| 1. |
- Geier, Max, et al.
(author)
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Fermion-parity qubit in a proximitized double quantum dot
- 2024
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In: Physical Review Research. - 2643-1564. ; 6:2
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Journal article (peer-reviewed)abstract
- Bound states in quantum dots coupled to superconductors can be in a coherent superposition of states with different electron number but with the same fermion parity. Electrostatic gating can tune this superposition to a sweet spot, where the quantum dot has the same mean electric charge independent of its electron-number parity. Here, we propose to encode quantum information in the local fermion parity of two tunnel-coupled quantum dots embedded in a Josephson junction. At the sweet spot, the qubit states have zero charge dipole moment. This protects the qubit from dephasing due to charge noise acting on the potential of each dot, as well as fluctuations of the (weak) interdot tunneling. At weak interdot tunneling, relaxation is suppressed because of disjoint qubit states. However, for strong interdot tunneling the system is protected against noise affecting each quantum dot separately (energy-level noise, dot-superconductor tunneling fluctuations, and hyperfine interactions). Finally, we describe initialization and readout as well as single-qubit and two-qubit gates by pulsing gate voltages.
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| 2. |
- Monsel, Juliette, 1994, et al.
(author)
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Non-geometric pumping effects on the performance of interacting quantum-dot heat engines
- 2023
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In: European Physical Journal: Special Topics. - 1951-6401 .- 1951-6355. ; 232:20-22, s. 3267-3272
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Journal article (peer-reviewed)abstract
- Periodically driven quantum dots can act as counterparts of cyclic thermal machines at the nanoscale. In the slow-driving regime of geometric pumping, such machines have been shown to operate in analogy to a Carnot cycle. For larger driving frequencies, which are required to increase the cooling power, the efficiency of the operation decreases. Up to which frequency a close-to-optimal performance is still possible depends on the magnitude and sign of on-site electron–electron interaction. Extending our previous detailed study on cyclic quantum-dot refrigerators [Phys. Rev. B 106, 035405 (2022)], we here find that the optimal cooling power remains constant up to weak interaction strength compared to the cold-bath temperature. By contrast, the work cost depends on the interaction via the dot’s charge relaxation rate, as the latter sets the typical driving frequency for the onset of non-geometric pumping contributions.
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| 3. |
- Nielsen, Ida Egholm, et al.
(author)
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Dynamics of parafermionic states in transport measurements
- 2023
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In: SciPost Physics. - 2542-4653. ; 15:5
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Journal article (peer-reviewed)abstract
- Advances in hybrid fractional quantum Hall (FQH)-superconductor platforms pave the way for realisation of parafermionic modes. We analyse signatures of these non-abelian anyons in transport measurements across devices with Z6 parafermions (PFs) coupled to an external electrode. Simulating the dynamics of these open systems by a stochastic quantum jump method, we show that a current readout over sufficiently long times constitutes a projective measurement of the fractional charge shared by two PFs. Interaction of these topological modes with the FQH environment, however, may cause poisoning events affecting this degree of freedom which we model by jump operators that describe incoherent coupling of PFs with FQH edge modes. We analyse how this gives rise to a characteristic three-level telegraph noise in the current, constituting a very strong signature of PFs. We discuss also other forms of poisoning and noise caused by interaction with fractional quasiparticles in the bulk of the Hall system. We conclude our work with an analysis of four-PF devices, in particular on how the PF fusion algebra can be observed in electrical transport experiments.
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| 4. |
- Schulenborg, Jens, 1988, et al.
(author)
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Detection of the relaxation rates of an interacting quantum dot by a capacitively coupled sensor dot
- 2014
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In: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 89:19, s. 195305-
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Journal article (peer-reviewed)abstract
- We present a theoretical study of the detection of the decay time scales for a single-level quantum dot by means of a capacitively coupled sensor dot, which acts as an electrometer. We investigate the measurement back-action on the quantum-dot decay rates and elucidate its mechanism. We explicitly show that the setup can be used to measure the bare quantum-dot relaxation rates by choosing gate pulses that minimize the back-action. Interestingly, we find that besides the charge relaxation rate, also the rate associated to the fermion parity in the dot can be accessed with this setup. © 2014 American Physical Society.
