Experimental demonstration of robustness under scaling errors for superadiabatic population transfer in a superconducting circuit

  1. Shruti Dogra,
  2. Antti Vepsäläinen,
  3. and Gheorghe Sorin Paraoanu
We study experimentally and theoretically the transfer of population between the ground state and the second excited state in a transmon circuit by the use of superadiabatic stimulated
Raman adiabatic passage (saSTIRAP). We show that the transfer is remarkably resilient against variations in the amplitudes of the pulses (scaling errors), thus demostrating that the superadiabatic process inherits certain robustness features from the adiabatic one. In particular, we put in evidence a new plateau that appears at high values of the counterdiabatic pulse strength, which goes beyond the usual framework of saSTIRAP.

Quantum simulation of parity-time symmetry breaking with a superconducting quantum processor

  1. Shruti Dogra,
  2. Artem A. Melnikov,
  3. and Gheorghe Sorin Paraoanu
The observation of genuine quantum effects in systems governed by non-Hermitian Hamiltonians has been an outstanding challenge in the field. Here we simulate the evolution under such
Hamiltonians in the quantum regime on a superconducting quantum processor by using a dilation procedure involving an ancillary qubit. We observe the parity-time ()-symmetry breaking phase transition at the exceptional points, obtain the critical exponent, and show that this transition is associated with a loss of state distinguishability. In a two-qubit setting, we show that the entanglement can be modified by local operations.

Protocol for temperature sensing using a three-level transmon circuit

  1. Aidar Sultanov,
  2. Marko Kuzmanović,
  3. Andrey V. Lebedev,
  4. and Gheorghe Sorin Paraoanu
We present a method for in situ temperature measurement of superconducting quantum circuits, by using the first three levels of a transmon device to which we apply a sequence of π
gates. Our approach employs projective dispersive readout and utilizes the basic properties of the density matrix associated with thermal states. This method works with an averaging readout scheme and does not require a single-shot readout setup. We validate this protocol by performing thermometry in the range of 50 mK – 200 mK, corresponding to a range of residual populations 1%−20% for the first excited state and 0.02%−3% for the second excited state.

Engineering dissipation with resistive elements in circuit quantum electrodynamics

  1. Marco Cattaneo,
  2. and Gheorghe Sorin Paraoanu
The importance of dissipation engineering ranges from universal quantum computation to non-equilibrium quantum thermodynamics. In recent years, more and more theoretical and experimental
studies have shown the relevance of this topic for circuit quantum electrodynamics, one of the major platforms in the race for a quantum computer. This article discusses how to simulate thermal baths by inserting resistive elements in networks of superconducting qubits. Apart from pedagogically reviewing the phenomenological and microscopic models of a resistor as thermal bath with Johnson-Nyquist noise, the paper introduces some new results in the weak coupling limit, showing that the most common examples of open quantum systems can be simulated through capacitively coupled superconducting qubits and resistors. The aim of the manuscript, written with a broad audience in mind, is to be both an instructive tutorial about how to derive and characterize the Hamiltonian of general dissipative superconducting circuits with capacitive coupling, and a review of the most relevant and topical theoretical and experimental works focused on resistive elements and dissipation engineering.

Listening to the quantum vacuum: a perspective on the dynamical Casimir effect

  1. Gheorghe Sorin Paraoanu,
  2. and Göran Johansson
Modern quantum field theory has offered us a very intriguing picture of empty space. The vacuum state is no longer an inert, motionless state. We are instead dealing with an entity
teeming with fluctuations that continuously produce virtual particles popping in and out of existence. The dynamical Casimir effect is a paradigmatic phenomenon, whereby these particles are converted into real particles (photons) by changing the boundary conditions of the field. It was predicted 50 years ago by Gerald T. Moore and it took more than 40 years until the first experimental verification.

Synchronization and subradiance as signatures of entangling bath between superconducting qubits

  1. Marco Cattaneo,
  2. Gian Luca Giorgi,
  3. Sabrina Maniscalco,
  4. Gheorghe Sorin Paraoanu,
  5. and Roberta Zambrini
A common environment acting on two superconducting qubits can give rise to a plethora of phenomena, such as the generation of entanglement between the qubits that, beyond its importance
for quantum computation tasks, also enforces a change of strategy in quantum error correction protocols. Further effects induced by a common bath are quantum synchronization and subradiance. Contrary to entanglement, for which full-state tomography is necessary, the latter can be assessed by detection of local observables only. In this work we explore different regimes to establish when synchronization and subradiance can be employed as reliable signatures of an entangling common bath. Moreover, we address a recently proposed measure of the collectiveness of the dynamics driven by the bath, and find that it almost perfectly witnesses the behavior of entanglement. Finally, we propose an implementation of the model based on two transmon qubits capacitively coupled to a common resistor, which may be employed as a versatile quantum simulation platform of the open system in general regimes.

Superadiabatic population transfer in a three-level superconducting circuit

  1. Antti Vepsäläinen,
  2. Sergey Danilin,
  3. and Gheorghe Sorin Paraoanu
Adiabatic manipulation of the quantum state is an essential tool in modern quantum information processing. Here we demonstrate the speed-up of the adiabatic population transfer in a
three-level superconducting transmon circuit by suppressing the spurious non-adiabatic excitations with an additional two-photon microwave pulse. We apply this superadiabatic method to the stimulated Raman adiabatic passage, realizing fast and robust population transfer from the ground state to the second excited state of the quantum circuit.

Dielectric losses in multi-layer Josephson junction qubits

  1. David Gunnarsson,
  2. Juha-Matti Pirkkalainen,
  3. Jian Li,
  4. Gheorghe Sorin Paraoanu,
  5. Pertti Hakonen,
  6. Mika Sillanpää,
  7. and Mika Prunnila
We have measured the excited state lifetimes in Josephson junction phase and transmon qubits, all of which were fabricated with the same scalable multi-layer process. We have compared
the lifetimes of phase qubits before and after removal of the isolating dielectric, SiNx, and find a four-fold improvement of the relaxation time after the removal. Together with the results from the transmon qubit and measurements on coplanar waveguide resonators, these measurements indicate that the lifetimes are limited by losses from the dielectric constituents of the qubits. We have extracted the individual loss contributions from the dielectrics in the tunnel junction barrier, AlOx, the isolating dielectric, SiNx, and the substrate, Si/SiO2, by weighing the total loss with the parts of electric field over the different dielectric materials. Our results agree well and complement the findings from other studies, demonstrating that superconducting qubits can be used as a reliable tool for high-frequency characterization of dielectric materials. We conclude with a discussion of how changes in design and material choice could improve qubit lifetimes up to a factor of four.