arXiv subjects

Joonas T. Peltonen

Publications and source records attributed to Joonas T. Peltonen.

At least 19 recordsLinked to original sources

arXiv2026

Electronic primary thermometry -- experimental comparison of the Coulomb Blockade and Shot Noise Thermometer

Since the redefinition of the kelvin in 2019, new methods of primary thermometry have been considered to replace the currently agreed temperature scale in the very high and low temperature limits. These new methods should provide improved uncertainties and, most importantly, a more direct link to the definition of the kelvin. We present an experimental comparison of two such primary thermometers working in the mK region: the Coulomb Blockade Thermometer (CBT) and the Shot Noise Thermometer (SNT). Both thermometers measure temperature hinging only on the natural constants $k_\mathrm{B}$, and $e$. Furthermore both of them are based on electron tunneling current and, thereby, need only electrical measurements, enhancing the practicality. CBT and the SNT are inter-compared in a range of 20 mK to 235 mK. The results show that the agreement of SNT and CBT is approximately within 2.5 % in this range. Basic measurement uncertainty is analyzed and we show that uncertainty in the measurement frequency can cause significant error to temperature measurement of the SNT at low temperatures where finite frequency plays a role.

cond-mat.mes-hall↗

arXiv2025

Heat measurement of quantum interference

Coherence is a key property of quantum systems, and it plays a central role in the operation and performance of quantum heat engines and refrigerators. Despite its importance for the fundamental understanding in quantum thermodynamics and its technological implications, coherence effects in heat transport have not been observed previously. Here, we measure quantum features in the heat transfer between a qubit and a thermal bath. The system is formed of a driven flux qubit galvanically coupled to a $λ/4$ coplanar-waveguide resonator that is coupled to a heat reservoir. This thermal bath is a normal-metal mesoscopic resistor, whose temperature can be measured and controlled. We detect interference patterns in the heat current due to driving-induced coherence. In particular, resonance peaks in the heat transferred to the bath are found at driving frequencies which are integer fractions of the resonator frequency. A selection rule on the even/odd parity of the peaks holds at the qubit symmetry point. We present a theoretical model based on Floquet theory that captures the experimental results. The studied system provides a platform for studying the role of coherence in quantum thermodynamics. Our work opens the possibility to demonstrate a true quantum thermal machine where heat is measured directly.

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arXiv2025

Strong-coupling quantum thermodynamics using a superconducting flux qubit

Thermodynamics in quantum circuits aims to find improved functionalities of thermal machines, highlight fundamental phenomena peculiar to quantum nature in thermodynamics, and point out limitations in quantum information processing due to coupling of the system to its environment. An important aspect to achieve some of these goals is the regime of strong coupling that has remained until now a domain of theoretical works only. Our aim is to demonstrate strong coupling features in heat transport using a superconducting flux qubit, which is capable of reaching strong to deep-ultra strong coupling regimes, as shown in previous studies. Here, we show experimental evidence of strong coupling by observing a hybridized state of the qubit with two cavities coupled to it, leading to a triplet-like thermal transport via this combined system around the minimum energy of the qubit, at power levels of tens of femtowatts, exceeding by an order of magnitude those in earlier experiments. We also demonstrate close to 100% on-off switching ratio of heat current mediated by photons by applying magnetic flux to the qubit. Our experiment opens a way towards testing debated questions in strong coupling thermodynamics such as what heat in this regime is. We also present a theoretical model that aligns with our experimental findings and explains the mechanism behind heat transport in our device. Furthermore, our experiment opens new possibilities for quantum thermodynamics, aiming to realize true quantum heat engines and refrigerators with enhanced power and efficiency, by leveraging ultra-strong coupling between the system and its environment in future experiments.

quant-ph↗

arXiv2025

Probing strongly driven and strongly coupled superconducting qubit-resonator system

We investigated a strongly driven qubit strongly connected to a quantum resonator. The measured system was a superconducting flux qubit coupled to a coplanar-waveguide resonator which is weakly coupled to a probing feedline. This hybrid qubit-resonator system was driven by a magnetic flux and probed with a weak probe signal through the feedline. We observed and theoretically described the quantum interference effects, deviating from the usual single-qubit Landau-Zener-Stückelberg-Majorana interferometry, because the strong coupling distorts the qubit energy levels.

quant-ph↗

arXiv2025

Quantum-Ready Microwave Detection with Scalable Graphene Bolometers in the Strong Localization Regime

