Papers by Alexandre Blais
Quantum theory predicts that empty space is not truly empty. Even in the absence of any particles... more Quantum theory predicts that empty space is not truly empty. Even in the absence of any particles or radiation, in pure vacuum, virtual particles are constantly created and annihilated. In an electromagnetic field, the presence of virtual photons manifests itself as a small renormalization of the energy of a quantum system, known as the Lamb shift. We present an experimental
Physical Review Letters, 2010
We show that controlling relative phases of electromagnetic fields driving an atom with a Delta-c... more We show that controlling relative phases of electromagnetic fields driving an atom with a Delta-configuration energy-level structure enables optical susceptibility to be engineered in novel ways. In particular, relative-phase control can yield electromagnetically induced transparency but with the benefit that the transparency window is sandwiched between an absorption and an amplification band rather than between two absorption bands in typical
Superconducting circuit implementations of cavity QED have enabled the exploration of various reg... more Superconducting circuit implementations of cavity QED have enabled the exploration of various regimes of light-matter interaction. In this work, we present theoretical aspects of the observation of quantum properties of the field emitted from a cavity without access to non-linear/single-photon detectors (which have not been demonstrated reliably in the microwave regime). In particular, we focus on how to perform the
From Nanomechanics to Quantum Superconducting Circuits, 2012
One of the most studied model systems in quantum optics is a two-level atom strongly coupled to a... more One of the most studied model systems in quantum optics is a two-level atom strongly coupled to a single mode of the electromagnetic field stored in a cavity, a research field named cavity quantum electrodynamics or CQED . This extremely simple quantum system has nevertheless nontrivial quantum dynamics described by the Jaynes-Cummings Hamiltonian (Jaynes and Cummings, 1963). In its implementation at microwave frequencies, it has allowed the observation of many basic concepts of quantum mechanics such as the quantum jumps of the electromagnetic field , as well as the generation and tomography of nonclassical states . In the context of quantum information, elementary quantum gates have been realized using the cavity as a catalyst for entanglement between atoms passing successively through it . CQED has recently received renewed attention due to its implementation with superconducting artificial atoms and coplanar resonators in the so-called circuit quantum electrodynamics (cQED) architecture . In cQED, the couplings can be much stronger than in CQED due to the design flexibility of superconducting circuits and to the enhanced field confinement in one-dimensional cavities, compensating the shorter coherence times of superconducting qubits. This enabled the realization of fundamental quantum physics and quantum information processing ) experiments with a degree of control comparable to that obtained in CQED. Even though the physical implementation is different, the system is described by the same Hamiltonian in cQED as in CQED.
Physical Review A, 2004
We propose a realizable architecture using one-dimensional transmission line resonators to reach ... more We propose a realizable architecture using one-dimensional transmission line resonators to reach the strong-coupling limit of cavity quantum electrodynamics in superconducting electrical circuits. The vacuum Rabi frequency for the coupling of cavity photons to quantized excitations of an adjacent electrical circuit (qubit) can easily exceed the damping rates of both the cavity and qubit. This architecture is attractive both as
Conference on Lasers and Electro-Optics/International Quantum Electronics Conference, 2009
Abstract We present the experimental implementation of two-qubit quantum algorithms in a supercon... more Abstract We present the experimental implementation of two-qubit quantum algorithms in a superconducting circuit. Our processor incorporates local and fast flux biasing of two transmon qubits within a circuit QED architecture. An off-resonant cavity bus shields the ...
We measure the quantum fluctuations of a pumped nonlinear resonator using a superconducting artif... more We measure the quantum fluctuations of a pumped nonlinear resonator using a superconducting artificial atom as an in situ probe. The qubit excitation spectrum gives access to the frequency and amount of excitation of the intracavity field fluctuations, from which we infer its effective temperature. These quantities are found to be in agreement with theoretical predictions; in particular, we experimentally observe the phenomenon of quantum heating.
Physical Review A, 2014
ABSTRACT In addition to their central role in quantum information processing, qubits have proven ... more ABSTRACT In addition to their central role in quantum information processing, qubits have proven to be useful tools in a range of other applications such as enhanced quantum sensing and as spectrometers of quantum noise. Here we show that a superconducting qubit strongly coupled to a nonlinear resonator can act as a probe of quantum fluctuations of the intraresonator field. Building on previous work [M. Boissoneault et al., Phys. Rev. A 85, 022305 (2012), 10.1103/PhysRevA.85.022305], we derive an effective master equation for the qubit which takes into account squeezing of the resonator field. We show how sidebands in the qubit excitation spectrum that are predicted by this model can reveal information about the squeezing factor r. The main results of this paper have already been successfully compared to experimental data [F. R. Ong et al., Phys. Rev. Lett. 110, 047001 (2013), 10.1103/PhysRevLett.110.047001], and we present here the details of the derivations.
