Showing papers by "Myungshik Kim published in 2021"
TL;DR: In this article, the authors proposed an error mitigation technique based on the assumption that noise in a deep quantum circuit is well described by global depolarizing error channels, and they used an error model ansatz to infer error-free results from noisy data.
Abstract: To get the best possible results from current quantum devices error mitigation is essential. In this work we present a simple but effective error mitigation technique based on the assumption that noise in a deep quantum circuit is well described by global depolarizing error channels. By measuring the errors directly on the device, we use an error model ansatz to infer error-free results from noisy data. We highlight the effectiveness of our mitigation via two examples of recent interest in quantum many-body physics: entanglement measurements and real-time dynamics of confinement in quantum spin chains. Our technique enables us to get quantitative results from the IBM quantum computers showing signatures of confinement, i.e., we are able to extract the meson masses of the confined excitations which were previously out of reach. Additionally, we show the applicability of this mitigation protocol in a wider setting with numerical simulations of more general tasks using a realistic error model. Our protocol is device-independent, simply implementable, and leads to large improvements in results if the global errors are well described by depolarization.
46 citations
01 Feb 2021
TL;DR: A review of the experimental state of the art and discuss promising routes towards quantum rotations can be found in this paper, where the rotations of levitated particles can show pronounced quantum effects enabling tests of quantum physics and torque measurements with unprecedented sensitivity.
Abstract: Rotations of microscale rigid bodies exhibit pronounced quantum phenomena that do not exist for their centre-of-mass motion. By levitating nanoparticles in ultra-high vacuum, researchers are developing a promising platform for observing and exploiting these quantum effects in an unexplored mass and size regime. Recent experimental and theoretical breakthroughs demonstrate exquisite control of nanoscale rotations, setting the stage for the first tabletop tests of rotational superpositions and for the next generation of ultra-precise torque sensors. Here, we review the experimental state of the art and discuss promising routes towards quantum rotations. The rotations of levitated particles can show pronounced quantum effects, enabling tests of quantum physics and torque measurements with unprecedented sensitivity. Breakthroughs in cooling and controlling nanorotors set the stage for such experiments.
31 citations
14 Oct 2021
TL;DR: In this paper, quantum geometric measures are presented to analyze parameterized quantum circuits, allowing one to identify improved quantum circuits and initialization techniques that enhance the performance of variational quantum algorithms.
Abstract: Quantum geometric measures are presented to analyze parameterized quantum circuits, allowing one to identify improved quantum circuits and initialization techniques that enhance the performance of variational quantum algorithms.
22 citations
28 Jun 2021
TL;DR: In this paper, a method combining restricted Boltzmann machines with feed-forward neural networks is devised for efficiently calculating observables of a measured quantum state, which can be used for quantum computing.
Abstract: A method combining restricted Boltzmann machines with feed-forward neural networks is devised for efficiently calculating observables of a measured quantum state.
13 citations
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TL;DR: In this article, the authors evaluate the capacity and trainability of parametrized quantum circuits using the geometric structure of the parameter space via the effective quantum dimension, which reveals the expressive power of circuits in general as well as of particular initialization strategies.
Abstract: To harness the potential of noisy intermediate-scale quantum devices, it is paramount to find the best type of circuits to run hybrid quantum-classical algorithms. Key candidates are parametrized quantum circuits that can be effectively implemented on current devices. Here, we evaluate the capacity and trainability of these circuits using the geometric structure of the parameter space via the effective quantum dimension, which reveals the expressive power of circuits in general as well as of particular initialization strategies. We assess the representation power of various popular circuit types and find striking differences depending on the type of entangling gates used. Particular circuits are characterized by scaling laws in their expressiveness. We identify a transition in the quantum geometry of the parameter space, which leads to a decay of the quantum natural gradient for deep circuits. For shallow circuits, the quantum natural gradient can be orders of magnitude larger in value compared to the regular gradient; however, both of them can suffer from vanishing gradients. By tuning a fixed set of circuit parameters to randomized ones, we find a region where the circuit is expressive, but does not suffer from barren plateaus, hinting at a good way to initialize circuits. Our results enhance the understanding of parametrized quantum circuits for improving variational quantum algorithms.
