43 research outputs found
Atomic-scale coexistence of short-range magnetic order and superconductivity in FeSeTe
The ground state of the parent compounds of many high temperature
superconductors is an antiferromagnetically (AFM) ordered phase, where
superconductivity emerges when the AFM phase transition is suppressed by doping
or application of pressure. This behaviour implies a close relation between the
two orders. Understanding the interplay between them promises a better
understanding of how the superconducting condensate forms from the AFM ordered
background. Here we explore this relation in real space at the atomic scale
using low temperature spin-polarized scanning tunneling microscopy (SP-STM) and
spectroscopy. We investigate the transition from antiferromagnetically ordered
via the spin glass phase in
to superconducting
. In
we observe an
atomic-scale coexistence of superconductivity and short-ranged bicollinear
antiferromagnetic order.Comment: 7 pages, 6 figure
Autoparametric resonance extending the bit-flip time of a cat qubit up to 0.3 s
Cat qubits, for which logical and are coherent states
of a harmonic mode, offer a promising route towards quantum
error correction. Using dissipation to our advantage so that photon pairs of
the harmonic mode are exchanged with single photons of its environment, it is
possible to stabilize the logical states and exponentially increase the
bit-flip time of the cat qubit with the photon number . Large
two-photon dissipation rate ensures fast qubit manipulation and
short error correction cycles, which are instrumental to correct the remaining
phase-flip errors in a repetition code of cat qubits. Here we introduce and
operate an autoparametric superconducting circuit that couples a mode
containing the cat qubit to a lossy mode whose frequency is set at twice that
of the cat mode. This passive coupling does not require a parametric pump and
reaches a rate . With such a strong
two-photon dissipation, bit-flip errors of the autoparametric cat qubit are
prevented for a characteristic time up to 0.3 s with only a mild impact on
phase-flip errors. Besides, we illustrate how the phase of a quantum
superposition between and can be arbitrarily
changed by driving the harmonic mode while keeping the engineered dissipation
active
Stochastic and resolvable gravitational waves from ultralight bosons
Ultralight scalar fields around spinning black holes can trigger superradiant instabilities, forming a long-lived bosonic condensate outside the horizon. We use numerical solutions of the perturbed field equations and astrophysical models of massive and stellar-mass black hole populations to compute, for the first time, the stochastic gravitational-wave background from these sources. In optimistic scenarios the background is observable by Advanced LIGO and LISA for field masses ms in the range 3c[2
710-13,10-12] and 3c5
7[10-19,10-16] eV, respectively, and it can affect the detectability of resolvable sources. Our estimates suggest that an analysis of the stochastic background limits from LIGO O1 might already be used to marginally exclude axions with mass 3c10-12.5 eV. Semicoherent searches with Advanced LIGO (LISA) should detect 3c15(5) to 200(40) resolvable sources for scalar field masses 3
710-13 (10-17) eV. LISA measurements of massive BH spins could either rule out bosons in the range 3c[10-18,2
710-13] eV, or measure ms with 10% accuracy in the range 3c[10-17,10-13] eV
Atomic-scale coexistence of short-range magnetic order and superconductivity in Fe1+ySe0.1Te0.9
Funding: UK EPSRC (EP/I031014/1) (HZ, J-PR, and PW)The ground state of the parent compounds of many high-temperature superconductors is an antiferromagnetically ordered phase, where superconductivity emerges when the antiferromagnetic phase transition is suppressed by doping or application of pressure. This behavior implies a close relation between the two orders. Examining the interplay between them promises a better understanding of how the superconducting condensate forms from the antiferromagnetically ordered background. Here we explore this relation in real space at the atomic scale using low-temperature spin-polarized scanning tunneling microscopy and spectroscopy. We investigate the transition from antiferromagnetically ordered Fe1+yTe via the spin-glass phase in Fe1+ySe0.1Te0.9 to superconducting Fe1+ySe0.15Te0.85. In Fe1+ySe0.1Te0.9 we observe an atomic-scale coexistence of superconductivity and short-ranged bicollinear antiferromagnetic order. However, a direct correlation between the two orders is not observed, supporting the scenario of s± superconducting symmetry in this material. Our work demonstrates a direct probe of the relation between the two orders, which is indispensable for our understanding of high-temperature superconductivity.Publisher PDFPeer reviewe
