Pokemon: Protected Logic Qubit Derived from the 0-π Qubit

  1. J. Q. You,
  2. and Franco Nori
We propose a new protected logic qubit called pokemon, which is derived from the 0-π qubit by harnessing one capacitively shunted inductor and two capacitively shunted Josephson junctions
embedded in a superconducting loop. Similar to the 0-π qubit, the two basis states of the proposed qubit are separated by a high barrier, but their wave functions are highly localized along both axis directions of the two-dimensional parameter space, instead of the highly localized wave functions along only one axis direction in the 0-π qubit. This makes the pokemon qubit more protected. For instance, the relaxation of the pokemon qubit is exponentially reduced by two equally important factors, while the relaxation of the 0-π qubit is exponentially reduced by only one factor. Moreover, we show that the inductor in the pokemon can be replaced by a nonlinear inductor using, e.g., a pair or two pairs of Josephson junctions. This offers an experimentally promising way to implement next-generation superconducting qubits with even higher quantum coherence.

A Cooper-Pair Box Architecture for Cyclic Quantum Heat Engines

  1. Andrew Guthrie,
  2. Christoforus Dimas Satrya,
  3. Yu-Cheng Chang,
  4. Paul Menczel,
  5. Franco Nori,
  6. and Jukka P. Pekola
Here we present an architecture for the implementation of cyclic quantum thermal engines using a superconducting circuit. The quantum engine consists of a gated Cooper-pair box, capacitively
coupled to two superconducting coplanar waveguide resonators with different frequencies, acting as thermal baths. We experimentally demonstrate the strong coupling of a charge qubit to two superconducting resonators, with the ability to perform voltage driving of the qubit at GHz frequencies. By terminating the resonators of the measured structure with normal-metal resistors whose temperature can be controlled and monitored, a quantum heat engine or refrigerator could be realized. Furthermore, we numerically evaluate the performance of our setup acting as a quantum Otto-refrigerator in the presence of realistic environmental decoherence.

Nonequilibrium heat transport and work with a single artificial atom coupled to a waveguide: emission without external driving

  1. Yong Lu,
  2. Neill Lambert,
  3. Anton Frisk Kockum,
  4. Ken Funo,
  5. Andreas Bengtsson,
  6. Simone Gasparinetti,
  7. Franco Nori,
  8. and Per Delsing
We observe the continuous emission of photons into a waveguide from a superconducting qubit without the application of an external drive. To explain this observation, we build a two-bath
model where the qubit couples simultaneously to a cold bath (the waveguide) and a hot bath (a secondary environment). Our results show that the thermal-photon occupation of the hot bath is up to 0.14 photons, 35 times larger than the cold waveguide, leading to nonequilibrium heat transport with a power of up to 132 zW, as estimated from the qubit emission spectrum. By adding more isolation between the sample output and the first cold amplifier in the output line, the heat transport is strongly suppressed. Our interpretation is that the hot bath may arise from active two-level systems being excited by noise from the output line. We also apply a coherent drive, and use the waveguide to measure thermodynamic work and heat, suggesting waveguide spectroscopy is a useful means to study quantum heat engines and refrigerators. Finally, based on the theoretical model, we propose how a similar setup can be used as a noise spectrometer which provides a new solution for calibrating the background noise of hybrid quantum systems.

Metrological characterisation of non-Gaussian entangled states of superconducting qubits

  1. Kai Xu,
  2. Yu-Ran Zhang,
  3. Zheng-Hang Sun,
  4. Hekang Li,
  5. Pengtao Song,
  6. Zhongcheng Xiang,
  7. Kaixuan Huang,
  8. Hao Li,
  9. Yun-Hao Shi,
  10. Chi-Tong Chen,
  11. Xiaohui Song,
  12. Dongning Zheng,
  13. Franco Nori,
  14. H. Wang,
  15. and Heng Fan
Multipartite entangled states are significant resources for both quantum information processing and quantum metrology. In particular, non-Gaussian entangled states are predicted to
achieve a higher sensitivity of precision measurements than Gaussian states. On the basis of metrological sensitivity, the conventional linear Ramsey squeezing parameter (RSP) efficiently characterises the Gaussian entangled atomic states but fails for much wider classes of highly sensitive non-Gaussian states. These complex non-Gaussian entangled states can be classified by the nonlinear squeezing parameter (NLSP), as a generalisation of the RSP with respect to nonlinear observables, and identified via the Fisher information. However, the NLSP has never been measured experimentally. Using a 19-qubit programmable superconducting processor, here we report the characterisation of multiparticle entangled states generated during its nonlinear dynamics. First, selecting 10 qubits, we measure the RSP and the NLSP by single-shot readouts of collective spin operators in several different directions. Then, by extracting the Fisher information of the time-evolved state of all 19 qubits, we observe a large metrological gain of 9.89[Math Processing Error] dB over the standard quantum limit, indicating a high level of multiparticle entanglement for quantum-enhanced phase sensitivity. Benefiting from high-fidelity full controls and addressable single-shot readouts, the superconducting processor with interconnected qubits provides an ideal platform for engineering and benchmarking non-Gaussian entangled states that are useful for quantum-enhanced metrology.

