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A. Sharafiev, G. Kirchmair, M. L. Juan, M. Cattaneo Leveraging collective effects for thermometry in waveguide quantum electrodynamics,
Phys. Rev. Lett. 134 213602 (2025-05-28),
http://dx.doi.org/10.1103/PhysRevLett.134.213602 doi:10.1103/PhysRevLett.134.213602 (ID: 721270)
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We report a proof-of-principle experiment for a new method of temperature measurements in waveguide quantum electrodynamics (wQED) experiments, allowing one to differentiate between global and local baths. The method takes advantage of collective states of two transmon qubits located in the center of a waveguide. The Hilbert space of such a system forms two separate subspaces (bright and dark) which are coupled differently to external noise sources. Measuring transmission through the waveguide allows one to extract separately the temperatures of the baths responsible for global and local excitations in the system. Such a system would allow for building a new type of primary temperature sensor capable of distinguishing between local and global baths.
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N. Diaz-Naufal, L. Deeg, D. Zoepfl, C. Schneider, M. L. Juan, G. Kirchmair, A. Metelmann Kerr enhanced optomechanical cooling in the unresolved sideband regime,
Phys. Rev. A 111 53505 (2025-05-25),
http://dx.doi.org/10.1103/PhysRevA.111.053505 doi:10.1103/PhysRevA.111.053505 (ID: 721277)
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Dynamical backaction cooling has been demonstrated to be a successful method for achieving the motional quantum ground state of a mechanical oscillator in the resolved-sideband regime, where the mechanical frequency is significantly larger than the cavity decay rate. Nevertheless, as mechanical systems increase in size, their frequencies naturally decrease, thus bringing them into the unresolved-sideband regime, where the effectiveness of the sideband cooling approach decreases. Here we demonstrate, however, that this cooling technique in the unresolved-sideband regime can be significantly enhanced by utilizing a nonlinear cavity as shown in the experimental work of Zoepfl et al. [Phys. Rev. Lett. 130, 033601 (2023)]. The above arises due to the increased asymmetry between the cooling and heating processes, thereby improving the cooling efficiency. In addition, we show that injecting a squeezed vacuum into the nonlinear cavity paves the way to ground-state cooling of the mechanical mode. Notably, the required squeezing parameters are far less stringent than in the linear case, simplifying experimental implementation.
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D. Atanasova, I. Yang, T. Hönigl-Decrinis, D. Gusenkova, I. M. Pop, G. Kirchmair In-situ tunable interaction with an invertible sign between a fluxonium and a post cavity,
PRX Quantum 6 20318 (2025-04-25),
http://dx.doi.org/10.1103/PRXQuantum.6.020318 doi:10.1103/PRXQuantum.6.020318 (ID: 721273)
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Quantum computation with bosonic modes presents a powerful paradigm for harnessing the principles of quantum mechanics to perform complex information processing tasks. In constructing a bosonic qubit with superconducting circuits, nonlinearity is typically introduced to a cavity mode through an ancillary two-level qubit. However, the ancilla's spurious heating has impeded progress towards fully fault-tolerant bosonic qubits. The ability to in situ decouple the ancilla when not in use would be beneficial but has so far only been realized with tunable couplers or additional parametric drives. This work presents a novel architecture for quantum information processing, comprising a 3D post cavity coupled to a fluxonium ancilla via a readout resonator. This system's intricate energy level structure results in a complex landscape of interactions whose sign can be tuned in situ by the magnetic field threading the fluxonium loop without the need of additional elements. Our results could significantly advance the lifetime and controllability of bosonic qubits.