13 citations
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TL;DR: In this paper, the generalized quantum natural gradient (GQN) is proposed to solve the problem of the barren plateau problem of gradient ascent for VQA. But, the gradient of the GQN does not vanish when the fidelity between the initial state and the state to be learned is bounded.
Abstract: Variational quantum algorithms (VQAs) promise efficient use of near-term quantum computers. However, training VQAs often requires an extensive amount of time and suffers from the barren plateau problem where the magnitude of the gradients vanishes with increasing number of qubits. Here, we show how to optimally train VQAs for learning quantum states. Parameterized quantum circuits can form Gaussian kernels, which we use to derive adaptive learning rates for gradient ascent. We introduce the generalized quantum natural gradient that features stability and optimized movement in parameter space. Both methods together outperform other optimization routines in training VQAs. Our methods also excel at numerically optimizing driving protocols for quantum control problems. The gradients of the VQA do not vanish when the fidelity between the initial state and the state to be learned is bounded from below. We identify a VQA for quantum simulation with such a constraint that thus can be trained free of barren plateaus. Finally, we propose the application of Gaussian kernels for quantum machine learning.
13 citations
TL;DR: In this article, the authors proposed an optimal quantum teleportation protocol for noisy quantum channels with a limited number of qubits by analyzing the generalized quantum measurement (GQM) problem.
Abstract: Quantum teleportation exemplifies how the transmission of quantum information starkly differs from that of classical information and serves as a key protocol for quantum communication and quantum computing. While an ideal teleportation protocol requires noiseless quantum channels to share a pure maximally entangled state, the reality is that shared entanglement is often severely degraded due to various decoherence mechanisms. Although the quantum noise induced by the decoherence is indeed a major obstacle to realizing a near-term quantum network or processor with a limited number of qubits, the methodologies considered thus far to address this issue are resource-intensive. Here, we demonstrate a protocol that allows optimal quantum teleportation via noisy quantum channels without additional qubit resources. By analyzing teleportation in the framework of generalized quantum measurement, we optimize the teleportation protocol for noisy quantum channels. In particular, we experimentally demonstrate that our protocol enables to teleport an unknown qubit even via a single copy of an entangled state under strong decoherence that would otherwise preclude any quantum operation. Our work provides a useful methodology for practically coping with decoherence with a limited number of qubits and paves the way for realizing noisy intermediate-scale quantum computing and quantum communication.
11 citations
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05 Jan 2021
10 citations
TL;DR: In this paper, the authors present a method to run quantum computers without qubit readout errors, which is a substantial barrier to running quantum alg.... But they do not discuss how to detect qubit errors.
Abstract: Quantum computers are becoming increasingly accessible and may soon outperform classical computers for useful tasks. However, qubit readout errors remain a substantial hurdle to running quantum alg...
10 citations
TL;DR: In this article, an optimisation method for variational quantum algorithms is introduced and experimentally demonstrated by obtaining multi-dimensional energy surfaces for small molecules and a spin model. But this method is not suitable for the next generation of variational problems with many physical degrees of freedom.
Abstract: We introduce an optimisation method for variational quantum algorithms and experimentally demonstrate a 100-fold improvement in efficiency compared to naive implementations. The effectiveness of our approach is shown by obtaining multi-dimensional energy surfaces for small molecules and a spin model. Our method solves related variational problems in parallel by exploiting the global nature of Bayesian optimisation and sharing information between different optimisers. Parallelisation makes our method ideally suited to the next generation of variational problems with many physical degrees of freedom. This addresses a key challenge in scaling-up quantum algorithms towards demonstrating quantum advantage for problems of real-world interest.
9 citations
TL;DR: In this article, it was shown that the Leggett-garg inequalities are violated by convex sums of coherent states, which is not a suitable class of descriptions for classical dynamics.
Abstract: In an endeavor to better define the distinction between classical macroscopic and quantum microscopic regimes, the Leggett-Garg inequalities were established as a test of macroscopic-realistic theories, which are commonly thought to be a suitable class of descriptions for classical dynamics. The relationship between their violation and nonclassicality is however not obvious. We show that classical states of light, which in the quantum optical sense are any convex sums of coherent states, may not satisfy the Leggett-Garg inequalities. After introducing a simple Mach-Zehnder setup and showing how to obtain a violation with a single photon using negative measurements, we focus on classical states of light, in particular those of low average photon number. We demonstrate how one can still perform negative measurements with an appropriate assignment of variables, and show that the inequalities are violable with coherent states. Finally, we abandon the initial phase reference and demonstrate that the violation is still possible, in particular with thermal states of light, and we investigate the effect of intermediate dephasing.