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| 5. |
- Schulenborg, Jens, 1988, et al.
(author)
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Duality for open fermion systems: Energy-dependent weak coupling and quantum master equations
- 2018
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In: Physical Review B. - 2469-9969 .- 2469-9950. ; 98:23
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Journal article (peer-reviewed)abstract
- Open fermion systems with energy-independent bilinear coupling to a fermionic environment have been shown to obey a general duality relation [J. Schulenborg, Phys. Rev. B 93, 081411 (2016)2469-995010.1103/PhysRevB.93.081411] which allows for a drastic simplification of time-evolution calculations. In the weak-coupling limit, such a system can be associated with a unique dual physical system in which all energies are inverted, in particular the internal interaction. This paper generalizes this fermionic duality in two ways: we allow for weak coupling with arbitrary energy dependence and describe both occupations and coherences coupled by a quantum master equation for the density operator. We also show that whenever generalized detailed balance holds (Kolmogorov criterion), the stationary probabilities for the dual system can be expressed explicitly in terms of the stationary recurrence times of the original system, even at large bias. We illustrate the generalized duality by a detailed analysis of the rate equation for a quantum dot with strong onsite Coulomb repulsion, going beyond the commonly assumed wide-band limit. We present predictions for (i) the decay rates for transient charge and heat currents after a gate-voltage quench and (ii) the thermoelectric linear-response coefficients in the stationary limit. We show that even for pronouncedly energy-dependent coupling, all nontrivial parameter dependence in these problems is entirely captured by just two well-understood stationary variables, the average charge of the system and of the dual system. Remarkably, it is the latter that often dictates the most striking features of the measurable quantities (e.g., positions of resonances), underscoring the importance of the dual system for understanding the actual one.
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| 6. |
- Schulenborg, Jens, 1988
(author)
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Dynamics of open fermionic nano-systems -- a fundamental symmetry and its application to electron transport in interacting quantum dots
- 2018
-
Doctoral thesis (other academic/artistic)abstract
- The study of electronic transport through strongly confined, interacting open quantum systems has regained considerable interest over the past years. One main motivation behind this concerns the possibility of time-dependently controlled operations on individual electrons, promising applications in, e.g., metrology and electron-based quantum computing. In particular, fundamental questions of quantum thermodynamics and the practical necessity to recover waste heat from nanocircuits have attracted attention towards electronic energy currents . The research articles covered by this thesis contribute to this topic by deriving and exploring a fundamental symmetry relation -- the fermionic duality . This duality applies to the quantum master equation of any locally interacting, fermionic open quantum system tunnel-coupled to non-interacting reservoirs. It yields a crosslink between modes and amplitudes corresponding to the evolution rates in the time-dependent decay of the open-system state. This crosslink involves a mapping between the system of interest and a dual system with inverted environment potentials, local energies, and thus especially inverted interactions. The duality thereby explains many, at first sight unintuitive, transport features and significantly improves their analytic accessability. In particular, we can understand why charge- and energy currents through quantum dots with strong local Coulomb repulsion in fact exhibit features of electron-electron attraction , both in the time-dependent decay after a sudden switch and in the stationary limit. More fundamental insights are obtained by identifying the duality to be rooted in Pauli's exclusion principle and the parity superselection principle. Namely, this implies that the duality is independent of, and hence combinable with many other general symmetries, including particle-hole symmetry, time-reversal symmetry, detailed balance and Onsager reciprocity. Especially the combination with the latter offers a novel perspective on the thermoelectric response of open, locally interacting electronic nanosystems.
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| 7. |
- Schulenborg, Jens, 1988, et al.