Exploiting quantum interference of charge carriers, epitaxial graphene grown on silicon carbide emerges as a game-changing platform for ultra-sensitive bolometric sensing, featuring an intrinsic resistive thermometer response unmatched by any other graphene variant. By achieving low and uniform carrier densities, we have accessed a new regime of strong charge localization that dramatically reduces thermal conductance, significantly enhancing bolometer performance. Here we present scalable graphene-based bolometers engineered for detecting GHz-range photons, a frequency domain essential for superconducting quantum processors. Our devices deliver a state-of-the-art noise equivalent power of 40 zW$/\sqrt{\rm Hz}$ at $T=40~$mK, enabled by the steep temperature dependence of thermal conductance, $G_{\rm th}\sim T^4$ for $T<100~$mK. These results establish epitaxial graphene bolometers as versatile and low-back-action detectors, unlocking new possibilities for next-generation quantum processors and pioneering investigations into the thermodynamics and thermalization pathways of strongly entangled quantum systems.

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arXiv2025

Thermal spectrometer for superconducting circuits

Superconducting circuits provide a versatile and controllable platform for studies of fundamental quantum phenomena as well as for quantum technology applications. A conventional technique to read out the state of a quantum circuit or to characterize its properties is based on RF measurement schemes. Here we demonstrate a simple DC measurement of a thermal spectrometer to investigate properties of a superconducting circuit, in this proof-of-concept experiment a coplanar waveguide resonator. A fraction of the microwave photons in the resonator is absorbed by an on-chip bolometer, resulting in a measurable temperature rise. By monitoring the DC signal of the thermometer due to this process, we are able to determine the resonance frequency and the lineshape (quality factor) of the resonator. The demonstrated scheme, which is a simple DC measurement, offers a wide frequency band potentially reaching up to 200 GHz, far exceeding that of the typical RF spectrometer. Moreover, the thermal measurement yields a highly frequency independent reference level of the Lorentzian absorption signal. In the low power regime, the measurement is fully calibration-free. Our technique offers an alternative spectrometer for quantum circuits.

quant-ph↗

arXiv2025

Thermometry Based on a Superconducting Qubit

We report temperature measurements using a transmon qubit by detecting the population of its first three energy levels, after applying a sequence of $π$-pulses and performing projective dispersive readout. We measure the effective temperature of the qubit and characterize its relaxation and coherence times $τ_{1,2}$ for three devices in the temperature range of $20-300$ mK. We analyze the process of qubit thermalization to its effective environment consisting of multiple heat baths and support it with experimental data. Signal-to-noise (SNR) ratio of the temperature measurement depends strongly on $τ_1$, which drops at higher temperatures due to quasiparticle excitations, adversely affecting the measurements and setting an upper bound of the dynamic temperature range of the thermometer. The measurement relies on coherent dynamics of the qubit during the $π$-pulses. The effective qubit temperature follows closely that of the cryostat in the range of $100 - 250$ mK. We present a numerical model of the qubit population distribution and compare it favorably with the experimental results. Finally, we compare our technique with previous works on qubit thermometry and discuss its application prospects.

quant-ph↗

arXiv2023

Bolometric detection of Josephson inductance in a highly resistive environment

The Josephson junction is a building block of quantum circuits. Its behavior, well understood when treated as an isolated entity, is strongly affected by coupling to an electromagnetic environment. In 1983, Schmid predicted that a Josephson junction shunted by a resistance exceeding the resistance quantum $\mathbf{\textit{R}}_\mathrm{Q} = h/4e^2 \approx 6.45$ k$\mathbfΩ$ for Cooper pairs would become insulating since the phase fluctuations would destroy the coherent Josephson coupling. However, recent microwave measurements have questioned this interpretation. Here, we insert a small Josephson junction in a Johnson-Nyquist-type setup where it is driven by weak current noise arising from thermal fluctuations. Our heat probe minimally perturbs the junction's equilibrium, shedding light on features not visible in charge transport. We find that the Josephson critical current completely vanishes in DC charge transport measurement, and the junction demonstrates Coulomb blockade in agreement with the theory. Surprisingly, thermal transport measurements show that the Josephson junction acts as an inductor at high frequencies, unambiguously demonstrating that a supercurrent survives despite the Coulomb blockade observed in DC measurements. The discrepancy between these two measurements highlights the difference between the low and the high frequency response of a junction and calls for further theoretical and experimental inputs on the dynamics of Josephson junctions \textcolor{black}{operating at high frequencies in highly resistive environments.

