Scalable Quantum Information Processors Research Articles

Full quantum tomography study of Google’s Sycamore gate on IBM’s quantum computers

The potential of achieving computational hardware with quantum advantage depends heavily on the quality of quantum gate operations. However, the presence of imperfect two-qubit gates poses a significant challenge and acts as a major obstacle in developing scalable quantum information processors. Google’s Quantum AI and collaborators claimed to have conducted a supremacy regime experiment. In this experiment, a new two-qubit universal gate called the Sycamore gate is constructed and employed to generate random quantum circuits (RQCs), using a programmable quantum processor with 53 qubits. These computations were carried out in a computational state space of size 9×1015\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$9 \ imes 10^{15}$\\end{document}. Nevertheless, even in strictly-controlled laboratory settings, quantum information on quantum processors is susceptible to various disturbances, including undesired interaction with the surroundings and imperfections in the quantum state. To address this issue, we conduct both quantum state tomography (QST) and quantum process tomography (QPT) experiments on Google’s Sycamore gate using different artificial architectural superconducting quantum computer. Furthermore, to demonstrate how errors affect gate fidelity at the level of quantum circuits, we design and conduct full QST experiments for the five-qubit eight-cycle circuit, which was introduced as an example of the programability of Google’s Sycamore quantum processor. These quantum tomography experiments are conducted in three distinct environments: noise-free, noisy simulation, and on IBM Quantum’s genuine quantum computer. Our results offer valuable insights into the performance of IBM Quantum’s hardware and the robustness of Sycamore gates within this experimental setup. These findings contribute to our understanding of quantum hardware performance and provide valuable information for optimizing quantum algorithms for practical applications.

Ultrafast coherent control of a hole spin qubit in a germanium quantum dot

Operation speed and coherence time are two core measures for the viability of a qubit. Strong spin-orbit interaction (SOI) and relatively weak hyperfine interaction make holes in germanium (Ge) intriguing candidates for spin qubits with rapid, all-electrical coherent control. Here we report ultrafast single-spin manipulation in a hole-based double quantum dot in a germanium hut wire (GHW). Mediated by the strong SOI, a Rabi frequency exceeding 540 MHz is observed at a magnetic field of 100 mT, setting a record for ultrafast spin qubit control in semiconductor systems. We demonstrate that the strong SOI of heavy holes (HHs) in our GHW, characterized by a very short spin-orbit length of 1.5 nm, enables the rapid gate operations we accomplish. Our results demonstrate the potential of ultrafast coherent control of hole spin qubits to meet the requirement of DiVincenzo’s criteria for a scalable quantum information processor.

Epitaxial superconductor-semiconductor two-dimensional systems for superconducting quantum circuits

Qubits on solid state devices could potentially provide the rapid control necessary for developing scalable quantum information processors. Materials innovation and design breakthroughs have increased functionality and coherence of qubits substantially over the past two decades. Here, we show by improving interface between InAs as a semiconductor and Al as a superconductor, one can reliably fabricate voltage-controlled Josephson junction field effect transistor (JJ-FET) that can be used as tunable qubits, resonators, and coupler switches. We find that bandgap engineering is crucial in realizing a two-dimensional electron gas near the surface. In addition, we show how the coupling between the semiconductor layer and the superconducting contacts can affect qubit properties. We present the anharmonicity and coupling strengths from one and two-photon absorption in a quantum two level system fabricated with a JJ-FET.

Optimal Control for Robust Photon State Transfer in Optomechanical Systems

AbstractCavity optomechanical systems converting quantum state between photons facilitate the development of scalable quantum information processors. The control of state transfer in such systems require producing fast and robust transfer to the target state with high fidelity. Shortcuts to adiabaticity (STA) has recently been proved a powerful method for performing fast quantum state conversion in optomechanical systems, which is, however, not robust enough against deviations in control parameters. Here a scheme to realize robust photon state transfer in a universal cavity optomechanical system by combining STA with optimal control technique (OCT) is proposed. Minimizing systematic error sensitivities with adjustable optimization parameters allows to control simultaneously the transfer speed, fidelity, and robustness against errors. Numerical results show that the optimized scheme is insensitive to deviations in optomechanical coupling and frequency detuning over a broad range. The effects of dissipation on the transfer fidelity are also examined for practical implementation of the scheme in realistic scenarios.

Teleporting a trapped-ion quantum gate

Quantum Physics Gating—controlling the state of one qubit conditioned on the state of another—is a key procedure in all quantum information processors. As the scale of quantum processors increases, the qubits will need to interact over larger and larger distances, which presents an experimental challenge in solid-state architectures. Wan et al. implemented the 20-year-old theoretical proposal of quantum gate teleportation that allows separated qubits to interact effectively. They deterministically teleported a controlled-NOT gate between two computational qubits in spatially separated zones in a segmented ion trap, demonstrating a feasible route toward scalable quantum information processors. Science , this issue p. [875][1] [1]: /lookup/doi/10.1126/science.aaw9415

Gate Tunable Hole Charge Qubit Formed in a Ge/Si Nanowire Double Quantum Dot Coupled to Microwave Photons.