Compteur de photon basé sur une mesure dispersive multiplexée
Lorsque lâon utilise un bit quantique (qubit) pour sonder lâĂ©tat dâun systĂšme, la stratĂ©gie habituelle consiste Ă poser une sĂ©rie de questions binaires, chaque question amĂ©liorant notre connaissance de lâĂ©tat du systĂšme. Cependant, cette stratĂ©gie nĂ©cessite de longs temps de mesure lorsque lâon considĂšre un grand systĂšme, comme par exemple un rĂ©sonateur Ă©lectromagnĂ©tique peuplĂ© dâun grand nombre de photons, car chaque question ne peut extraire quâun bit dâinformation. Dans cette thĂšse, nous proposons une nouvelle stratĂ©gie qui permet dâobtenir un temps de mesure indĂ©pendant de la taille du systĂšme. Cette nouvelle approche est basĂ©e sur lâutilisation dâun qubit comme routeur, ce qui permet dâencoder lâinformation sur lâĂ©tat du systĂšme dans les nombreux modes dâune ligne de transmission.Dans le cas dâun dĂ©tecteur idĂ©al, nous montrons Ă lâaide dâune expĂ©rience de pensĂ©e que cette stratĂ©gie permet de mesurer le nombre de photons contenu dans une cavitĂ© en un temps constant, indĂ©pendant de la taille du systĂšme. Pour dĂ©montrer la faisabilitĂ© de cette mesure idĂ©ale, nous appliquons cette stratĂ©gie Ă la mesure du nombre de photons contenu dans un rĂ©sonateur micro-onde couplĂ© dispersivement Ă un qubit supraconducteur. Dans un premier temps, la fluorescence du qubit est mesurĂ©e lorsque ce dernier est sondĂ© Ă lâaide dâun ton micro-onde monochromatique. Lâaction en retour de cette mesure dispersive est Ă©tudiĂ©e, nous dĂ©montrons Ă travers la post-sĂ©lection que la fluorescence du qubit encode effectivement le nombre de photons contenu dans le rĂ©sonateur. Nous mesurons le taux de dĂ©phasage induit par la mesure entre deux Ă©tats de Fock du rĂ©sonateur et le comparons Ă un modĂšle thĂ©orique. Ce dernier nous permet alors dâĂ©tudier le comportement non-linĂ©aire du taux de dĂ©phasage induit par la mesure avec lâamplitude du ton micro-onde.Dans un deuxiĂšme temps, la fluorescence du qubit est sondĂ©e Ă lâaide dâun peigne de frĂ©quence. Des mesures hĂ©tĂ©rodynes multiplexĂ©es Ă tous les tons du peigne de frĂ©quence nous permettent alors de mesurer le nombre de photons contenus dans le rĂ©sonateur. Cette mesure multiplexĂ©e est rendue possible grĂące aux rĂ©centes amĂ©liorations sur la bande passante des amplificateurs limitĂ©s quantiquement. Le temps de vie du rĂ©sonateur et une efficacitĂ© de mesure limitĂ©s nous empĂȘchent d'atteindre un rapport signal sur bruit permettant de dĂ©coder toute l'information contenue dans notre mesure hĂ©tĂ©rodyne multiplexĂ©e. Cependant, contrairement Ă une mesure sĂ©quentielle, notre approche fournit en parallĂšle une information partielle sur la population de chaque Ă©tat de Fock. Lâaction en retour de cette mesure dispersive multiplexĂ©e est Ă©tudiĂ©e Ă lâaide de tomographies de Wigner du rĂ©sonateur. Nous sommes ainsi capables de mesurer le taux de dĂ©phasage induit pas la mesure multiplexĂ©e et mettons en Ă©vidence une amplitude optimale du peigne de frĂ©quence qui maximise le taux de dĂ©phasage. Un modĂšle thĂ©orique basĂ© sur lâapproximation que le peigne de frĂ©quence est infini nous permet de prĂ©dire lâamplitude optimale du peigne, et ce en accord avec lâexpĂ©rience.When a two-level system â a qubit â is used to probe a larger system, it naturally leads to answering a single yes-no question about the system state. Identifying what is the state of a system thus comes down to ask a series of binary questions iteratively to refine our knowledge. However, this approach leads to long measurement times for large systems, such as a resonator containing a large number of photons. In this thesis, we propose a new approach which enables us to make a measurement in a time, which is independent of the system size. This new measurement uses the qubit as an encoder of information about the system state into the many propagating modes of a transmission line. Assuming an ideal detector, we show that photon counting can then be implemented in a fixed time whatever the number of photons. We demonstrate the practicality of this approach by counting the number of photons in a microwave resonator coupled dispersively to a single superconducting qubit. We observe the qubit fluorescence dependence on the resonator photon number when the qubit is driven by a microwave monochromatic tone. Using the backaction of this dispersive