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J. Yang, T. E. Agrenius Gustafsson, V. Usova, O. Romero-Isart, G. Kirchmair Hot Schrödinger Cat States,
Sci. Adv. 11 (2025-04-04),
http://dx.doi.org/10.1126/sciadv.adr4492 doi:10.1126/sciadv.adr4492 (ID: 721262)
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The observation of quantum phenomena often necessitates sufficiently pure states, a requirement that can be challenging to achieve. In this study, our goal is to prepare a non-classical state originating from a mixed state, utilizing dynamics that preserve the initial low purity of the state. We generate a quantum superposition of displaced thermal states within a microwave cavity using only unitary interactions with a transmon qubit. We measure the Wigner functions of these ``hot'' Schrödinger cat states for an initial purity as low as 0.06. This corresponds to a cavity mode temperature of up to 1.8 Kelvin, sixty times hotter than the cavity's physical environment. Our realization of highly mixed quantum superposition states could be implemented with other continuous-variable systems e.g. nanomechanical oscillators, for which ground-state cooling remains challenging.
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L. Deeg, D. Zoepfl, N. Diaz-Naufal, M. L. Juan, A. Metelmann, G. Kirchmair Optomechanical Backaction in the Bistable Regime,
Phys. Rev. Applied 23 (2025-01-31),
http://dx.doi.org/10.1103/PhysRevApplied.23.014082 doi:10.1103/PhysRevApplied.23.014082 (ID: 721263)
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With a variety of realizations, optomechanics utilizes its light-matter interaction to test fundamental physics. By coupling the phonons of a mechanical resonator to the photons in a high-quality cavity, control of increasingly macroscopic objects has become feasible. In such systems, state manipulation of the mechanical mode is achieved by driving the cavity. To be able to achieve high drive powers the system is typically designed such that it remains in a linear response regime when driven. A nonlinear response, and especially bistability, in a driven cavity is often considered detrimental to cooling and state preparation in optomechanical systems and is avoided in experiments. Here we show that with an intrinsic nonlinear cavity backaction cooling of a mechanical resonator is feasible operating deep within the nonlinear regime of the cavity. With our theory taking the nonlinearity into account, precise predictions on backaction cooling can be achieved even with a cavity beyond the bifurcation point, where the cavity photon number spectrum starts to deviate from a typical Lorentzian shape.
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R. Holzinger, R. Gutierrez Jauregui, T. Hönigl-Decrinis, G. Kirchmair, A. Asenjo Garcia, Helmut Ritsch Control of Localized Single- and Many-Body Dark States in Waveguide QED,
Phys. Rev. Lett. 129 (2022-12-14),
http://dx.doi.org/10.1103/PhysRevLett.129.253601 doi:10.1103/PhysRevLett.129.253601 (ID: 720918)
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Subradiant states in a finite chain of two-level quantum emitters coupled to a one-dimensional reservoir are a resource for superior photon storage and their controlled release. As one can maximally store one energy quantum per emitter, storing multiple excitations requires delocalized states, which typically exhibit fermionic correlations and antisymmetric wave functions, thus making them hard to access experimentally. Here we identify a new class of quasilocalized dark states with up to half of the qubits excited, which only appear for lattice constants of an integer multiple of the wavelength. These states allow for a high-fidelity preparation and minimally invasive readout in state-of-the-art setups. In particular, we suggest an experimental implementation using a coplanar waveguide coupled to superconducting transmon qubits on a chip. With minimal free space and intrinsic losses, virtually perfect dark states can be achieved for a low number of qubits featuring fast preparation and precise manipulation.
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T. Orell, M. Zanner, M. L. Juan, A. Sharafiev, R. Albert, S. Oleschko, G. Kirchmair, M. Silveri Collective bosonic effects in an array of transmon devices,
Phys. Rev. A 105 (2022-06-01),
http://dx.doi.org/10.1103/PhysRevA.105.063701 doi:10.1103/PhysRevA.105.063701 (ID: 720925)
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Multiple emitters coherently interacting with an electromagnetic mode give rise to collective effects such as correlated decay and coherent exchange interaction, depending on the separation of the emitters. By diagonalizing the effective non-Hermitian many-body Hamiltonian we reveal the complex-valued eigenvalue spectrum encoding the decay and interaction characteristics. We show that there are significant differences in the emerging collective effects for an array of interacting anharmonic oscillators compared to those of two-level systems and harmonic oscillators. The bosonic decay rate of the most superradiant state increases linearly as a function of the filling factor and exceeds that of two-level systems in magnitude. Furthermore, with bosonic systems, dark states are formed at each filling factor. These are in strong contrast with two-level systems, where the maximal superradiance is observed at half-filling and with larger filling factors superradiance diminishes and no dark states are formed. As an experimentally relevant setup of bosonic waveguide QED, we focus on arrays of transmon devices embedded inside a rectangular waveguide. Specifically, we study the setup of two transmon pairs realized experimentally in Zanner et al. [Nat. Phys. 18, 538 (2022)] and show that it is necessary to consider transmons as bosonic multilevel emitters to accurately recover correct collective effects for the higher excitation manifolds.