Science, 2013
Photon-mediated interactions between atoms are of fundamental importance in quantum optics, quant... more Photon-mediated interactions between atoms are of fundamental importance in quantum optics, quantum simulations and quantum information processing. The exchange of real and virtual photons between atoms gives rise to non-trivial interactions the strength of which decreases rapidly with distance in three dimensions.
Science, 2008
Quantum theory predicts that empty space is not truly empty. Even in the absence of any particles... more Quantum theory predicts that empty space is not truly empty. Even in the absence of any particles or radiation, in pure vacuum, virtual particles are constantly created and annihilated. In an electromagnetic field, the presence of virtual photons manifests itself as a small renormalization of the energy of a quantum system, known as the Lamb shift. We present an experimental observation of the Lamb shift in a solid-state system. The strong dispersive coupling of a superconducting electronic circuit acting as a quantum bit (qubit) to the vacuum field in a transmission-line resonator leads to measurable Lamb shifts of up to 1.4% of the qubit transition frequency. The qubit is also observed to couple more strongly to the vacuum field than to a single photon inside the cavity, an effect that is explained by taking into account the limited anharmonicity of the higher excited qubit states.
Science, 2007
In quantum information science, the phase of a wavefunction plays an important role in encoding i... more In quantum information science, the phase of a wavefunction plays an important role in encoding information. While most experiments in this field rely on dynamic effects to manipulate this information, an alternative approach is to use geometric phase, which has been argued to have potential fault tolerance. We demonstrate the controlled accumulation of a geometric phase, Berry's phase, in a superconducting qubit, manipulating the qubit geometrically using microwave radiation, and observing the accumulated phase in an interference experiment. We find excellent agreement with Berry's predictions, and also observe a geometry dependent contribution to dephasing.
Physical Review Letters, 2005
We have spectroscopically measured the energy level separation of a superconducting charge qubit ... more We have spectroscopically measured the energy level separation of a superconducting charge qubit coupled non-resonantly to a single mode of the electromagnetic field of a superconducting on-chip resonator. The strong coupling leads to large shifts in the energy levels of both the qubit and the resonator in this circuit quantum electrodynamics system. The dispersive shift of the resonator frequency is used to non-destructively determine the qubit state and to map out the dependence of its energy levels on the bias parameters. The measurement induces an ac-Stark shift of 0.6 MHz per photon in the qubit level separation. Fluctuations in the photon number (shot noise) induce level fluctuations in the qubit leading to dephasing which is the characteristic back-action of the measurement. A cross-over from lorentzian to gaussian line shape with increasing measurement power is observed and theoretically explained. For weak measurement a long intrinsic dephasing time of T2 > 200 ns of the qubit is found.
Physical Review Letters, 2011
Creating a train of single photons and monitoring its propagation and interaction is challenging ... more Creating a train of single photons and monitoring its propagation and interaction is challenging in most physical systems, as photons generally interact very weakly with other systems. However, when confining microwave frequency photons in a transmission line resonator, effective photonphoton interactions can be mediated by qubits embedded in the resonator. Here, we observe the phenomenon of photon blockade through second-order correlation function measurements. The experiments clearly demonstrate antibunching in a continuously pumped source of single microwave photons measured using microwave beam splitters, linear amplifiers, and quadrature amplitude detectors. We also investigate resonance fluorescence and Rayleigh scattering in Mollow-triplet-like spectra.
Physical Review Letters, 2012
We demonstrate the realization of a hybrid solid-state quantum device, in which a semiconductor d... more We demonstrate the realization of a hybrid solid-state quantum device, in which a semiconductor double quantum dot is dipole coupled to the microwave field of a superconducting coplanar waveguide resonator. The double dot charge stability diagram extracted from measurements of the amplitude and phase of a microwave tone transmitted through the resonator is in good agreement with that obtained from transport measurements. Both the observed frequency shift and linewidth broadening of the resonator are explained considering the double dot as a charge qubit coupled with a strength of several tens of MHz to the resonator.
Physical Review Letters, 2011
We present a superconducting qubit for the circuit quantum electrodynamics architecture that has ... more We present a superconducting qubit for the circuit quantum electrodynamics architecture that has a tunable coupling strength g. We show that this coupling strength can be tuned from zero to values that are comparable with other superconducting qubits. At g = 0 the qubit is in a decoherence free subspace with respect to spontaneous emission induced by the Purcell effect. Furthermore we show that in the decoherence free subspace the state of the qubit can still be measured by either a dispersive shift on the resonance frequency of the resonator or by a cycling-type measurement.
Physical Review Letters, 2013
We measure the quantum fluctuations of a pumped nonlinear resonator, using a superconducting arti... more We measure the quantum fluctuations of a pumped nonlinear resonator, using a superconducting artificial atom as an in-situ probe. The qubit excitation spectrum gives access to the frequency and temperature of the intracavity field fluctuations. These are found to be in agreement with theoretical predictions; in particular we experimentally observe the phenomenon of quantum heating.
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Papers by Alexandre Blais