10 Mar 2021
TL;DR: This paper proposes a scheme for unconditionally secure qubit-commitment that is a quantum cryptographic primitive forbidden by the recently proven no-masking theorem in the standard model, and demonstrates an operational setting where neither maximally classically-correlated state nor maximally entangled state is more valuable resource.
Abstract: The commodity-based cryptography is an alternative approach to realize conventionally impossible cryptographic primitives such as unconditionally secure bit-commitment by consuming pre-established correlation between distrustful participants. A unit of such classical correlation is known as the one-time table (OTT). In this paper, we introduce a new example besides quantum key distribution in which quantum correlation is useful for cryptography. We propose a scheme for unconditionally secure qubit-commitment, a quantum cryptographic primitive forbidden by the recently proven no-masking theorem in the standard model, based on the consumption of the quantum generalization of the OTT, the bipartite quantum state we named quantum one-time tables (QOTT). The construction of the QOTT is based on the newly analyzed internal structure of quantum masker and the quantum secret sharing schemes. Our qubit-commitment scheme is shown to be universally composable. We propose to measure the randomness cost of preparing a (Q)OTT in terms of its entropy, and show that the QOTT with superdense coding can increase the security level with half the cost of OTTs for unconditionally secure bit-commitment. The QOTT exemplifies an operational setting where neither maximally classically correlated state nor maximally entangled state, but rather a well-structured partially entangled mixed state is more valuable resource.
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TL;DR: In this paper, a natural parameterized quantum circuit (NPQC) with a euclidean quantum geometry is introduced, which can be used for various tasks including quantum state preparation and variational quantum algorithms.
Abstract: Noisy intermediate scale quantum computers are useful for various tasks including quantum state preparation, quantum metrology and variational quantum algorithms. However, the non-euclidean quantum geometry of parameterized quantum circuits is detrimental for these applications. Here, we introduce the natural parameterized quantum circuit (NPQC) with a euclidean quantum geometry. The initial training of variational quantum algorithms is substantially sped up as the gradient is equivalent to the quantum natural gradient. NPQCs can also be used as highly accurate multi-parameter quantum sensors. For a general class of quantum circuits, the NPQC has the minimal quantum Cramer-Rao bound. We provide an efficient sensing protocol that only requires sampling in the computational basis. Finally, we show how to generate tailored superposition states without training. These applications can be realized for any number of qubits with currently available quantum processors.
04 Jun 2021
TL;DR: In this paper, a nanocrystal matter-wave interferometer was used to create a state-of-the-art quantum sensor to detect low accelerations and discuss how it can be used to probe quantum aspects of gravity in a laboratory.
Abstract: The authors show that a nanocrystal matter-wave interferometer can be used to create a state-of-the-art quantum sensor to detect low accelerations and discuss how it can be used to probe quantum aspects of gravity in a laboratory.
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TL;DR: In this article, the quantum Fisher information metric is used to encode high-dimensional data into quantum computers with the number of features scaling linearly with the circuit depth, which is characterized by the radial basis function kernel.
Abstract: Quantum computers promise to enhance machine learning for practical applications. Quantum machine learning for real-world data has to handle extensive amounts of high-dimensional data. However, conventional methods for measuring quantum kernels are impractical for large datasets as they scale with the square of the dataset size. Here, we measure quantum kernels using randomized measurements to gain a quadratic speedup in computation time and quickly process large datasets. Further, we efficiently encode high-dimensional data into quantum computers with the number of features scaling linearly with the circuit depth. The encoding is characterized by the quantum Fisher information metric and is related to the radial basis function kernel. We demonstrate the advantages of our methods by classifying images with the IBM quantum computer. To achieve further speedups we distribute the quantum computational tasks between different quantum computers. Our approach is exceptionally robust to noise via a complementary error mitigation scheme. Using currently available quantum computers, the MNIST database can be processed within 220 hours instead of 10 years which opens up industrial applications of quantum machine learning.