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M. Zanner, T. Orell, C. Schneider, R. Albert, S. Oleschko, M. L. Juan, M. Silveri, G. Kirchmair Coherent control of a symmetry-engineered multi-qubit dark state in waveguide quantum electrodynamics,
Nature Phys. 18 (2022-03-14),
http://dx.doi.org/10.1038/s41567-022-01527-w doi:10.1038/s41567-022-01527-w (ID: 720659)
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Quantum information is typically encoded in the state of a qubit that is decoupled from the environment. In contrast, waveguide quantum electrodynamics studies qubits coupled to a mode continuum, exposing them to a loss channel and causing quantum information to be lost before coherent operations can be performed. Here we restore coherence by realizing a dark state that exploits symmetry properties and interactions between four qubits. Dark states decouple from the waveguide and are thus a valuable resource for quantum information but also come with a challenge: they cannot be controlled by the waveguide drive. We overcome this problem by designing a drive that utilizes the symmetry properties of the collective state manifold allowing us to selectively drive both bright and dark states. The decay time of the dark state exceeds that of the waveguide-limited single qubit by more than two orders of magnitude. Spectroscopy on the second excitation manifold provides further insight into the level structure of the hybridized system. Our experiment paves the way for implementations of quantum many-body physics in waveguides and the realization of quantum information protocols using decoherence-free subspaces.
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P. Heidler, C. Schneider, K. Kustura, C. Gonzalez-Ballestero, O. Romero-Isart, G. Kirchmair Non-Markovian Effects of Two-Level Systems in a Niobium Coaxial Resonator with a Single-Photon Lifetime of 10 ms,
Phys. Rev. Applied 16 34024 (2021-09-13),
http://dx.doi.org/10.1103/PhysRevApplied.16.034024 doi:10.1103/PhysRevApplied.16.034024 (ID: 720630)
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Understanding and mitigating loss channels due to two-level systems (TLS) is one of the main corner stones in the quest of realizing long photon lifetimes in superconducting quantum circuits. Typically, the TLS to which a circuit couples are modelled as a large bath without any coherence. Here we demonstrate that the coherence of TLS has to be considered to accurately describe the ring-down dynamics of a coaxial quarter-waver resonator with an internal quality factor of $0.5\times10^9$ at the single-photon level. The transient analysis reveals an effective non-markovian dynamics of the combined TLS and cavity system, which we can accurately fit by introducing a comprehensive TLS model. The fit returns relaxation times around $T_1=0.8\,\mathrm{\mu s}$ for a total of $N\approx 2 \times 10^{8}$ TLS with power-law distributed coupling strengths. Despite the short-lived TLS excitations, we observe long-term effects on the cavity decay due to coherent elastic scattering between the resonator field and the TLS. The presented method is applicable to various systems and allows for a simple characterization of TLS properties.
