arXiv subjects

Anatoly Kulikov

Publications and source records attributed to Anatoly Kulikov.

10 recordsLinked to original sources

arXiv2026

A Modular Cryogenic Link for Microwave Quantum Communication Over Distances of Tens of Meters

Quantum technologies promise a radically new way to solve classically intractable computing problems. Superconducting circuits as a platform are at the forefront of this field. The cryogenic operation temperatures of superconducting circuits however impose challenges for the further scaling to many connected quantum information processing units into a local area or global network. In this work, we present a hardware solution for connecting quantum devices operating at microwave frequencies into local area networks, which enable the exchange of quantum information between spatially separated parties. Specifically, we demonstrate a modular system spanning distances of 5, 10 and 30 meters operated at cryogenic temperatures and connecting two superconducting circuit systems, located in individual dilution refrigerators, through a quantum communication channel. We develop a thermal model to evaluate the heat transfer processes in the setup, optimize the design and select appropriate materials for its construction. The assembled 30-meter-long system achieves operating temperatures of below 50 mK after a cooldown time of about six and a half days. This link enables the execution of distributed quantum computing and communication algorithms. It also adds the resource of non-locality, certified by a loophole-free Bell test, to the field of quantum science and technology with superconducting circuits.

quant-ph↗

arXiv2026

Emission and Absorption of Microwave Photons in Orthogonal Temporal Modes across a 30-Meter Two-Node Network

The tunable interaction between stationary quantum bits and propagating modes of light allows for the encoding of quantum information in the state of itinerant photons. This ability fulfills a central requirement for quantum networking, enabling quantum state transfer between distant quantum devices. Conventionally, a symmetric envelope of the photon wavepacket is used for such purposes. Yet, the use of alternative \textit{temporal modes} enables multiple applications in waveguide quantum electrodynamics that remain unexplored experimentally. Here, we use superconducting quantum circuits to generate individual itinerant microwave photons shaped in three mutually orthogonal temporal modes. We transfer the created photons across a 30-m cryogenic link, showing that the orthogonality allows us to decide at the receiver which mode to absorb, reflecting the other two with a selectivity ratio of 40. This experimental capability extends the microwave-frequency quantum communication toolbox, enabling a new photonic degree of freedom.

quant-ph↗

arXiv2024

Device-Independent Randomness Amplification

Successful realization of Bell tests has settled an 80-year-long debate, proving the existence of correlations which cannot be explained by a local realistic model. Recent experimental progress allowed to rule out any possible loopholes in these tests, and opened up the possibility of applications in cryptography envisaged more than three decades ago. A prominent example of such an application is device-independent quantum key distribution, which has recently been demonstrated. One remaining gap in all existing experiments, however, is that access to perfect randomness is assumed. To tackle this problem, the concept of randomness amplification has been introduced, allowing to generate such randomness from a weak source -- a task impossible in classical physics. In this work, we demonstrate the amplification of imperfect randomness coming from a physical source. It is achieved by building on two recent developments: The first is a theoretical protocol implementing the concept of randomness amplification within an experimentally realistic setup, which however requires a combination of the degree of Bell inequality violation (S-value) and the amount of data not attained previously. The second is experimental progress enabling the execution of a loophole-free Bell test with superconducting circuits, which offers a platform to reach the necessary combination. Our experiment marks an important step in achieving the theoretical physical limits of privacy and randomness generation.

quant-ph↗

arXiv2024

Complete Self-Testing of a System of Remote Superconducting Qubits

Self-testing protocols enable the certification of quantum systems in a device-independent manner, i.e. without knowledge of the inner workings of the quantum devices under test. Here, we demonstrate this high standard for characterization routines with superconducting circuits, a prime platform for building large-scale quantum computing systems. We first develop the missing theory allowing for the self-testing of Pauli measurements. We then self-test Bell pair generation and measurements at the same time, performing a complete self-test in a system composed of two entangled superconducting circuits operated at a separation of 30 meters. In an experiment based on 17 million trials, we measure an average CHSH (Clauser-Horne-Shimony-Holt) S-value of 2.236. Without relying on additional assumptions on the experimental setup, we certify an average Bell state fidelity of at least 58.9% and an average measurement fidelity of at least 89.5% in a device-independent manner, both with 99% confidence. This enables applications in the field of distributed quantum computing and communication with superconducting circuits, such as delegated quantum computing.

quant-ph↗

arXiv2024

How Real is Incomputability in Physics?