measurement and post-selection, we evidence the photon counting ability of the measurement. The dephasing rate between two Fock states induced by the photon number measurement is measured and compared to theory. The latter allows us the study the non-linear dependence of the dephasing rate on the microwave drive amplitude.In a second time, the qubit fluorescence is probed using a frequency comb. Multiplexed heterodyne detections are simultaneously performed at each comb frequency and allow us to measure the photon number in the microwave resonator. This multiplexed measurement benefits from the recent bandwidth improvements of near quantum limited amplifiers. The limited cavity lifetime and detector efficiency prevented us from reaching single shot readout of the photon number in this proof-of-principle experiment. However, unlike in sequential measurement schemes, a single run of our experiment does provide, in parallel, partial information about the occupancy of each Fock state. Besides, we manage to observe the multiplexed measurement backaction on the resonator using direct Wigner tomography, which allowed us to measure the decoherence rate of the resonator induced by the measurement. We evidence an optimal qubit drive amplitude for information extraction, which matches the expected dynamics of a qubit under a multifrequency drive
Photon counting with a multiplexed dispersive readout
When a two-level system â a qubit â is used to probe a larger system, it naturally leads to answering a single yes-no question about the system state. Identifying what is the state of a system thus comes down to ask a series of binary questions iteratively to refine our knowledge. However, this approach leads to long measurement times for large systems, such as a resonator containing a large number of photons. In this thesis, we propose a new approach which enables us to make a measurement in a time, which is independent of the system size. This new measurement uses the qubit as an encoder of information about the system state into the many propagating modes of a transmission line. Assuming an ideal detector, we show that photon counting can then be implemented in a fixed time whatever the number of photons. We demonstrate the practicality of this approach by counting the number of photons in a microwave resonator coupled dispersively to a single superconducting qubit. We observe the qubit fluorescence dependence on the resonator photon number when the qubit is driven by a microwave monochromatic tone. Using the backaction of this dispersive measurement and post-selection, we evidence the photon counting ability of the measurement. The dephasing rate between two Fock states induced by the photon number measurement is measured and compared to theory. The latter allows us the study the non-linear dependence of the dephasing rate on the microwave drive amplitude.In a second time, the qubit fluorescence is probed using a frequency comb. Multiplexed heterodyne detections are simultaneously performed at each comb frequency and allow us to measure the photon number in the microwave resonator. This multiplexed measurement benefits from the recent bandwidth improvements of near quantum limited amplifiers. The limited cavity lifetime and detector efficiency prevented us from reaching single shot readout of the photon number in this proof-of-principle experiment. However, unlike in sequential measurement schemes, a single run of our experiment does provide, in parallel, partial information about the occupancy of each Fock state. Besides, we manage to observe the multiplexed measurement backaction on the resonator using direct Wigner tomography, which allowed us to measure the decoherence rate of the resonator induced by the measurement. We evidence an optimal qubit drive amplitude for information extraction, which matches the expected dynamics of a qubit under a multifrequency drive.Lorsque lâon utilise un bit quantique (qubit) pour sonder lâĂ©tat dâun systĂšme, la stratĂ©gie habituelle consiste Ă poser une sĂ©rie de questions binaires, chaque question amĂ©liorant notre connaissance de lâĂ©tat du systĂšme. Cependant, cette stratĂ©gie nĂ©cessite de longs temps de mesure lorsque lâon considĂšre un grand systĂšme, comme par exemple un rĂ©sonateur Ă©lectromagnĂ©tique peuplĂ© dâun grand nombre de photons, car chaque question ne peut extraire quâun bit dâinformation. Dans cette thĂšse, nous proposons une nouvelle stratĂ©gie qui permet dâobtenir un temps de mesure indĂ©pendant de la taille du systĂšme. Cette nouvelle approche est basĂ©e sur lâutilisation dâun qubit comme routeur, ce qui