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arXiv2023

Microwave quantum diode

The fragile nature of quantum circuits is a major bottleneck to scalable quantum applications. Operating at cryogenic temperatures, quantum circuits are highly vulnerable to amplifier backaction and external noise. Non-reciprocal microwave devices such as circulators and isolators are used for this purpose. These devices have a considerable footprint in cryostats, limiting the scalability of quantum circuits. We present a compact microwave diode architecture, which exploits the non-linearity of a superconducting flux qubit. At the qubit degeneracy point we experimentally demonstrate a significant difference between the power levels transmitted in opposite directions. The observations align with the proposed theoretical model. At -99 dBm input power, and near the qubit-resonator avoided crossing region, we report the transmission rectification ratio exceeding 90% for a 50 MHz wide frequency range from 6.81 GHz to 6.86 GHz, and over 60% for the 250 MHz range from 6.67 GHz to 6.91 GHz. The presented architecture is compact, and easily scalable towards multiple readout channels, potentially opening up diverse opportunities in quantum information, microwave read-out and optomechanics.

quant-ph↗

arXiv2022

Suppression of back-tunnelling events in hybrid single-electron turnstiles by source-drain bias modulation

Accuracy of single-electron currents produced in hybrid turnstiles at high operation frequencies is, among other errors, limited by electrons tunnelling in the wrong direction. Increasing the barrier transparency between the island and the leads, and the source-drain bias helps to suppress these events in a larger frequency range, although they lead to some additional errors. We experimentally demonstrate a driving scheme that suppresses tunnelling in the wrong direction hence extending the range of frequencies for generating accurate single-electron currents. The main feature of this approach is an additional AC signal applied to the bias with twice the frequency as the one applied to the gate electrode. This allows additional modulation of the island chemical potential. By using the new approach under certain parameters, we improve the single-electron current accuracy by one order of magnitude. Finally, we show through experimentally-contrasted calculations that our method can improve accuracy even in devices for which the usual gate driving gives errors $\sim 10^{-3}$ at high frequencies and can bring them under $5\times 10^{-4}$.

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arXiv2022

Zero-average bias bidirectional single-electron current generation in a hybrid turnstile

Hybrid turnstiles have proven to generate accurate single-electron currents. The usual operation consists of applying a periodic modulation to a capacitively coupled gate electrode and requires a non-zero DC source-drain bias voltage. Under this operation, a current of the same magnitude and opposite direction can be generated by flipping the polarity of the bias. Here, we demonstrate that accurate single-electron currents can be generated under zero average bias voltage. We achieve this by applying an extra periodic modulation with twice the frequency of the gate signal and zero DC level to the source electrode. This creates a time interval, which is otherwise zero, between the crossings of tunnelling thresholds that enable single-electron tunnelling. Furthermore, we show that within this operation the current direction can be reversed by only shifting the phase of the source signal.

cond-mat.mes-hall↗

arXiv2022

Robust strong coupling architecture in circuit quantum electrodynamics

We report on a robust method to achieve strong coupling between a superconducting flux qubit and a high-quality quarter-wavelength coplanar waveguide resonator. We demonstrate the progression from the strong to ultrastrong coupling regime by varying the length of a shared inductive coupling element, ultimately achieving a qubit-resonator coupling strength of 655 MHz, $10\%$ of the resonator frequency. We derive an analytical expression for the coupling strength in terms of circuit parameters and also discuss the maximum achievable coupling within this framework. We experimentally characterize flux qubits coupled to superconducting resonators using one and two-tone spectroscopy methods, demonstrating excellent agreement with the proposed theoretical model.

quant-ph↗

arXiv2021

Photonic heat transport in three terminal superconducting circuit

Quantum heat transport devices are currently intensively studied in theory. Experimental realization of quantum heat transport devices is a challenging task. So far, they have been mostly investigated in experiments with ultra-cold atoms and single atomic traps. Experiments with superconducting qubits have also been carried out and heat transport and heat rectification has been studied in two terminal devices. The structures with three independent terminals offer additional opportunities for realization of heat transistors, heat switches, on-chip masers and even more complicated devices. Here we report an experimental realization of a three-terminal photonic heat transport device based on a superconducting quantum circuit. Its central element is a flux qubit made of a superconducting loop containing three Josephson junctions, which is connected to three resonators terminated by resistors. By heating one of the resistors and monitoring the temperatures of the other two, we determine photonic heat currents in the system and demonstrate their tunability by magnetic field at the level of 1 aW. We determine system parameters by performing microwave transmission measurements on a separate nominally identical sample and, in this way, demonstrate clear correlation between the level splitting of the qubit and the heat currents flowing through it. Our experiment is an important step in the development of on-chip quantum heat transport devices. On the one hand, such devices are of great interest for fundamental science because they allow one to investigate the effect of quantum interference and entanglement on the transport of heat. On the other hand, they also have great practical importance for the rapidly developing field of quantum computing, in which management of heat generated by qubits is a problem.