Mirror, mirror: Landau-Zener-Stuckelberg-Majorana interferometry of a superconducting qubit in front of a mirror

  1. P. Y. Wen,
  2. O. V. Ivakhnenko,
  3. M. A. Nakonechnyi,
  4. B. Suri,
  5. J.-J. Lin,
  6. W.-J. Lin,
  7. J. C. Chen,
  8. S. N. Shevchenko,
  9. Franco Nori,
  10. and I.-C. Hoi
We investigate the Landau-Zener-Stuckelberg-Majorana interferometry of a superconducting qubit in a semi-infinite transmission line terminated by a mirror. The transmon-type qubit is
at the node of the resonant electromagnetic (EM) field, hiding from the EM field. „Mirror, mirror“ briefly describes this system, because the qubit acts as another mirror. We modulate the resonant frequency of the qubit by applying a sinusoidal flux pump. We probe the spectroscopy by measuring the reflection coefficient of a weak probe in the system. Remarkable interference patterns emerge in the spectrum, which can be interpreted as multi-photon resonances in the dressed qubit. Our calculations agree well with the experiments.

Gauge freedom, quantum measurements, and time-dependent interactions in cavity and circuit QED

  1. Alessio Settineri,
  2. Omar Di Stefano,
  3. David Zueco,
  4. Stephen Hughes,
  5. Salvatore Savasta,
  6. and Franco Nori
The interaction between the electromagnetic field inside a cavity and natural or artificial atoms has played a crucial role in developing our understanding of light-matter interaction,
and is central to various quantum technologies. Recently, new regimes beyond the weak and strong light-matter coupling have been explored in several settings. These regimes, where the interaction strength is comparable (ultrastrong) or even higher (deep-strong) than the transition frequencies in the system, can give rise to new physical effects and applications. At the same time, they challenge our understanding of cavity QED. When the interaction strength is so high, fundamental issues like the proper definition of subsystems and of their quantum measurements, the structure of light-matter ground states, or the analysis of time-dependent interactions are subject to ambiguities leading to even qualitatively distinct predictions. The resolution of these ambiguities is also important for understanding and designing next-generation quantum devices that will exploit the ultrastrong coupling regime. Here we discuss and provide solutions to these issues.

Probing the dynamical phase transition with a superconducting quantum simulator

  1. Kai Xu,
  2. Zheng-Hang Sun,
  3. Wuxin Liu,
  4. Yu-Ran Zhang,
  5. Hekang Li,
  6. Hang Dong,
  7. Wenhui Ren,
  8. Pengfei Zhang,
  9. Franco Nori,
  10. Dongning Zheng,
  11. Heng Fan,
  12. and H. Wang
Non-equilibrium quantum many-body systems, which are difficult to study via classical computation, have attracted wide interest. Quantum simulation can provide insights into these problems.
Here, using a programmable quantum simulator with 16 all-to-all connected superconducting qubits, we investigate the dynamical phase transition in the Lipkin-Meshkov-Glick model with a quenched transverse field. Clear signatures of the dynamical phase transition, merging different concepts of dynamical criticality, are observed by measuring the non-equilibrium order parameter, nonlocal correlations, and the Loschmidt echo. Moreover, near the dynamical critical point, we obtain the optimal spin squeezing of −7.0±0.8 decibels, showing multipartite entanglement useful for measurements with precision five-fold beyond the standard quantum limit. Based on the capability of entangling qubits simultaneously and the accurate single-shot readout of multi-qubit states, this superconducting quantum simulator can be used to study other problems in non-equilibrium quantum many-body systems.

Quantum bits with Josephson junctions

  1. Anton Frisk Kockum,
  2. and Franco Nori
Already in the first edition of this book (Barone and Paterno, „Fundamentals and Physics and Applications of the Josephson Effect“, Wiley 1982), a great number of interesting
and important applications for Josephson junctions were discussed. In the decades that have passed since then, several new applications have emerged. This chapter treats one such new class of applications: quantum optics and quantum information processing (QIP) based on superconducting circuits with Josephson junctions. In this chapter, we aim to explain the basics of superconducting quantum circuits with Josephson junctions and demonstrate how these systems open up new prospects, both for QIP and for the study of quantum optics and atomic physics.

Ideal Quantum Nondemolition Readout of a Flux Qubit Without Purcell Limitations

  1. Xin Wang,
  2. Adam Miranowicz,
  3. and Franco Nori
Dispersive coupling based on the Rabi model with large detuning is widely used for quantum nondemolition (QND) qubit readout in quantum computation. However, the measurement speed and
fidelity are usually significantly limited by the Purcell effects, i.e.: Purcell decay, critical photon numbers, and qubit-dependent Kerr nonlinearity. To avoid these effects, we propose how to realize an ideal QND readout of a gradiometric flux qubit with a tunable gap via its direct dispersive coupling to a boundary-tunable measurement resonator. We show that this novel readout mechanism is free of dipole-field interactions, and that the qubit-QND measurement is not deteriorated by intracavity photons. Both qubit-readout speed and fidelity can avoid the Purcell limitations. Moreover, this direct dispersive coupling can be conveniently turned on and off via an external control flux. We show how to extend this proposal to a multi-qubit architecture for a joint qubit readout.

Simple preparation of Bell and GHZ states using ultrastrong-coupling circuit QED

  1. Vincenzo Macrì,
  2. Franco Nori,
  3. and Anton Frisk Kockum
The ability to entangle quantum systems is crucial for many applications in quantum technology, including quantum communication and quantum computing. Here, we propose a new, simple,
and versatile setup for deterministically creating Bell and Greenberger-Horne-Zeilinger (GHZ) states between photons of different frequencies in a two-step protocol. The setup consists of a quantum bit (qubit) coupled ultrastrongly to three photonic resonator modes. The only operations needed in our protocol are to put the qubit in a superposition state, and then tune its frequency in and out of resonance with sums of the resonator-mode frequencies. By choosing which frequency we tune the qubit to, we select which entangled state we create. We show that our protocol can be implemented with high fidelity using feasible experimental parameters in state-of-the-art circuit quantum electrodynamics. One possible application of our setup is as a node distributing entanglement in a quantum network.