Qubit Error Rate Research Articles

Dynamics of measurement-induced state transitions in superconducting qubits

We have investigated temporal fluctuation of superconducting qubits via the time-resolved measurement for an IBM Quantum system. We found that the qubit error rate abruptly changes during specific time intervals. Each high error state persists for several tens of seconds and exhibits an on-off behavior. The observed temporal instability can be attributed to qubit transitions induced by a measurement stimulus. Resonant transition between fluctuating dressed states of the qubits coupled with high-frequency resonators can be responsible for the error-rate change.

High-fidelity four-photon GHZ states on chip

Mutually entangled multi-photon states are at the heart of all-optical quantum technologies. While impressive progresses have been reported in the generation of such quantum light states using free space apparatus, high-fidelity high-rate on-chip entanglement generation is crucial for future scalability. In this work, we use a bright quantum-dot based single-photon source to demonstrate the high fidelity generation of 4-photon Greenberg-Horne-Zeilinger (GHZ) states with a low-loss reconfigurable glass photonic circuit. We reconstruct the density matrix of the generated states using full quantum-state tomography reaching an experimental fidelity to the target state of FGHZ4=(86.0±0.4)%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{{{\\mathcal{F}}}}}_{{{{{\\rm{GHZ}}}}}_{4}}=(86.0\\pm 0.4)\\, \\%$$\\end{document}, and a purity of PGHZ4=(76.3±0.6)%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{{{\\mathcal{P}}}}}_{{{{{\\rm{GHZ}}}}}_{4}}=(76.3\\pm 0.6)\\, \\%$$\\end{document}. The entanglement of the generated states is certified with a semi device-independent approach through the violation of a Bell-like inequality by more than 39 standard deviations. Finally, we carry out a four-partite quantum secret sharing protocol on-chip where a regulator shares with three interlocutors a sifted key with up to 1978 bits, achieving a qubit-error rate of 10.87%. These results establish that the quantum-dot technology combined with glass photonic circuitry offers a viable path for entanglement generation and distribution.

A Remote Quantum Error-Correcting Code Preparation Protocol on Cluster States

The blind quantum computation (BQC) protocol allows for privacy-preserving remote quantum computations. In this paper, we introduce a remote quantum error correction code preparation protocol for BQC using a cluster state and analyze its blindness in the measurement-based quantum computation model. Our protocol requires fewer quantum resources than previous methods, as it only needs weak coherent pulses, eliminating the need for quantum memory and limited quantum computing. The results of our theoretical analysis and simulations show that our protocol requires fewer quantum resources compared to non-coding methods with the same qubit error rate.

Phase estimation of local Hamiltonians on NISQ hardware

In this work we investigate a binned version of quantum phase estimation (QPE) set out by Somma (2019 New J. Phys. 21 123025) and known as the quantum eigenvalue estimation problem (). Specifically, we determine whether the circuit decomposition techniques we set out in previous work, Clinton et al (2021 Nat. Commun. 12 1–10), can improve the performance of in the noisy intermediate scale quantum (NISQ) regime. To this end we adopt a physically motivated abstraction of NISQ device capabilities as in Clinton et al (2021 Nat. Commun. 12 1–10). Within this framework, we find that our techniques reduce the threshold at which it becomes possible to perform the minimum two-bin instance of this algorithm by an order of magnitude. This is for the specific example of a two dimensional spin Fermi-Hubbard model. For example, we estimate that the depolarizing single qubit error rate required to implement a minimum two bin example of —with a Fermi-Hubbard model and up to a precision of —can be reduced from 10−7 to 10−5. We explore possible modifications to this protocol and propose an application, which we dub randomized quantum eigenvalue estimation problem (). outputs estimates on the fraction of eigenvalues which lie within randomly chosen bins and upper bounds the total deviation of these estimates from the true values. One use case we envision for this algorithm is resolving density of states features of local Hamiltonians.

