npj Quantum Information最新文献

Silicon-based spin qubits offer a potential pathway toward realizing a scalable quantum computer owing to their compatibility with semiconductor manufacturing technologies. Recent experiments in this system have demonstrated crucial technologies, including high-fidelity quantum gates and multiqubit operation. However, the realization of a fault-tolerant quantum computer requires a high-fidelity spin measurement faster than decoherence. To address this challenge, we characterize and optimize the initialization and measurement procedures using the parity-mode Pauli spin blockade technique. Here, we demonstrate a rapid (with a duration of a few μs) and accurate (with >99% fidelity) parity spin measurement in a silicon double quantum dot. These results represent a significant step forward toward implementing measurement-based quantum error correction in silicon.

Employing spins in quantum dots for fault-tolerant quantum computing in large-scale qubit arrays with on-chip control electronics requires high-fidelity qubit operation at elevated temperature. This poses a challenge for single spin initialization and readout. Existing schemes rely on Zeeman splitting or Pauli spin blockade with typical energy scales of 0.1 or 1 meV for electron-based qubits, so that sufficient fidelity is obtained only at temperatures around or below 0.1 or 1 K, respectively. Here we describe a method to achieve high temperature spin selectivity in a quantum dot using a reservoir with a spin accumulation, which deterministically sets the spin of a single electron on the dot. Since spin accumulation as large as 10 meV is achievable in silicon, spin selection with electrically adjustable error rates below 10⁻⁴ is possible even in a liquid He bath at 4 K. Via the reservoir spin accumulation, induced and controlled by a nearby ferromagnet, classical information (magnetization direction) is mapped onto a spin qubit. These features provide the prospect of spin qubit operation at elevated temperatures and connect the worlds of quantum computing and spintronics.

To advance the full potential of quantum networks one should be able to distribute quantum resources over long distances at appreciable rates. As a consequence, all components in such networks need to have large multimode capacity to manipulate photonic quantum states. Towards this end, a photonic quantum memory with a large multimode capacity, especially one operating at telecom wavelength, remains an important challenge. Here we optimize the preparation of atomic frequency combs and demonstrate a spectro-temporally multiplexed quantum memory in a 10-m-long cryogenically cooled erbium doped silica fibre. Our multiplexing storage has five spectral channels - each 10 GHz wide with 5 GHz separation - with up to 330 temporal modes in each, thus resulting in a simultaneous storage of 1,650 modes of heralded single photons with a 1000-fold increasing in coincidence detection rate with respect to single mode storage. Our results could pave the way for high speed quantum networks compatible with the infrastructure of fibre optical communication.

Random numbers are a basic ingredient of simulation algorithms and cryptography, and play a significant part in computer simulation and information processing. One prominent feature of a squeezed light is its lower fluctuation and more randomness in a pair of orthogonal oriented quadratures, thus it prompts a significant application in not only quantum information and quantum precision measurement but also an excellent entropy source for true random number generation. Here we report a generation of a high-efficiency semi-device-independent quantum random number based on a broadband squeezed light, where a reliable randomness source is unnecessary and a noisy local oscillator is allowed for homodyne detection. The equivalent generation of private random bits is at a rate of 580.7 Mbps. In addition, the use of squeezed light at 1.3 μm enables the transmission of entropy sources and local oscillators at the metropolitan scale, thus expanding the potential applications of quantum random number generators based on non-classical state of light.

Variational quantum eigensolvers (VQEs) are leading candidates to demonstrate near-term quantum advantage. Here, we conduct density-matrix simulations of leading gate-based VQEs for a range of molecules. We numerically quantify their level of tolerable depolarizing gate-errors. We find that: (i) The best-performing VQEs require gate-error probabilities between 10⁻⁶ and 10⁻⁴ (10⁻⁴ and 10⁻² with error mitigation) to predict, within chemical accuracy, ground-state energies of small molecules with 4 − 14 orbitals. (ii) ADAPT-VQEs that construct ansatz circuits iteratively outperform fixed-circuit VQEs. (iii) ADAPT-VQEs perform better with circuits constructed from gate-efficient rather than physically-motivated elements. (iv) The maximally-allowed gate-error probability, p_c, for any VQE to achieve chemical accuracy decreases with the number N_II of noisy two-qubit gates as ({p}_{c}mathop{propto }limits_{displaystyle{ sim }}{N}_{{{{rm{II}}}}}^{-1}). Additionally, p_c decreases with system size, even with error mitigation, implying that larger molecules require even lower gate-errors. Thus, quantum advantage via gate-based VQEs is unlikely unless gate-error probabilities are decreased by orders of magnitude.