A controllable and coherent light-matter interface is an essential element for a scalable quantum information processor. Strong coupling to an on-chip cavity has been accomplished in various electron quantum dot systems, but rarely explored in the hole systems. Here we demonstrate a hybrid architecture comprising a microwave transmission line resonator controllably coupled to a hole charge qubit formed in a Ge/Si core/shell nanowire (NW), which is a natural one-dimensional hole gas with a strong spin-orbit interaction (SOI) and lack of nuclear spin scattering, potentially enabling fast spin manipulation by electric manners and long coherence times. The charge qubit is established in a double quantum dot defined by local electrical gates. Qubit transition energy can be independently tuned by the electrochemical potential difference and the tunnel coupling between the adjacent dots, opening transverse (σ x) and longitudinal (σ z) degrees of freedom for qubit operation and interaction. As the qubit energy is swept across the photon level, the coupling with resonator is thus switched on and off, as detected by resonator transmission spectroscopy. The observed resonance dynamics is replicated by a complete quantum numerical simulation considering an efficient charge dipole-photon coupling with a strength up to 2π × 55 MHz, yielding an estimation of the spin-resonator coupling rate through SOI to be about 10 MHz. The results inspire the future researches on the coherent hole-photon interaction in Ge/Si nanowires.

Tunable Coupling Scheme for Implementing High-Fidelity Two-Qubit Gates

The prospect of computational hardware with quantum advantage relies critically on the quality of quantum gate operations. Imperfect two-qubit gates is a major bottleneck for achieving scalable quantum information processors. Here, we propose a generalizable and extensible scheme for a two-qubit coupler switch that controls the qubit-qubit coupling by modulating the coupler frequency. Two-qubit gate operations can be implemented by operating the coupler in the dispersive regime, which is non-invasive to the qubit states. We investigate the performance of the scheme by simulating a universal two-qubit gate on a superconducting quantum circuit, and find that errors from known parasitic effects are strongly suppressed. The scheme is compatible with existing high-coherence hardware, thereby promising a higher gate fidelity with current technologies.

Optimizing microwave photodetection: input–output theory

High fidelity microwave photon counting is an important tool for various areas from background radiation analysis in astronomy to the implementation of circuit quantum electrodynamic architectures for the realization of a scalable quantum information processor. In this work we describe a microwave photon counter coupled to a semi-infinite transmission line. We employ input–output theory to examine a continuously driven transmission line as well as traveling photon wave packets. Using analytic and numerical methods, we calculate the conditions on the system parameters necessary to optimize measurement and achieve high detection efficiency. With this we can derive a general matching condition depending on the different system rates, under which the measurement process is optimal.

Robust and compact entanglement generation from diode-laser-pumped four-wave mixing

Four-wave-mixing processes are now routinely used to demonstrate multi-spatial-mode Einstein-Podolsky-Rosen entanglement and intensity difference squeezing. Diode-laser-pumped four-wave mixing processes have recently been shown to provide an affordable, compact, and stable source for intensity difference squeezing, but it was unknown if excess phase noise present in power amplifier pump configurations would be an impediment to achieving quadrature entanglement. Here, we demonstrate the operating regimes under which these systems are capable of producing entanglement and under which excess phase noise produced by the amplifier contaminates the output state. We show that Einstein-Podolsky-Rosen entanglement in two mode squeezed states can be generated by a four-wave-mixing source deriving both the pump field and the local oscillators from a tapered-amplifier diode-laser. This robust continuous variable entanglement source is highly scalable and amenable to miniaturization, making it a critical step toward the development of integrated quantum sensors and scalable quantum information processors, such as spatial comb cluster states.

Guiding light through optical bound states in the continuum for ultrahigh‐Q microresonators

AbstractLight is usually confined in photonic structures with a band gap or relatively high refractive index for broad scientific and technical applications. Here, a light confinement mechanism is proposed based on the photonic bound state in the continuum (BIC). In a low‐refractive‐index waveguide on a high‐refractive‐index thin membrane, optical dissipation is forbidden because of the destructive interference of various leakage channels. The BIC‐based low‐mode‐area waveguide and high‐Q microresonator can be used to enhance light–matter interaction for laser, nonlinear optical and quantum optical applications. For example, a polymer structure on a diamond membrane shows excellent optical performance that can be achieved with large fabrication tolerance. It can induce strong coupling between photons and the nitrogen–vacancy center in diamond for scalable quantum information processors and networks. image

Infrared-dressed entanglement of cold open-shell polar molecules for universal matchgate quantum computing

Implementing a scalable quantum information processor using polar molecules in optical lattices requires precise control over the long-range dipole–dipole interaction between molecules in selected lattice sites. We present here a scheme using trapped open-shell polar molecules that allows dipolar exchange processes between nearest and next-nearest neighbors to be controlled in order to construct a generalized transverse Ising spin Hamiltonian with tunable XX, YY and XY couplings in the rotating frame of the driving lasers. The scheme requires a moderately strong bias magnetic field together with near-infrared light to provide local tuning of the qubit energy gap, and mid-infrared pulses to perform rotational state transfer via stimulated Raman adiabatic passage. No interaction between qubits occurs in the absence of the infrared driving. We analyze the fidelity of the resulting two-qubit matchgate, and demonstrate its robustness as a function of the driving parameters. We discuss a realistic application of the system for universal matchgate quantum computing in optical lattices.

