Talks Tuesday June 28

Faculty @ Washington U. in St. Louis, US

Tuesday June 28 – 18.30 BST

Energetic cost of measurements using quantum, coherent, and thermal light

In the quantum information literature, the basic concepts are unitary operators, quantum gates, entangling operations, and measurements. These are all applied to accomplish tasks in quantum information processing. However, much like classical computers, all logical operations demand resources. In particular, dissipative operations, such as measurement and erasure, are nonconservative operations, and must also generate entropy – and are therefore connected to and constrained by the laws of thermodynamics. I will present our recent experimental work investigating quantum measurement from the perspective of thermodynamical and quantum resources. Using a superconducting qubit and the physics of cavity quantum electrodynamics, we have characterized measurement efficiency and effectiveness for different thermal, coherent, and quantum states of light. In particular, we find that thermal light is capable of performing quantum measurements with comparable efficiency to coherent light, both being outperformed by single-photon light. These experiments elucidate the physics of quantum measurement, deepening our understanding of the thermodynamics of quantum information. 

Faculty @ Universidad de Buenos Aires, Argentina

Tuesday June 28 – 17.45 BST

TBA

Undergraduate @ Institute Néel 

Tuesday June 28 – 17.15 BST

The thermodynamic cost of quantum measurements in the circuit quantum electrodynamics architecture

Since work can be extracted from coherences in the eigenenergy basis [1], conversely, measuring a state in this basis can have a cost. Theoretically, this cost is proportional to the QC-mutual information obtained from the measurement~[2], but this bound is not always reachable in the experimental context. Since some resources, such as thermal states, are much easier to produce than others, one could wonder about their measuring capabilities. Here, in a circuit quantum electrodynamics setup, we investigate the resources required to perform quantum measurements of a qubit. We compare the measurement backaction and signal-to-noise ratio of single-photon, coherent and thermal fields. We find that in the strong dispersive limit the thermal light is capable of performing quantum measurements with comparable figure of merit to coherent light. Furthermore, we analyze the energetic and entropic costs of these quantum measurements at different measurement strengths and investigate the fundamental reasons behind these experimental results. This work demonstrates a new, efficient approach to quantum measurements in circuit quantum electrodynamics and provides a new point of view to the energetic cost of a measurement. 
 
[1] P. Kammerlander, J. Anders, “Coherence and measurement in quantum thermodynamics”, Scientific Reports 6, (2016). 
[2] T. Sagawa, M. Ueda, “Minimal Energy Cost for Thermodynamic Information Processing: Measurement and Information Erasure”, Phys. Rev. Let. 102. (2009). 
*This work was supported by the John Templeton Foundation, Grant No. 61835 

Postdoc @ University of Vienna

Tuesday June 28 – 16.45 BST

Experimental nonequilibrium memory erasure beyond Landauer’s bound 

Logically irreversible transformations unavoidably consume power and cause heat dissipation. On a fundamental level, Landauer’s bound provides the minimum value of these energetic costs when erasing one bit of information in equilibrium with its environment [1]. Several proof-of-concept experiments have successfully reached this limit [2-6]. Yet, real devices operate out of equilibrium, calling for a deeper understanding of the underlying thermodynamic laws in this regime [7-9]. Here, we demonstrate the possibility to control the thermodynamics of a two-state memory by separating nonequilibrium memory preparation from memory processing [10]. Specifically, we report the first experimental demonstration of the full reset of one bit of information that evades the seemingly inevitable heat dissipation when stored in an out-of-equilibrium state. To achieve this, we developed an electro-optical double-well trap for levitated nanoparticles that can be shaped dynamically, allowing for precise and fast memory state preparation and processing. Our results indicate that far-from-equilibrium thermodynamics can offer a route for heat management in computational architectures paving the road for the extension of the research question to the quantum regime. 

[1] Landauer, R. Irreversibility and heat generation in the computing process. IBM J. Res. Dev. 5, 183 191 (1961). 
[2] Bérut, A. et al. Experimental verification of Landauer’s principle linking information and thermodynamics. Nature 484, 187 189 (2012). 
[3] Orlov, A. O., Lent, C. S., Thorpe, C. C., Boechler, G. P. & Snider, G. L. Experimental test of Landauer’s principle at the sub-kBT level. Japanese Journal of Applied Physics 51, 06FE10 (2012). 
[4] Jun, Y., Gavrilov, M. & Bechhoefer, J. High-precision test of Landauer’s principle in a feedback trap. Phys. Rev. Lett. 113, 190601 (2014). 
[5] Hong, J., Lambson, B., Dhuey, S. & Bokor, J. Experimental test of Landauer’s principle in single-bit operations on nanomagnetic memory bits. Science Advances 2, 3 (2016). 
[6] Dago, S., Pereda, J., Barros, N., Ciliberto, S. & Bellon, L. Information and thermodynamics: Fast and precise approach to landauer’s bound in an underdamped micromechanical oscillator. Phys. Rev. Lett. 126, 170601 (2021). 
[7] Esposito, M. & den Broeck, C. V. Second law and Landauer principle far from equilibrium. EPL (Europhysics Letters) 95, 40004 (2011). 
[8] Wolpert, D. H. The stochastic thermodynamics of computation. J. Phys. A: Math. Theor. 52, 193001 (2019). 
[9] Konopik, M., Friedenberger, A., Kiesel, N. & Lutz, E. Nonequilibrium information erasure below kTln2. EPL 131, 6004 (2020) 
[10] Ciampini, M.A. et al. Experimental nonequilibrium memory erasure beyond Landauer’s bound, arXiv:2107.04429 