Exploring the scaling limitations of the variational quantum eigensolver with the bond dissociation of hydride diatomic molecules

AbstractMaterials simulations involving strongly correlated electrons pose fundamental challenges to state‐of‐the‐art electronic structure methods but are hypothesized to be the ideal use case for quantum computing algorithms. To date, no quantum computer has simulated a molecule of a size and complexity relevant to real‐world applications, despite the fact that the variational quantum eigensolver (VQE) algorithm can predict chemically accurate total energies. Nevertheless, because of the many applications of moderately sized, strongly correlated systems, such as molecular catalysts, the successful use of the VQE stands as an important waypoint in the advancement toward useful chemical modeling on near‐term quantum processors. In this paper, we take a significant step in this direction. We lay out the steps, write, and run parallel code for an (emulated) quantum computer to compute the bond dissociation curves of the TiH, LiH, NaH, and KH diatomic hydride molecules using the VQE. TiH was chosen as a relatively simple chemical system that incorporates d orbitals and strong electron correlation. Because current VQE implementations on existing quantum hardware are limited by qubit error rates, the number of qubits available, and the allowable gate depth, recent studies using it have focused on chemical systems involving s and p block elements. Through VQE + UCCSD calculations of TiH, we evaluate the near‐term feasibility of modeling a molecule with d‐orbitals on real quantum hardware. We demonstrate that the inclusion of d‐orbitals and the use of the UCCSD ansatz, which are both necessary to capture the correct TiH physics, dramatically increase the cost of this problem. We estimate the approximate error rates necessary to model TiH on current quantum computing hardware using VQE + UCCSD and show them to likely be prohibitive until significant improvements in hardware and error correction algorithms are available.

Entanglement-based quantum key distribution with a blinking-free quantum dot operated at a temperature up to 20 K

Entanglement-based quantum key distribution (QKD) promises enhanced robustness against eavesdropping and compatibility with future quantum networks. Among other sources, semiconductor quantum dots (QDs) can generate polarization-entangled photon pairs with near-unity entanglement fidelity and a multiphoton emission probability close to zero even at maximum brightness. These properties have been demonstrated under resonant two-photon excitation (TPE) and at operation temperatures below 10 K. However, source blinking is often reported under TPE conditions, limiting the maximum achievable photon rate. In addition, operation temperatures reachable with compact cryocoolers could facilitate the widespread deployment of QDs, e.g., in satellite-based quantum communication. We demonstrate blinking-free emission of highly entangled photon pairs from GaAs QDs embedded in a p-i-n diode. High fidelity entanglement persists at temperatures of at least 20 K, which we use to implement fiber-based QKD between two buildings with an average raw key rate of 55 bits / s and a qubit error rate of 8.4%. We are confident that by combining electrical control with already demonstrated photonic and strain engineering, QDs will keep approaching the ideal source of entangled photons for real world applications.

Key rates for quantum key distribution protocols with asymmetric noise

We consider the asymptotic key rates achieved in the simplest quantum key distribution protocols, namely the BB84 and the six-state protocols, when non-uniform noise is present in the system. We first observe that higher qubit error rates do not necessarily imply lower key rates. Secondly, we consider protocols with advantage distillation and show that it can be advantageous to use the basis with higher quantum bit error rate for the key generation. We then discuss the relation between advantage distillation and entanglement distillation protocols. We show that applying advantage distillation to a string of bits formed by the outcomes of measurements in the basis with higher quantum bit error rate is closely connected to the two-to-one entanglement distillation protocol DEJMPS. Finally, we discuss the implications of these results for implementations of quantum key distribution.

Benchmarking noise extrapolation with the OpenPulse control framework

Distilling precise estimates from noisy intermediate scale quantum (NISQ) data has recently attracted considerable attention. In order to augment digital qubit metrics, such as gate fidelity, we discuss analog error mitigability, i.e. the ability to accurately distill precise observable estimates, as a hybrid quantum-classical computing benchmarking task. Specifically, we characterize single qubit error rates on IBM's Poughkeepsie superconducting quantum hardware, incorporate control-mediated noise dependence into a generalized rescaling protocol, and analyze how noise characteristics influence Richardson extrapolation-based error mitigation. Our results identify regions in the space of Hamiltonian control fields and circuit-depth which are most amenable to reliable noise extrapolation, as well as shedding light on how low-level hardware characterization can be used as a predictive tool for uncertainty quantification in error mitigated NISQ computations.

Blind information reconciliation with variable step sizes for quantum key distribution

Quantum key distribution (QKD) generates symmetric keys between two authenticated parties with the guarantee of information-theoretically security. A vital step in QKD to obtain fully-matched key between two parties is information reconciliation. The blind reconciliation protocol provides a useful tool that corrects the mismatch in a wide range of qubit error rate (QBER) but without a prior error estimation. However, there is a contradiction between the reconciliation efficiency and the processing time in this protocol. In this work, we propose a blind reconciliation protocol with variable step sizes to relieve this contradiction. The analysis and simulation results show that the improved protocol inherits all the advantages of the original blind reconciliation protocol and can obtain better reconciliation efficiency with less operation time. The improved blind reconciliation protocol enhances the final secret key rate and accelerates the processing speed of a QKD system.