Initialization and readout of spin chains for quantum information transport

Linear chains of spins acting as quantum wires are a promising approach for achieving scalable quantum information processors. Nuclear spins in apatite crystals provide an ideal test bed for the experimental study of quantum information transport, as they closely emulate a one-dimensional spin chain, while magnetic resonance techniques can be used to drive the spin chain dynamics and probe the accompanying transport mechanisms. Here we demonstrate initialization and readout capabilities in these spin chains, even in the absence of single-spin addressability. These control schemes enable preparing desired states for quantum information transport and probing their evolution under the transport Hamiltonian. We further optimize the control schemes by a detailed analysis of 19F NMR lineshape.

Scalable architecture for a room temperature solid-state quantum information processor

The realization of a scalable quantum information processor has emerged over the past decade as one of the central challenges at the interface of fundamental science and engineering. Here we propose and analyse an architecture for a scalable, solid-state quantum information processor capable of operating at room temperature. Our approach is based on recent experimental advances involving nitrogen-vacancy colour centres in diamond. In particular, we demonstrate that the multiple challenges associated with operation at ambient temperature, individual addressing at the nanoscale, strong qubit coupling, robustness against disorder and low decoherence rates can be simultaneously achieved under realistic, experimentally relevant conditions. The architecture uses a novel approach to quantum information transfer and includes a hierarchy of control at successive length scales. Moreover, it alleviates the stringent constraints currently limiting the realization of scalable quantum processors and will provide fundamental insights into the physics of non-equilibrium many-body quantum systems.

Tripartite interactions between two phase qubits and a resonant cavity

Micrometre-scale superconducting circuits are at present explored as the building blocks for scalable quantum information processors. In a system where two such qubits are coupled to a resonant cavity, tripartite interactions and controlled coherent dynamics have now been demonstrated. This platform should allow for a fuller exploration of multipartite quantum states and their deterministic preparation. Multipartite entanglement is essential for quantum computation1 and communication2,3,4, and for fundamental tests of quantum mechanics5 and precision measurements6. It has been achieved with various forms of quantum bits (qubits), such as trapped ions7,8, photons9 and atoms passing through microwave cavities10. Quantum systems based on superconducting circuits, which are potentially more scalable, have been used to control pair-wise interactions of qubits11,12,13,14,15,16 and spectroscopic evidence for three-particle entanglement was observed17,18. Here, we report the demonstration of coherent interactions in the time domain for three directly coupled superconducting quantum systems, two phase qubits and one resonant cavity. We provide evidence for the deterministic evolution from a simple product state, through a tripartite W state, into a (bipartite) Bell state. The cavity can be thought of as a multiphoton register or an entanglement bus, and arbitrary preparation of multiphoton states in this cavity using one of the qubits19 and subsequent interactions for entanglement distribution should allow for the deterministic creation of another class of entanglement, a Greenberger–Horne–Zeilinger state.

Coherence and Control of Quantum Registers Based on Electronic Spin in a Nuclear Spin Bath

We consider a protocol for the control of few-qubit registers comprising one electronic spin embedded in a nuclear spin bath. We show how to isolate a few proximal nuclear spins from the rest of the bath and use them as building blocks for a potentially scalable quantum information processor. We describe how coherent control techniques based on magnetic resonance methods can be adapted to these solid-state spin systems, to provide not only efficient, high fidelity manipulation but also decoupling from the spin bath. As an example, we analyze feasible performances and practical limitations in the realistic setting of nitrogen-vacancy centers in diamond.

TWO-DIMENSIONAL ARRAY OF ION TRAPS ON A PLANAR CHIP

We investigate a planar ion chip design with a two-dimensional array of linear ion traps for the scalable quantum information processor. The segmented electrodes reside in a single plane on a substrate and a grounded metal plate, a combination of appropriate rf and DC potentials are applied to them for stable ion confinement, and the trap axes are located above the surface at a distance controlled by the electrodes' lateral extent and the substrate's height as discussed. The potential distributions are calculated using static electric field qualitatively. This architecture is conceptually simple and many current microfabrication techniques are feasible for the basic structure. It may provide a promising route for scalable quantum computers.

Counting Individual Trapped Electrons on Liquid Helium

We show that small numbers of electrons, including a single isolated electron, can be held in an electrostatic trap above the surface of superfluid helium. A potential well is created using microfabricated electrodes in a 5 μm diameter pool of helium. Electrons are injected into the trap from an electron reservoir on a helium microchannel. They are individually detected using a superconducting single-electron transistor as an electrometer. A Coulomb staircase is observed as electrons leave the trap one–by–one until the trap is empty. A design for a scalable quantum information processor using an array of electron traps is presented.