permet dâencoder lâinformation sur lâĂ©tat du systĂšme dans les nombreux modes dâune ligne de transmission.Dans le cas dâun dĂ©tecteur idĂ©al, nous montrons Ă lâaide dâune expĂ©rience de pensĂ©e que cette stratĂ©gie permet de mesurer le nombre de photons contenu dans une cavitĂ© en un temps constant, indĂ©pendant de la taille du systĂšme. Pour dĂ©montrer la faisabilitĂ© de cette mesure idĂ©ale, nous appliquons cette stratĂ©gie Ă la mesure du nombre de photons contenu dans un rĂ©sonateur micro-onde couplĂ© dispersivement Ă un qubit supraconducteur. Dans un premier temps, la fluorescence du qubit est mesurĂ©e lorsque ce dernier est sondĂ© Ă lâaide dâun ton micro-onde monochromatique. Lâaction en retour de cette mesure dispersive est Ă©tudiĂ©e, nous dĂ©montrons Ă travers la post-sĂ©lection que la fluorescence du qubit encode effectivement le nombre de photons contenu dans le rĂ©sonateur. Nous mesurons le taux de dĂ©phasage induit par la mesure entre deux Ă©tats de Fock du rĂ©sonateur et le comparons Ă un modĂšle thĂ©orique. Ce dernier nous permet alors dâĂ©tudier le comportement non-linĂ©aire du taux de dĂ©phasage induit par la mesure avec lâamplitude du ton micro-onde.Dans un deuxiĂšme temps, la fluorescence du qubit est sondĂ©e Ă lâaide dâun peigne de frĂ©quence. Des mesures hĂ©tĂ©rodynes multiplexĂ©es Ă tous les tons du peigne de frĂ©quence nous permettent alors de mesurer le nombre de photons contenus dans le rĂ©sonateur. Cette mesure multiplexĂ©e est rendue possible grĂące aux rĂ©centes amĂ©liorations sur la bande passante des amplificateurs limitĂ©s quantiquement. Le temps de vie du rĂ©sonateur et une efficacitĂ© de mesure limitĂ©s nous empĂȘchent d'atteindre un rapport signal sur bruit permettant de dĂ©coder toute l'information contenue dans notre mesure hĂ©tĂ©rodyne multiplexĂ©e. Cependant, contrairement Ă une mesure sĂ©quentielle, notre approche fournit en parallĂšle une information partielle sur la population de chaque Ă©tat de Fock. Lâaction en retour de cette mesure dispersive multiplexĂ©e est Ă©tudiĂ©e Ă lâaide de tomographies de Wigner du rĂ©sonateur. Nous sommes ainsi capables de mesurer le taux de dĂ©phasage induit pas la mesure multiplexĂ©e et mettons en Ă©vidence une amplitude optimale du peigne de frĂ©quence qui maximise le taux de dĂ©phasage. Un modĂšle thĂ©orique basĂ© sur lâapproximation que le peigne de frĂ©quence est infini nous permet de prĂ©dire lâamplitude optimale du peigne, et ce en accord avec lâexpĂ©rience
Photon counting with a multiplexed dispersive readout
When a two-level system â a qubit â is used to probe a larger system, it naturally leads to answering a single yes-no question about the system state. Identifying what is the state of a system thus comes down to ask a series of binary questions iteratively to refine our knowledge. However, this approach leads to long measurement times for large systems, such as a resonator containing a large number of photons. In this thesis, we propose a new approach which enables us to make a measurement in a time, which is independent of the system size. This new measurement uses the qubit as an encoder of information about the system state into the many propagating modes of a transmission line. Assuming an ideal detector, we show that photon counting can then be implemented in a fixed time whatever the number of photons. We demonstrate the practicality of this approach by counting the number of photons in a microwave resonator coupled dispersively to a single superconducting qubit. We observe the qubit fluorescence dependence on the resonator photon number when the qubit is driven by a microwave monochromatic tone. Using the backaction of this dispersive measurement and post-selection, we evidence the photon counting ability of the measurement. The dephasing rate between two Fock states induced by the photon number measurement is measured and compared to theory. The latter allows us the study the non-linear dependence of the dephasing rate on the microwave drive amplitude.In a second time, the qubit fluorescence is probed using a frequency comb. Multiplexed heterodyne detections are simultaneously performed at each comb frequency and allow us to measure the photon number in the microwave resonator. This multiplexed measurement benefits from the recent bandwidth improvements of near quantum limited amplifiers. The limited cavity lifetime and detector efficiency prevented us from reaching single shot readout of the photon number in this proof-of-principle experiment. However, unlike in sequential measurement schemes, a single run of our experiment does provide, in parallel, partial