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R. Asensio-Perea, A. Parra-Rodriguez, G. Kirchmair, E. Solano, E. Rico Ortega Chiral states and nonreciprocal phases in a Josephson junction ring,
Phys. Rev. B 103 224525 (2021-06-22),
http://dx.doi.org/10.1103/PhysRevB.103.224525 doi:10.1103/PhysRevB.103.224525 (ID: 720583)
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A. Sharafiev, M. L. Juan, O. Gargiulo, M. Zanner, S. Wögerer, J. García-Ripoll, G. Kirchmair Visualizing the emission of a single photon with frequency and time resolved spectroscopy,
Quantum 5 474 (2021-06-10),
http://dx.doi.org/10.22331/q-2021-06-10-474 doi:10.22331/q-2021-06-10-474 (ID: 720494)
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At the dawn of Quantum Physics, Wigner and Weisskopf obtained a full analytical description (a photon portrait) of the emission of a single photon by a two-level system, using the basis of frequency modes (Weisskopf and Wigner, "Zeitschrift für Physik", 63, 1930). A direct experimental reconstruction of this portrait demands an accurate measurement of a time resolved fluorescence spectrum, with high sensitivity to the off-resonant frequencies and ultrafast dynamics describing the photon creation. In this work we demonstrate such an experimental technique in a superconducting waveguide Quantum Electrodynamics (wQED) platform, using single transmon qubit and two coupled transmon qubits as quantum emitters. In both scenarios, the photon portraits agree quantitatively with the predictions of the input-output theory and qualitatively with Wigner-Weisskopf theory. We believe that our technique allows not only for interesting visualization of fundamental principles, but may serve as a tool, e.g. to realize multi-dimensional spectroscopy in waveguide Quantum Electrodynamics.
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E. I. Rosenthal, C. Schneider, M. Malnou, Z. Zhao, F. Leditzky, B. J. Chapman, W. Wustmann, X. Ma, D. A. Palken, M. Zanner, L. R. Vale, G. C. Hilton, J. Gao, G. Smith, G. Kirchmair, K. Lehnert Efficient and low-backaction quantum measurement using a chip-scale detector,
Phys. Rev. Lett. 126 90503 (2021-03-03),
http://dx.doi.org/10.1103/PhysRevLett.126.090503 doi:10.1103/PhysRevLett.126.090503 (ID: 720580)
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Superconducting qubits are a leading platform for scalable quantum computing and quantum error correction. One feature of this platform is the ability to perform projective measurements orders of magnitude more quickly than qubit decoherence times. Such measurements are enabled by the use of quantum-limited parametric amplifiers in conjunction with ferrite circulators - magnetic devices which provide isolation from noise and decoherence due to amplifier backaction. Because these non-reciprocal elements have limited performance and are not easily integrated on-chip, it has been a longstanding goal to replace them with a scalable alternative. Here, we demonstrate a solution to this problem by using a superconducting switch to control the coupling between a qubit and amplifier. Doing so, we measure a transmon qubit using a single, chip-scale device to provide both parametric amplification and isolation from the bulk of amplifier backaction. This measurement is also fast, high fidelity, and has 70% efficiency, comparable to the best that has been reported in any superconducting qubit measurement. As such, this work constitutes a high-quality platform for the scalable measurement of superconducting qubits.
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O. Gargiulo, S. Oleschko, J. Prat-Camps, M. Zanner, G. Kirchmair Fast flux control of 3D transmon qubits using a magnetic hose,
Appl. Phys. Lett. 118 (2021-01-04),
http://dx.doi.org/10.1063/5.0032615 doi:10.1063/5.0032615 (ID: 720099)
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N. Friis, A. Melnikov, G. Kirchmair, H. J. Briegel Coherent controlization using superconducting qubits,
Sci. Rep. 5 18036 (2015-12-15),
http://dx.doi.org/10.1038/srep18036 doi:10.1038/srep18036 (ID: 719310)
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Coherent controlization, i.e., coherent conditioning of arbitrary single- or multi-qubit operations on the state of one or more control qubits, is an important ingredient for the flexible implementation of many algorithms in quantum computation. This is of particular significance when certain subroutines are changing over time or when they are frequently modified, such as in decision-making algorithms for learning agents. We propose a scheme to realize coherent controlization for any number of superconducting qubits coupled to a microwave resonator. For two and three qubits, we present an explicit construction that is of high relevance for quantum learning agents. We demonstrate the feasibility of our proposal, taking into account loss, dephasing, and the cavity self-Kerr effect.