Quantum communication networks rely on quantum cryptographic protocols including quantum key distribution (QKD) based on single photons. A critical element regarding the security of QKD protocols is the photon number coherence (PNC), i.e., the phase relation between the vacuum and one-photon Fock state. To obtain single photons with the desired properties for QKD protocols, optimal excitation schemes for quantum emitters need to be selected. As emitters, we consider semiconductor quantum dots, that are known to generate on-demand single photons with high purity and indistinguishability. Exploiting two-photon excitation of a quantum dot combined with a stimulation pulse, we demonstrate the generation of high-quality single photons with a controllable degree of PNC. The main tuning knob is the pulse area giving full control from minimal to maximal PNC, while without the stimulating pulse the PNC is negligible in our setup for all pulse areas. Our approach provides a viable route toward secure communication in quantum networks.

State preparation is a necessary component of many quantum algorithms. In this work, we combine a method for efficiently representing smooth differentiable probability distributions using matrix product states with recently discovered techniques for initializing quantum states to approximate matrix product states. Using this, we generate quantum states encoding a class of normal probability distributions in a trapped ion quantum computer for up to 20 qubits. We provide an in depth analysis of the different sources of error which contribute to the overall fidelity of this state preparation procedure. Our work provides a study in quantum hardware for scalable distribution loading, which is the basis of a wide range of algorithms that provide quantum advantage.

We demonstrate three-dimensional magnetic resonance tomography with a resolution down to 5.9 ± 0.1 nm. Our measurements use lithographically fabricated microwires as a source of three-dimensional magnetic field gradients, which we use to image NV centers in a densely doped diamond by Fourier-accelerated magnetic resonance tomography. We also demonstrate a compressed sensing scheme, which allows for direct visual interpretation without numerical optimization and implements an effective zoom into a spatially localized volume of interest, such as a localized cluster of NV centers. It is based on aliasing induced by equidistant undersampling of k-space. The resolution achieved in our work is comparable to the best existing schemes of super-resolution microscopy and approaches the positioning accuracy of site-directed spin labeling, paving the way to three-dimensional structure analysis by magnetic-gradient based tomography.

Detecting entanglement in multipartite quantum states is an inherently probabilistic process, typically with a few measured samples. The level of confidence in entanglement detection quantifies the scheme’s validity via the probability that the signal comes from a separable state, offering a meaningful figure of merit for big datasets. Yet, with limited samples, avoiding experimental data misinterpretations requires considering not only the probabilities concerning separable states but also the probability that the signal came from an entangled state, i.e. the detection scheme’s efficiency. We demonstrate this explicitly and apply a general method to optimize both the validity and the efficiency in small data sets providing examples using at most 20 state copies. The method is based on an analytical model of finite statistics effects on correlation functions which takes into account both a Frequentist as well as a Bayesian approach and is applicable to arbitrary entanglement witnesses.

We propose a scheme for generating highly indistinguishable single photons from an active quantum Su-Schrieffer-Heeger chain composed of a collection of noisy quantum emitters. Strikingly, the single photon emission spectrum of the active quantum chain is exceedingly narrow relative to that of a single emitter or a topologically trivial chain. Furthermore, this effect is amplified dramatically in proximity to the non-trivial-to-trivial phase transition point. Exploiting this effect, we demonstrate that the single-photon linewidth of a long topological quantum chain can be arbitrarily reduced, rendering it an ideal source of indistinguishable single photons. Finally, by analyzing the most critical parameters concerning experimental realization and providing a microscopic and quantitative analysis of our model, we take concrete examples of actual quantum emitters to establish the viability of our proposal.