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arXiv2021

Radio frequency Coulomb blockade thermometry

We present a scheme and demonstrate measurements of a Coulomb blockade thermometer (CBT) in a microwave transmission setup. The sensor is embedded in an $LCR$ resonator, where $R$ is determined by the conductance of the junction array of the CBT. A transmission measurement yields a signal that is directly proportional to the conductance of the CBT, thus enabling the calibration-free operation of the thermometer. This is verified by measuring an identical sensor simultaneously in the usual dc setup. The important advantage of the rf measurement is its speed: the whole bias dependence of the CBT conductance can now be measured in a time of about 100\,ms, which is thousand times faster than in a standard dc measurement. The achieved noise equivalent temperature of this first rf primary measurement is about 1 mK/$\sqrt{\rm Hz}$ at the bath temperature $T=200\,$mK.

cond-mat.mes-hall↗

arXiv2021

Frequency to power conversion by an electron turnstile

Direct frequency to power conversion (FPC), to be presented here, links both quantities through a known energy, like single-electron transport relates an operation frequency $f$ to the emitted current $I$ through the electron charge $e$ as $I=ef$. FPC is a natural candidate for a power standard resorting to the most basic definition of the watt -- energy, which is traceable to Planck's constant $h$, emitted per unit of time. This time is in turn traceable to the unperturbed ground state hyperfine transition frequency of the caesium 133 atom $Δν_\mathrm{Cs}$; hence, FPC comprises a simple and elegant way to realize the watt. In this spirit, single-photon emission and detection at known rates have been proposed and experimented as radiometric standard. However, nowadays power standards are only traceable to electrical units, i.e., volt and ohm. In this letter, we demonstrate the feasibility of an alternative proposal based on solid-state direct FPC using a SINIS (S = superconductor, N = normal metal, I = insulator) single-electron transistor (SET) accurately injecting $N$ (integer) quasiparticles (qps) per cycle to both leads with discrete energies close to their superconducting gap $Δ$, even at zero drain-source voltage. Furthermore, the bias voltage plays an important role in the distribution of the power among the two leads, allowing for an almost equal injection $NΔf$ to the two. We estimate that under appropriate conditions errors can be well below $1\%$.

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arXiv2020

Electron-phonon coupling of epigraphene at millikelvin temperatures

We investigate the basic charge and heat transport properties of charge neutral epigraphene at sub-kelvin temperatures, demonstrating nearly logarithmic dependence of electrical conductivity over more than two decades in temperature. Using graphene's sheet conductance as in-situ thermometer, we present a measurement of electron-phonon heat transport at mK temperatures and show that it obeys the $T^4$ dependence characteristic for clean two-dimensional conductor. Based on our measurement we predict the noise-equivalent power of $\sim 10^{-22}~{\rm W}/\sqrt{\rm Hz}$ of epigraphene bolometer at the low end of achievable temperatures.

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arXiv2020

Electric field control of radiative heat transfer in a superconducting circuit

Heat is detrimental for the operation of quantum systems, yet it fundamentally behaves according to quantum mechanics, being phase coherent and universally quantum-limited regardless of its carriers. Due to their robustness, superconducting circuits integrating dissipative elements are ideal candidates to emulate many-body phenomena in quantum heat transport, hitherto scarcely explored experimentally. However, their ability to tackle the underlying full physical richness is severely hindered by the exclusive use of a magnetic flux as a control parameter and requires complementary approaches. Here, we introduce a dual, magnetic field-free circuit where charge quantization in a superconducting island enables thorough electric field control. We thus tune the thermal conductance, close to its quantum limit, of a single photonic channel between two mesoscopic reservoirs. We observe heat flow oscillations originating from the competition between Cooper-pair tunnelling and Coulomb repulsion in the island, well captured by a simple model. Our results demonstrate that the duality between charge and flux extends to heat transport, with promising applications in thermal management of quantum devices.

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arXiv2020

Active quasiparticle suppression in a non-equilibrium superconductor

Quasiparticle (qp) poisoning is a major issue that impairs the operation of various superconducting devices. Even though these devices are often operated at temperatures well below the critical point where the number density of excitations is expected to be exponentially suppressed, their bare operation and stray microwave radiation excite the non-equilibrium qps. Here we use voltage-biased superconducting junctions to demonstrate and quantify qp extraction in the turnstile operation of a superconductor-insulator-normal metal-insulator-superconductor single-electron transistor. In this operation regime excitations are injected into the superconducting leads at a rate proportional to the driving frequency. We reach a reduction of density by an order of magnitude even for the highest injection rate of $2.4\times 10^8$ qps per second when extraction is turned on.

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