Polarization-Multiplexed Phase Modulation of Single Photons in a Sagnac Interferometer

Polarization-multiplexed phase modulation was demonstrated in a Sagnac interferometer for polarization-auto-compensating quantum key distribution. The Sagnac interferometer was composed of a polarization beam splitter to split the signal pulses into clockwise and counter-clockwise beams of different polarizations, a Faraday rotator to rotate the polarizations of the split clockwise and counter-clockwise beams, and two phase-modulators asymmetrically set in the Sagnac ring. With this structure, the delays of the electronic triggering signals for the two phase-modulators could be set separately, reducing the difficulty in the setup and achieving more precise phase modulation on different polarized beams. Such polarization-multiplexed phase modulation was appropriately gated in sequential time gates to ensure the long-term stability of automatic polarization compensation in the long-distance quantum key distribution system. Polarization-multiplexed phase modulation was implemented in a protocol quantum key distribution system of 25-km communicating fiber length, generating 20-kHz sifted key rate with a qubit error rate lower than 5%.

Multi-partite entanglement can speed up quantum key distribution in networks

The laws of quantum mechanics allow for the distribution of a secret random key between two parties. Here we analyse the security of a protocol for establishing a common secret key between N parties (i.e. a conference key), using resource states with genuine N-partite entanglement. We compare this protocol to conference key distribution via bipartite entanglement, regarding the required resources, achievable secret key rates and threshold qubit error rates. Furthermore we discuss quantum networks with bottlenecks for which our multipartite entanglement-based protocol can benefit from network coding, while the bipartite protocol cannot. It is shown how this advantage leads to a higher secret key rate.

Security analysis of BB84 protocol in the collective-rotation noise channel

Most of quantum cryptography protocols are designed under the ideal conditions without considering the impact of noise in actual communication; thus they may result in that the confidential information cannot be transmitted to the receiver accurately or eavesdroppers can steal the confidential information by mixing in noise. Therefore, analyzing the security of quantum cryptography protocols under noise conditions is of great significance. For the purpose of analyzing the security of quantum BB84 protocol in collective-rotation noise, firstly this paper introduces the quantum BB84 protocol, and considers the influence of environmental noise on it. An explanation should be stated that in a noise environment, the effects of noise and eavesdropping cannot be distinguished between each other. So the mechanism for which the error bit is simply used as the criterion to judge whether there exists eavesdropping in the BB84 protocol, cannot be used in the noise environment. The mechanism to judge whether there exists eavesdropping in quantum noise channel needs to be modified and improved for protecting the information. An initial qubit error rate can be set according to the noisy quantum channel. If the qubit error rate of the quantum communication channel is larger than that, it can be determined that the quantum channel is not secure and exists eavesdropping, no matter what the reason is. And on this basis, the collective-rotation noise model will be established in quantum channel by using the particle deflection model and distinguish the noise from the eavesdropping in quantum channel quantitatively, and the relationship of the amount of information that eavesdroppers can steal, the quantum bits error rate and the noise level will be analyzed by using the von Neumann entropy. Finally, the noise critical point will be calculated by using the collective noise model and the relationship between the amount of information that eavesdroppers can steal, at the quantum bits error rate, and the noise level. Through the analysis, we can know that in the existing noise level, the most of the eavesdropping can steal 25% of the key from the communication. However, the Eve's eavesdropping behavior will be detected, so that Alice and Bob will give up the current consultation key, and restart the key negotiation. This result shows that the quantum BB84 protocol is safe and secure in the collective-rotation noise channel. The research results of this paper will enrich the theory of quantum cryptography, and the innovation of security detection methods in quantum cryptographic protocols will help promote the process of practical quantum cryptography.

Robust quantum secure direct communication and authentication protocol against decoherence noise based on six-qubit DF state**Project supported by the National Natural Science Foundation of China (Grant No. 61402058), the Science and Technology Support Project of Sichuan Province of China (Grant No. 2013GZX0137), the Fund for Young Persons Project of Sichuan Province of China (Grant No. 12ZB017), and the Foundation of

By using six-qubit decoherence-free (DF) states as quantum carriers and decoy states, a robust quantum secure direct communication and authentication (QSDCA) protocol against decoherence noise is proposed. Four six-qubit DF states are used in the process of secret transmission, however only the |0′⟩ state is prepared. The other three six-qubit DF states can be obtained by permuting the outputs of the setup for |0′⟩. By using the |0′⟩ state as the decoy state, the detection rate and the qubit error rate reach 81.3%, and they will not change with the noise level. The stability and security are much higher than those of the ping–pong protocol both in an ideal scenario and a decoherence noise scenario. Even if the eavesdropper measures several qubits, exploiting the coherent relationship between these qubits, she can gain one bit of secret information with probability 0.042.