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E. T. Holland, B. Vlastakis, R. W. Heeres, M. J. Reagor, U. Vool, Z. Leghtas, L. Frunzio, G. Kirchmair, M. H. Devoret, M. Mirrahimi, R. Schoelkopf Single-photon Resolved Cross-Kerr Interaction for Autonomous Stabilization of Photon-number States,
Phys. Rev. Lett. 115 180501 (2015-04-26),
http://dx.doi.org/10.1103/PhysRevLett.115.180501 doi:10.1103/PhysRevLett.115.180501 (ID: 719226)
Toggle Abstract
Quantum states can be stabilized in the presence of intrinsic and environmental losses by either applying active feedback conditioned on an ancillary system or through reservoir engineering. Reservoir engineering maintains a desired quantum state through a combination of drives and designed entropy evacuation. We propose and implement a quantum reservoir engineering protocol that stabilizes Fock states in a microwave cavity. This protocol is realized with a circuit quantum electrodynamics platform where a Josephson junction provides direct, nonlinear coupling between two superconducting waveguide cavities. The nonlinear coupling results in a single photon resolved cross-Kerr effect between the two cavities enabling a photon number dependent coupling to a lossy environment. The quantum state of the microwave cavity is discussed in terms of a net polarization and is analyzed by a measurement of its steady state Wigner function.
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G. Via, G. Kirchmair, O. Romero-Isart Strong Single-Photon Coupling in Superconducting Quantum Magnetomechanics,
Phys. Rev. Lett. 114 143602 (2015-04-07),
http://dx.doi.org/10.1103/PhysRevLett.114.143602 doi:10.1103/PhysRevLett.114.143602 (ID: 719099)
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We show that the inductive coupling between the quantum mechanical motion of a superconducting microcantilever and a flux-dependent microwave quantum circuit can attain the strong single-photon nanomechanical coupling regime with feasible experimental parameters. We propose to use a superconducting strip, which is in the Meissner state, at the tip of a cantilever. A pick-up coil collects the flux generated by the sheet currents induced by an external quadrupole magnetic field centered at the strip location. The position-dependent magnetic response of the superconducting strip, enhanced by both diamagnetism and demagnetizing effects, leads to a strong magnetomechanical coupling to quantum circuits.
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M. Dalmonte, S. Mirzaei, P. R. Muppalla, D. Marcos, P. Zoller, G. Kirchmair Dipolar Spin Models with Arrays of Superconducting Qubits,
Phys. Rev. B 92 174507 (2015-01-13),
http://dx.doi.org/10.1103/PhysRevB.92.174507 doi:10.1103/PhysRevB.92.174507 (ID: 719116)
Toggle Abstract
We propose a novel platform for quantum many body simulations of dipolar spin models using current circuit QED technology. Our basic building blocks are 3D Transmon qubits where we use the naturally occurring dipolar interactions to realize interacting spin systems. This opens the way toward the realization of a broad class of tunable spin models in both two- and one-dimensional geometries. We illustrate the potential offered by these systems in the context of dimerized Majumdar-Ghosh-type phases, archetypical examples of quantum magnetism, showing how such phases are robust against disorder and decoherence, and could be observed within state-of-the-art experiments.
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B. Vlastakis, G. Kirchmair, Z. Leghtas, S. E. Nigg, L. Frunzio, S. M. Girvin, M. Mirrahimi, M. H. Devoret, R. Schoelkopf Deterministically encoding quantum information in 100-photon Schrödinger cat states,
Science 342 610 (2013-09-26),
http://dx.doi.org/10.1126/science.1243289 doi:10.1126/science.1243289 (ID: 718631)
Toggle Abstract
In contrast to a single quantum bit, an oscillator can store multiple excitations and coherences, provided one has the ability to generate and manipulate complex multiphoton states. We demonstrate multiphoton control using a superconducting transmon qubit coupled to a waveguide cavity resonator with a highly ideal off-resonant coupling. This dispersive interaction is much greater than decoherence rates and higher-order nonlinearities to allow simultaneous manipulation of hundreds of photons. With a toolset of conditional qubit-photon logic, we map an arbitrary qubit state to a superposition of coherent states, known as a "cat state." We create cat states as large as 111 photons and extend this protocol to create superpositions of up to four coherent states. This control creates a powerful interface between discrete and continuous variable quantum computation and could enable applications in metrology and quantum information processing.