(author)
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Fermion-parity duality and energy relaxation in interacting open systems
- 2016
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In: Physical Review B. - 2469-9969 .- 2469-9950. ; 93:8, s. 081411-
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Journal article (peer-reviewed)abstract
- We study the transient heat current out of a confined electron system into a weakly coupled electrode in response to a voltage switch. We show that the decay of the Coulomb interaction energy for this repulsive system exhibits signatures of electron-electron attraction, and is governed by an interaction-independent rate. This can only be understood from a general duality that relates the non-unitary evolution of a quantum system to that of a dual model with inverted energies. Deriving from the fermion-parity superselection postulate, this duality applies to a large class of open systems.
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| 8. |
- Schulenborg, Jens, 1988, et al.
(author)
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Spectroscopy of hot electron pair emission from a driven quantum dot
- 2024
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In: Physical Review B. - 2469-9969 .- 2469-9950. ; 109:11
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Journal article (peer-reviewed)abstract
- On-demand emission of individual electrons for the implementation of flying qubits and quantum electron-optics experiments requires precise knowledge and tunability of emission times and energies. Crucially, for confined electron sources such as driven quantum dots, the effect of local Coulomb interaction on these emission properties needs to be understood, in particular if multiple particles are emitted close in time or near simultaneously. This paper theoretically analyzes electron pair emission from an ac driven quantum dot, detailing the competing effects of the electron-electron interaction, the time-dependent potential forming the quantum dot, and of the quantum-state properties, such as degeneracy, on the emission times and energies. We complement a numerical analysis of the coherent Schrödinger evolution of two particles in a driven potential with a master-equation description for strongly interacting electrons tunneling stochastically into a weakly coupled conductor. This captures a broad range of different influences on the emitted particles and thereby guides the development of single-electron sources with higher control over two-particle emission properties.
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| 9. |
- Schulenborg, Jens, 1988, et al.
(author)
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Thermoelectrics of Interacting Nanosystems -- Exploiting Superselection Instead of Time-Reversal Symmetry
- 2017
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In: Entropy. - : MDPI AG. - 1099-4300. ; 19:12
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Journal article (peer-reviewed)abstract
- Thermoelectric transport is traditionally analyzed using relations imposed by time-reversal symmetry, ranging from Onsager’s results to fluctuation relations in counting statistics. In this paper, we show that a recently discovered duality relation for fermionic systems—deriving from the fundamental fermion-parity superselection principle of quantum many-particle systems—provides new insights into thermoelectric transport. Using a master equation, we analyze the stationary charge and heat currents through a weakly coupled, but strongly interacting single-level quantum dot subject to electrical and thermal bias. In linear transport, the fermion-parity duality shows that features of thermoelectric response coefficients are actually dominated by the average and fluctuations of the charge in a dual quantum dot system, governed by attractive instead of repulsive electron-electron interaction. In the nonlinear regime, the duality furthermore relates most transport coefficients to much better understood equilibrium quantities. Finally, we naturally identify the fermion-parity as the part of the Coulomb interaction relevant for both the linear and nonlinear Fourier heat. Altogether, our findings hence reveal that next to time-reversal, the duality imposes equally important symmetry restrictions on thermoelectric transport. As such, it is also expected to simplify computations and clarify the physical understanding for more complex systems than the simplest relevant interacting nanostructure model studied here.
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| 10. |
- Schulenborg, Jens, 1988, et al.
(author)
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Thermovoltage in quantum dots with attractive interaction
- 2020
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 116:24
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Journal article (peer-reviewed)abstract
- We study the linear and nonlinear thermovoltage of a quantum dot with effective attractive electron-electron interaction and weak, energy-dependent tunnel coupling to electronic contacts. Remarkably, we find that the thermovoltage shows signatures of repulsive interaction, which can be rationalized. These thermovoltage characteristics are robust against large potential and temperature differences well into the nonlinear regime, which we expect can be demonstrated in current state-of-the-art experiments. Furthermore, under nonlinear operation, we find extended regions of large power production at efficiencies on the order of the Curzon-Ahlborn bound interrupted only by a characteristic sharp dip.
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