition is not a limitation at present, since the best existing quantum memories have very short coherence times.

Device-independent quantum key distribution (DIQKD) in its current design requires a violation of a Bell’s inequality between two parties, Alice and Bob, who are connected by a quantum channel. However, in reality, quantum channels are lossy and current DIQKD protocols are thus vulnerable to attacks exploiting the detection loophole of the Bell test. Here, we propose a novel approach to DIQKD that overcomes this limitation. In particular, we propose a protocol where the Bell test is performed entirely on two casually independent devices situated in Alice’s laboratory. As a result, the detection loophole caused by the losses in the channel is avoided.

We describe a many-body quantum system that can be made to quantum compute by the adiabatic application of a large applied field to the system. Prior to the application of the field, quantum information is localized on one boundary of the device, and after the application of the field, this information propagates to the other side of the device, with a quantum circuit applied to the information. The applied circuit depends on the many-body Hamiltonian of the material, and the computation takes place in a degenerate ground space with symmetry-protected topological order. Such “adiabatic quantum transistors” are universal adiabatic quantum computing devices that have the added benefit of being modular. Here, we describe this model, provide arguments for why it is an efficient model of quantum computing, and examine these many-body systems in the presence of a noisy environment.

Making a system state follow a prescribed trajectory despite fluctuations and errors commonly consists of monitoring an observable (temperature, blood-glucose level, etc.) and reacting on its controllers (heater power, insulin amount, etc.). In the quantum domain, there is a change of paradigm in feedback, since measurements modify the state of the system, most dramatically when the trajectory goes through superpositions of measurement eigenstates. Here, we demonstrate the stabilization of an arbitrary trajectory of a superconducting qubit by measurement-based feedback. The protocol benefits from the long coherence time ($T_2>10\mu \mathrm{s}$) of the 3D transmon qubit, the high efficiency ($82\%$) of the phase-preserving Josephson amplifier, and fast electronics that ensure less than 500 ns total delay. At discrete time intervals, the state of the qubit is measured and corrected in case an error is detected. For Rabi oscillations, where the discrete measurements occur when the qubit is supposed to be in the measurement pointer states, we demonstrate an average fidelity of $85\%$ to the targeted trajectory. For Ramsey oscillations, which do not go through pointer states, the average fidelity reaches $76\%$. Incidentally, we demonstrate a fast reset protocol that allows us to cool a 3D transmon qubit down to $0.6\%$ in the excited state.

The conflict between classical and quantum physics can be identified through a series of yes-no tests on quantum systems, without it being necessary that these systems be in special quantum states. Kochen-Specker (KS) sets of yes-no tests have this property and provide a quantum-versus-classical advantage that is free of the initialization problem that affects some quantum computers. Here, we report the first experimental implementation of a complete KS set that consists of 18 yes-no tests on four-dimensional quantum systems and show how to use the KS set to obtain a state-independent quantum advantage. We first demonstrate the unique power of this KS set for solving a task while avoiding the problem of state initialization. Such a demonstration is done by showing that, for 28 different quantum states encoded in the orbital-angular-momentum and polarization degrees of freedom of single photons, the KS set provides an impossible-to-beat solution. In a second experiment, we generate maximally contextual quantum correlations by performing compatible sequential measurements of the polarization and path of single photons. In this case, state independence is demonstrated for 15 different initial states. Maximum contextuality and state independence follow from the fact that the sequences of measurements project any initial quantum state onto one of the KS set’s eigenstates. Our results show that KS sets can be used for quantum-information processing and quantum computation and pave the way for future developments.

In this work, we present theoretical tools suitable for quantitative modeling of large superconducting circuits that include one-dimensional Josephson-junction arrays. The large number of low-energy degrees of freedom and the peculiar interactions between them induced by flux quantization present a considerable challenge to the detailed modeling of such circuits. For the concrete example of the fluxonium device, we show how to address this challenge. Starting from the complete degrees of freedom of the circuit, we employ the relevant collective modes and circuit symmetries to obtain a systematic approximation scheme. Important circuit symmetries include approximate invariance under the symmetric group and lead to considerable simplifications of the theory. Selection rules restrict the possible coupling among different collective modes and help explain the remarkable accuracy of previous simplified models. Using this strategy, we obtain new predictions for the energy spectrum of the fluxonium device that can be tested with current experimental technology.

We report cooling of a single neutral atom to its three-dimensional vibrational ground state in an optical tweezer. After employing Raman sideband cooling for tens of milliseconds, we measure via sideband spectroscopy a three-dimensional ground-state occupation of about $90\%$. We further observe coherent control of the spin and motional state of the trapped atom. Our demonstration shows that an optical tweezer, formed simply by a tightly focused beam of light, creates sufficient confinement for efficient sideband cooling. This source of ground-state neutral atoms will be instrumental in numerous quantum simulation and logic applications that require a versatile platform for storing and manipulating ultracold single neutral atoms. For example, these results will improve current optical-tweezer experiments studying atom-photon coupling and Rydberg quantum logic gates, and could provide new opportunities such as rapid production of single dipolar molecules or quantum simulation in tweezer arrays.