High-fidelity quantum control using ion crystals in a penning trap

We provide an introduction to the use of ion crystals in a Penning trap [1, 2, 3, 4] forexperiments in quantum information. Macroscopic Penning traps allow for the contain-ment of a few to a few million atomic ions whose internal states may be used in quantuminformation experiments. Ions are laser Doppler cooled [1], and the mutual Coulomb re-pulsion of the ions leads to the formation of crystalline arrays [5, 6, 7, 8]. The structureand dimensionality of the resulting ion crystals may be tuned using a combination ofcontrol laser beams and external potentials [9, 10]. We discuss the use of two-dimensional 9Be+ ion crystals for experimental tests ofquantum control techniques. Our primary qubit is the 124 GHz ground-state electronspin flip transition, which we drive using microwaves [11, 12]. An ion crystal representsa spatial ensemble of qubits, but the effects of inhomogeneities across a typical crystalare small, and as such we treat the ensemble as a single effective spin. We are able toinitialize the qubits in a simple state and perform a projective measurement [1] on thesystem. We demonstrate full control of the qubit Bloch vector, performing arbitrary high-fidelity rotations (τπ ∼200 µs). Randomized Benchmarking [13] demonstrates an errorper gate (a Pauli-randomized π=2 and π pulse pair) of 8±1×10-4. Ramsey interferome-try and spin-locking [14] measurements are used to elucidate the limits of qubit coherencein the system, yielding a typical free-induction decay coherence time of T2 ∼2 ms, and alimiting T1ρ ∼688 ms. These experimental specifications make ion crystals in a Penningtrap ideal candidates for novel experiments in quantum control. As such, we brieflydescribe recent efforts aimed at studying the error-suppressing capabilities of dynamicaldecoupling pulse sequences [15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26], demonstratingan ability to extend qubit coherence and suppress phase errors [11, 12]. We concludewith a discussion of future avenues for experimental exploration, including the use of ad-ditional nuclear-spin-flip transitions for effective multiqubit protocols, the potential forhardware refinements to suppress qubit error rates well-below predicted fault-tolerancethresholds, and possibilities for single-ion addressing useful for the realization of complexentangled states.

OPTIMAL EAVESDROPPING ON NOISY STATES IN QUANTUM KEY DISTRIBUTION

We study eavesdropping in quantum key distribution with the six state protocol, when the signal states are mixed with white noise. This situation may arise either when Alice deliberately adds noise to the signal states before they leave her lab, or in a realistic scenario where Eve cannot replace the noisy quantum channel by a noiseless one. We find Eve's optimal mutual information with Alice, for individual attacks, as a function of the qubit error rate. Our result is that added quantum noise reduces Eve's mutual information more than Bob's.

Optimized dynamical decoupling in a model quantum memory

Any quantum system, such as those used in quantum information or magnetic resonance, is subject to random phase errors that can dramatically affect the fidelity of a desired quantum operation or measurement. In the context of quantum information, quantum error correction techniques have been developed to correct these errors, but resource requirements are extraordinary. The realization of a physically tractable quantum information system will therefore be facilitated if qubit (quantum bit) error rates are far below the so-called fault-tolerance error threshold, predicted to be of the order of 10(-3)-10(-6). The need to realize such low error rates motivates a search for alternative strategies to suppress dephasing in quantum systems. Here we experimentally demonstrate massive suppression of qubit error rates by the application of optimized dynamical decoupling pulse sequences, using a model quantum system capable of simulating a variety of qubit technologies. We demonstrate an analytically derived pulse sequence, UDD, and find novel sequences through active, real-time experimental feedback. The latter sequences are tailored to maximize error suppression without the need for a priori knowledge of the ambient noise environment, and are capable of suppressing errors by orders of magnitude compared to other existing sequences (including the benchmark multi-pulse spin echo). Our work includes the extension of a treatment to predict qubit decoherence under realistic conditions, yielding strong agreement between experimental data and theory for arbitrary pulse sequences incorporating nonidealized control pulses. These results demonstrate the robustness of qubit memory error suppression through dynamical decoupling techniques across a variety of qubit technologies.