TL;DR: In this paper, it was shown that the revolutions of symmetric nanorotors can be strongly affected by a small number of intrinsic spins, and the resulting dynamics are observable with freely rotating nanodiamonds with embedded nitrogen-vacancy centers and persist for realistically shaped near-symmetric particles.
Abstract: Spin angular momentum and mechanical rotation both contribute to the total angular momentum of rigid bodies, leading to spin-rotational coupling via the Einstein--de Haas and Barnett effects. Here, we show that the revolutions of symmetric nanorotors can be strongly affected by a small number of intrinsic spins. The resulting dynamics are observable with freely rotating nanodiamonds with embedded nitrogen-vacancy centers and persist for realistically shaped near-symmetric particles, opening the door to torque-free schemes to control their rotations at the quantum level.
TL;DR: This work uses a photon-number-resolving detector, the transition-edge sensor, to discriminate binary-phase-shifted coherent states at a telecom wavelength and achieves a bit error probability that unconditionally exceeds the standard quantum limit (SQL) by up to 7.7 dB.
Abstract: The discrimination of coherent states is a key task in optical communication and quantum key distribution protocols. In this work, we use a photon-number-resolving detector, the transition-edge sensor, to discriminate binary-phase-shifted coherent states at a telecom wavelength. Owing to its dynamic range and high efficiency, we achieve a bit error probability that unconditionally exceeds the standard quantum limit (SQL) by up to 7.7 dB. The improvement to the SQL persists for signals containing up to approximately seven photons on average and is achieved in a single shot (i.e., without measurement feedback), thus making our approach compatible with larger bandwidths.
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TL;DR: In this article, a semi-classical model of an atom interferometer with a mesoscopic oscillator has been proposed to detect entanglement in a quantum system, where the oscillator is subject to a random unitary channel.
Abstract: Combining gravity with quantum mechanics remains one of the biggest challenges of physics. In the past years, experiments with opto-mechanical systems have been proposed that may give indirect clues about the quantum nature of gravity. In a recent variation of such tests [D. Carney et al., Phys.Rev.X Quantum 2, 030330 (2021)], the authors propose to gravitationally entangle an atom interferometer with a mesoscopic oscillator. The interaction results in periodic drops and revivals of the interferometeric visibility, which under specific assumptions indicate the gravitational generation of entanglement. Here we study semi-classical models of the atom interferometer that can reproduce the same effect. We show that the core signature -- periodic collapses and revivals of the visibility -- can appear if the atom is subject to a random unitary channel, including the case where the oscillator is fully classical and situations even without explicit modelling of the oscillator. We also show that the non-classicality of the oscillator vanishes unless the system is very close to its ground state, and even when the system is in the ground state, the non-classicality is limited by the coupling strength. Our results thus indicate that deducing entanglement from the proposed experiment is very challenging, since fulfilling and verifying the non-classicality assumptions is a significant challenge on its own right.
TL;DR: In this article, a photon-number-resolving detector, the transition-edge sensor, is used to discriminate binary-phase-shifted coherent states at a telecom wavelength, achieving a bit error probability that unconditionally exceeds the standard quantum limit by up to 7.7 dB.
Abstract: The discrimination of coherent states is a key task in optical communication and quantum key distribution protocols. In this work, we use a photon-number-resolving detector, the transition-edge sensor, to discriminate binary-phase-shifted coherent states at a telecom wavelength. Owing to its dynamic range and high efficiency, we achieve a bit error probability that unconditionally exceeds the standard quantum limit (SQL) by up to 7.7 dB. The improvement to the SQL persists for signals containing up to approximately seven photons on average and is achieved in a single shot (i.e. without measurement feedback), thus making our approach compatible with larger bandwidths.
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TL;DR: Although quantum mechanics is essential to understand microscopic systems, it has little effect on heavier objects as discussed by the authors, and experiments have now put strict constraints on theories that use gravity to explain the absence of large-scale quantum effects.
Abstract: Although quantum mechanics is essential to understand microscopic systems, it has little effect on heavier objects. Experiments have now put strict constraints on theories that use gravity to explain the absence of large-scale quantum effects.