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Z. Leghtas, G. Kirchmair, B. Vlastakis, R. Schoelkopf, M. H. Devoret, M. Mirrahimi Hardware-efficient autonomous quantum memory protection,
Phys. Rev. Lett. 111 120501 (2013-09-20),
http://dx.doi.org/10.1103/PhysRevLett.111.120501 doi:10.1103/PhysRevLett.111.120501 (ID: 718630)
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We propose to encode a quantum bit of information in a superposition of coherent states of an oscillator, with four different phases. Our encoding in a single cavity mode, together with a protection protocol, significantly reduces the error rate due to photon loss. This protection is ensured by an efficient quantum error correction scheme employing the nonlinearity provided by a single physical qubit coupled to the cavity. We describe in detail how to implement these operations in a circuit quantum electrodynamics system. This proposal directly addresses the task of building a hardware-efficient quantum memory and can lead to important shortcuts in quantum computing architectures.
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Z. Leghtas, G. Kirchmair, B. Vlastakis, M. H. Devoret, R. Schoelkopf, M. Mirrahimi Deterministic protocol for mapping a qubit to coherent state superpositions in a cavity,
Phys. Rev. A 87 042315 (2013-04-15),
http://dx.doi.org/10.1103/PhysRevA.87.042315 doi:10.1103/PhysRevA.87.042315 (ID: 718629)
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We propose and analyze a quantum gate that transfers an arbitrary state of a qubit into a superposition of two quasiorthogonal coherent states of a cavity mode (qcMAP), with opposite phases. This qcMAP gate is based on conditional qubit and cavity operations exploiting the energy-level dispersive shifts in the regime where they are much stronger than the cavity and qubit linewidths. The generation of multicomponent superpositions of quasiorthogonal coherent states, nonlocal entangled states of two resonators, and multiqubit Greenberger-Horne-Zeilinger states can be efficiently achieved by this gate.
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G. Kirchmair, B. Vlastakis, Z. Leghtas, S. E. Nigg, H. Paik, E. Ginossar, M. Mirrahimi, L. Frunzio, S. M. Girvin, R. Schoelkopf Observation of quantum state collapse and revival due to the single-photon Kerr effect,
Nature 495 209 (2013-03-13),
http://dx.doi.org/10.1038/nature11902 doi:10.1038/nature11902 (ID: 718628)
Toggle Abstract
To create and manipulate non-classical states of light for quantum information protocols, a strong, nonlinear interaction at the single-photon level is required. One approach to the generation of suitable interactions is to couple photons to atoms, as in the strong coupling regime of cavity quantum electrodynamic systems. In these systems, however, the quantum state of the light is only indirectly controlled by manipulating the atoms. A direct photon–photon interaction occurs in so-called Kerr media, which typically induce only weak nonlinearity at the cost of significant loss. So far, it has not been possible to reach the single-photon Kerr regime, in which the interaction strength between individual photons exceeds the loss rate. Here, using a three-dimensional circuit quantum electrodynamic architecture, we engineer an artificial Kerr medium that enters this regime and allows the observation of new quantum effects. We realize a gedanken experiment in which the collapse and revival of a coherent state can be observed. This time evolution is a consequence of the quantization of the light field in the cavity and the nonlinear interaction between individual photons. During the evolution, non-classical superpositions of coherent states (that is, multi-component ‘Schrödinger cat’ states) are formed. We visualize this evolution by measuring the Husimi Q function and confirm the non-classical properties of these transient states by cavity state tomography. The ability to create and manipulate superpositions of coherent states in such a high-quality-factor photon mode opens perspectives for combining the physics of continuous variables with superconducting circuits. The single-photon Kerr effect could be used in quantum non-demolition measurement of photons, single-photon generation, autonomous quantum feedback schemes and quantum logic operations.
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