A physical system is determined by a finite set of initial conditions and "laws" represented by equations. The system is computable if we can solve the equations in all instances using a "finite body of mathematical knowledge". In this case, if the laws of the system can be coded into a computer program, then given the initial conditions of the system, one can compute the system's evolution. Are there incomputable physical systems? This question has been theoretically studied in the last 30-40 years. In this paper, we experimentally show for the first time the strong incomputability of a quantum experiment, namely the outputs of a quantum random number generator. Moreover, the experimental results are robust and statistically significant.

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arXiv2021

Neural networks for on-the-fly single-shot state classification

Neural networks have proven to be efficient for a number of practical applications ranging from image recognition to identifying phase transitions in quantum physics models. In this paper we investigate the application of neural networks to state classification in a single-shot quantum measurement. We use dispersive readout of a superconducting transmon circuit to demonstrate an increase in assignment fidelity for both two and three state classification. More importantly, our method is ready for on-the-fly data processing without overhead or need for large data transfer to a hard drive. In addition we demonstrate the capacity of neural networks to be trained against experimental imperfections, such as phase drift of a local oscillator in a heterodyne detection scheme.

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arXiv2020

Measuring effective temperatures of qubits using correlations

Initialization of a qubit in a pure state is a prerequisite for quantum computer operation. Qubits are commonly initialized by cooling to their ground states through passive thermalization or by using active reset protocols. To accurately quantify the initialization one requires a tool to measure the excited state population with sufficient accuracy given that the spurious excited state population may not exceed a fraction of a percent. In this Letter we propose a new technique of finding the excited state population of a qubit using correlations between two sequential measurements. We experimentally implement the proposed technique using a circuit QED platform and compare its performance with previously developed techniques. Unlike other techniques, our method does not require high-fidelity readout and does not involve the excited levels of the system outside of the qubit subspace. We experimentally demonstrated measurement of the spurious qubit population with accuracy of up to $0.01\%$. This accuracy enabled us to perform "temperature spectroscopy" of the qubit which helps to shed light on sources of the decoherence.

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arXiv2018

In situ characterization of qubit control lines: a qubit as a vector network analyzer

We present a technique to measure the transfer function of a control line using a qubit as a vector network analyzer. Our method requires coupling the line under test to the the longitudinal component of the Hamiltonian of the qubit and the ability to induce Rabi oscillations through simultaneous driving of the transversal component. We used this technique to characterize the 'flux' control of a superconducting Transmon qubit in the range of 8 to 400\,MHz. Our method can be used for the qubit 'flux' line calibration to increase the fidelity of entangling gates for the quantum processor. The qubit can be also used as a microscopic probe of the electro-magnetic fields on a chip.

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arXiv2017

Realization of a quantum random generator certified with the Kochen-Specker theorem

Random numbers are required for a variety of applications from secure communications to Monte-Carlo simulation. Yet randomness is an asymptotic property and no output string generated by a physical device can be strictly proven to be random. We report an experimental realization of a quantum random number generator (QRNG) with randomness certified by quantum contextuality and the Kochen-Specker theorem. The certification is not performed in a device-independent way but through a rigorous theoretical proof of each outcome being value-indefinite even in the presence of experimental imperfections. The analysis of the generated data confirms the incomputable nature of our QRNG.

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arXiv2016

On the chromatic numbers of small-dimensional Euclidean spaces

The paper is devoted to the study of graph sequence G_n = (V_n, E_n) where V_n is the set of all vectors v in R^n with coordinates from {-1, 0, 1} such that |v| = sqrt(3), and E_n consists of all pairs of vertices with the scalar product 1. We find exactly the independence number of G_n. As a corollary we get some new lower bounds of chi(\R^n) and chi(\Q^n) for small values of n.

math.CO↗