information about the occupancy of each Fock state. Besides, we manage to observe the multiplexed measurement backaction on the resonator using direct Wigner tomography, which allowed us to measure the decoherence rate of the resonator induced by the measurement. We evidence an optimal qubit drive amplitude for information extraction, which matches the expected dynamics of a qubit under a multifrequency drive.Lorsque lâon utilise un bit quantique (qubit) pour sonder lâĂ©tat dâun systĂšme, la stratĂ©gie habituelle consiste Ă poser une sĂ©rie de questions binaires, chaque question amĂ©liorant notre connaissance de lâĂ©tat du systĂšme. Cependant, cette stratĂ©gie nĂ©cessite de longs temps de mesure lorsque lâon considĂšre un grand systĂšme, comme par exemple un rĂ©sonateur Ă©lectromagnĂ©tique peuplĂ© dâun grand nombre de photons, car chaque question ne peut extraire quâun bit dâinformation. Dans cette thĂšse, nous proposons une nouvelle stratĂ©gie qui permet dâobtenir un temps de mesure indĂ©pendant de la taille du systĂšme. Cette nouvelle approche est basĂ©e sur lâutilisation dâun qubit comme routeur, ce qui permet dâencoder lâinformation sur lâĂ©tat du systĂšme dans les nombreux modes dâune ligne de transmission.Dans le cas dâun dĂ©tecteur idĂ©al, nous montrons Ă lâaide dâune expĂ©rience de pensĂ©e que cette stratĂ©gie permet de mesurer le nombre de photons contenu dans une cavitĂ© en un temps constant, indĂ©pendant de la taille du systĂšme. Pour dĂ©montrer la faisabilitĂ© de cette mesure idĂ©ale, nous appliquons cette stratĂ©gie Ă la mesure du nombre de photons contenu dans un rĂ©sonateur micro-onde couplĂ© dispersivement Ă un qubit supraconducteur. Dans un premier temps, la fluorescence du qubit est mesurĂ©e lorsque ce dernier est sondĂ© Ă lâaide dâun ton micro-onde monochromatique. Lâaction en retour de cette mesure dispersive est Ă©tudiĂ©e, nous dĂ©montrons Ă travers la post-sĂ©lection que la fluorescence du qubit encode effectivement le nombre de photons contenu dans le rĂ©sonateur. Nous mesurons le taux de dĂ©phasage induit par la mesure entre deux Ă©tats de Fock du rĂ©sonateur et le comparons Ă un modĂšle thĂ©orique. Ce dernier nous permet alors dâĂ©tudier le comportement non-linĂ©aire du taux de dĂ©phasage induit par la mesure avec lâamplitude du ton micro-onde.Dans un deuxiĂšme temps, la fluorescence du qubit est sondĂ©e Ă lâaide dâun peigne de frĂ©quence. Des mesures hĂ©tĂ©rodynes multiplexĂ©es Ă tous les tons du peigne de frĂ©quence nous permettent alors de mesurer le nombre de photons contenus dans le rĂ©sonateur. Cette mesure multiplexĂ©e est rendue possible grĂące aux rĂ©centes amĂ©liorations sur la bande passante des amplificateurs limitĂ©s quantiquement. Le temps de vie du rĂ©sonateur et une efficacitĂ© de mesure limitĂ©s nous empĂȘchent d'atteindre un rapport signal sur bruit permettant de dĂ©coder toute l'information contenue dans notre mesure hĂ©tĂ©rodyne multiplexĂ©e. Cependant, contrairement Ă une mesure sĂ©quentielle, notre approche fournit en parallĂšle une information partielle sur la population de chaque Ă©tat de Fock. Lâaction en retour de cette mesure dispersive multiplexĂ©e est Ă©tudiĂ©e Ă lâaide de tomographies de Wigner du rĂ©sonateur. Nous sommes ainsi capables de mesurer le taux de dĂ©phasage induit pas la mesure multiplexĂ©e et mettons en Ă©vidence une amplitude optimale du peigne de frĂ©quence qui maximise le taux de dĂ©phasage. Un modĂšle thĂ©orique basĂ© sur lâapproximation que le peigne de frĂ©quence est infini nous permet de prĂ©dire lâamplitude optimale du peigne, et ce en accord avec lâexpĂ©rience
Deep Reinforcement Learning for Quantum State Preparation with Weak Nonlinear Measurements
International audienceQuantum control has been of increasing interest in recent years, e.g. for tasks like state initialization and stabilization. Feedback-based strategies are particularly powerful, but also hard to find, due to the exponentially increased search space. Deep reinforcement learning holds great promise in this regard. It may provide new answers to difficult questions, such as whether nonlinear measurements can compensate for linear, constrained control. Here we show that reinforcement learning can successfully discover such feedback strategies, without prior knowledge. We illustrate this for state preparation in a cavity subject to quantum-non-demolition detection of photon number, with a simple linear drive as control. Fock states can be produced and stabilized at very high fidelity. It is even possible to reach superposition states, provided the measurement rates for different Fock states can be controlled as well