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TL;DR: In this article, the authors present a scheme to more efficiently mitigate readout errors on quantum hardware and numerically show that their method consistently gives advantage over previous mitigation schemes and can be combined with other mitigation methods allowing tractable mitigation even for large numbers of qubits.
Abstract: Quantum computers are becoming increasingly accessible, and may soon outperform classical computers for useful tasks. However, qubit readout errors remain a significant hurdle to running quantum algorithms on current devices. We present a scheme to more efficiently mitigate these errors on quantum hardware and numerically show that our method consistently gives advantage over previous mitigation schemes. Our scheme removes biases in the readout errors allowing a general error model to be built with far fewer calibration measurements. Specifically, for reading out $n$-qubits we show a factor of $2^n$ reduction in the number of calibration measurements without sacrificing the ability to compensate for correlated errors. Our approach can be combined with, and simplify, other mitigation methods allowing tractable mitigation even for large numbers of qubits.
TL;DR: In this article, the authors review the experimental state of the art and discuss promising routes towards macroscopic quantum rotations, including table-top tests of rotational superpositions and for the next generation of ultra-precise torque sensors.
Abstract: Rotations of microscale rigid bodies exhibit pronounced quantum phenomena that do not exist for their center-of-mass motion By levitating nanoparticles in ultra-high vacuum, researchers are developing a promising platform for observing and exploiting these quantum effects in an unexplored mass and size regime Recent experimental and theoretical breakthroughs demonstrate exquisite control of nanoscale rotations, setting the stage for the first table-top tests of rotational superpositions and for the next generation of ultra-precise torque sensors Here, we review the experimental state of the art and discuss promising routes towards macroscopic quantum rotations
Posted Content•
TL;DR: In this paper, it was shown that the revolutions of symmetric nanorotors can be strongly affected by a small number of intrinsic spins, and the resulting dynamics are observable with freely rotating nanodiamonds with embedded nitrogen-vacancy centers.
Abstract: Spin angular momentum and mechanical rotation both contribute to the total angular momentum of rigid bodies, leading to spin-rotational coupling via the Einstein-de Haas and Barnett effects. Here we show that the revolutions of symmetric nanorotors can be strongly affected by a small number of intrinsic spins. The resulting dynamics are observable with freely rotating nanodiamonds with embedded nitrogen-vacancy centers and persist for realistically-shaped near-symmetric particles, opening the door to torque-free schemes to control their rotations at the quantum level.
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TL;DR: In this article, a scheme based on intensity interferometry is proposed to measure the joint spectral mode of photon pairs produced by spontaneous parametric down-conversion, which does not require phase stability, nonlinearities, or spectral shaping, and is an experimentally simple way of measuring the modal structure of quantum light.
Abstract: The time-frequency structure of quantum light can be manipulated for information processing and metrology. Characterizing this structure is also important for developing quantum light sources with high modal purity that can interfere with other independent sources. Here, we present and experimentally demonstrate a scheme based on intensity interferometry to measure the joint spectral mode of photon pairs produced by spontaneous parametric down-conversion. We observe correlations in the spectral phase of the photons due to chirp in the pump. We also show that our scheme can be combined with stimulated emission tomography to quickly measure their mode using bright classical light. Our scheme does not require phase stability, nonlinearities, or spectral shaping, and thus is an experimentally simple way of measuring the modal structure of quantum light.
TL;DR: In this article, a new optimisation method for variational quantum algorithms is introduced and experimentally demonstrated a 100-fold improvement in efficiency compared to naive implementations. But the method is not suitable for the next generation of variational problems with many physical degrees of freedom.
Abstract: We introduce a new optimisation method for variational quantum algorithms and experimentally demonstrate a 100-fold improvement in efficiency compared to naive implementations. The effectiveness of our approach is shown by obtaining multi-dimensional energy surfaces for small molecules and a spin model. Our method solves related variational problems in parallel by exploiting the global nature of Bayesian optimisation and sharing information between different optimisers. Parallelisation makes our method ideally suited to next generation of variational problems with many physical degrees of freedom. This addresses a key challenge in scaling-up quantum algorithms towards demonstrating quantum advantage for problems of real-world interest.