Postdoc @ Institute for Quantum Studies, Chapman University, Orange, California, 92866, USA

Tuesday June 28 – 15.45 BST

Continuous measurement boosted adiabatic quantum thermal machines 

We present a unified approach to study continuous measurement based quantum thermal machines in static as well as adiabatically driven systems. We investigate both steady state and transient dynamics for the time-independent case. In the adiabatically driven case, we show how measurement based thermodynamic quantities can be attributed geometric characteristics. We also provide the appropriate definition for heat transfer and dissipation owing to continous measurement in the presence and absence of adiabatic driving. We illustrate the aforementioned ideas and study the phenomena of refrigeration in two different paradigmatic examples: a coupled quantum dot and a coupled qubit system, both undergoing continuous measurement and slow driving. In the time-independent case, we show that quantum coherence can improve the cooling power of measurement based quantum refrigerators. Exclusively for the case of coupled qubits, we consider linear as well as non-linear system-bath couplings. We observe that non-linear coupling produces cooling effects in certain regime where otherwise heating is expected. In the adiabatically driven case, we observe that quantum measurement can provide significant boost to the power of adiabatic quantum refrigerators. The measurement based refrigerators can have similar or better coefficient of performance (COP) in the driven case compared to the static one in the regime where heat extraction is maximum. Our results have potential significance for future application in devices ranging from measurement based quantum thermal machines to refrigeration in quantum processing networks.  

B. Bhandari and A. N. Jordan arXiv:2112.03971 (2021) 

Postdoc @ University of Geneva 

Tuesday June 28 – 15.15 BST

Fundamental limits in Bayesian thermometry and attainability via adaptive strategies 

We investigate the limits of quantum thermometry using quantum probes at thermal equilibrium within the Bayesian approach. We consider the possibility of engineering interactions between the probes in order to enhance their sensitivity, as well as feedback during the measurement process, i.e., adaptive protocols. On the one hand, we obtain an ultimate bound on thermometry precision in the Bayesian setting, valid for arbitrary interactions and measurement schemes, which lower bounds the error with a quadratic (Heisenberg-like) scaling with the number of probes. We develop an adaptive strategy that can saturate this limit. On the other hand, we derive a no-go theorem for non-adaptive protocols that does not allow for better than linear (shot-noise-like) scaling even if one has unlimited control over the probes, namely access to arbitrary many-body interactions. Our work highlights the crucial role of both feedback and many-body interactions in quantum thermometry. This talk is based on [1]. 

[1] Fundamental limits in Bayesian thermometry and attainability via adaptive strategies, Mohammad Mehboudi, Mathias R. Jørgensen, Stella Seah, Jonatan B. Brask, Jan Kołodyński, Martí Perarnau-Llobet,   arXiv:2108.05932 (2021).

Postgraduate @ Aalto University

Tuesday June 28 – 14.45 BST

An electron turnstile for frequency-to-power conversion

Nanometric normal-metal islands coupled to superconducting leads through insulating tunnel junctions are suitable as single-electron turnstiles, when its excess charge is periodically driven through a capacitively coupled gate electrode. The superconducting leads extend the stability zone along the whole gate-voltage parameter space making it possible to create single-electron currents by only allowing one tunnelling event per cycle per junction [1]. In this device, electric charge is carried by superconducting excitations which are injected into the leads in each tunnelling event. These excitations are created close to the superconducting gap edge. Consequently, the energy current, that is, power injected to the leads is given by a simple frequency-to-power conversion relation, namely the superconducting energy gap times the driving signal frequency $\left( P=\Delta f\right)$. Such a simple relation enables this device as a candidate for a power standard, analogue to the frequency-to-current conversion of single-electron turnstiles. This is experimentally demonstrated [2] as a first proof-of-concept. The power production is shown to be possible even in the absence of particle current. Moreover, the dynamics of power repartition among both junctions is studied. Further improvements in the accuracy of the power emission are proposed. 

[1] Pekola, J. P. Vartiainen, J. J. Möttönen, M. Saira, O-P. Meschke, M. Averin, D. Hybrid single-electron transistor as a source of quantized electric current. Nat. Phys. 4, 2007. 
[2] Marín-Suárez, M. Peltonen, J. T. Golubev, D. S. Pekola, J. P. An electron turnstile for frequency-to-power conversion. Nat. Nanotechnol., 2022. 

Faculty @ CNRS, University Paris-Sud, University Paris-Saclay, France

Tuesday June 28 – 14.00 BST

Coherence-powered work exchanges between a solid-state qubit and light fields 

We explore how quantum coherence impacts energy exchanges between a solid-state quantum bit and light fields. Following pioneering theoretical frameworks, we first experimentally study the work transferred during the spontaneous emission of a solid-state qubit into a reservoir of modes of the electromagnetic field. This step energetically corresponds to the charging of a quantum battery and we show that the amount of transferred work is proportional to the initial quantum coherence of the qubit, and is reduced at higher temperatures. In a second step, we study the discharge of the battery and its energy transfer to a classical- i.e.- laser field using homodyne-type measurements. We demonstrate that the amount of energy and work transferred to the laser field is controlled by the relative classical optical phase between the two fields, the quantum purity of the charged battery field as theoretically predicted, as well as long-term fluctuations in the qubit solid-state environment.