Abstract:We introduce a magnetic-flux-tunable phase shifter for propagating microwave photons, based on three equidistant superconducting quantum interference devices (SQUIDs) on a transmission line. We experimentally implement the phase shifter and demonstrate that it produces a broad range of phase shifts and full transmission within the experimental uncertainty. Together with previously demonstrated beam splitters, this phase shifter can be utilized to implement arbitrary single-qubit gates for qubits based on propa… Show more
“…Our device does not internally lose photons upon detection and is built with separate detection and readout lines, which provides easy access to the reflected radiation field. This allows one to take advantage of the nondemolition nature of the detector and use the device as a mediator of photon-photon interactions for allphotonic quantum computation [32][33][34][35][36]. Other applications include heralded entanglement [26] with a high rate, without the need to shape the photons or to perform Bell state analysis [13].…”
Single-photon detection is an essential component in many experiments in quantum optics, but it remains challenging in the microwave domain. We realize a quantum nondemolition detector for propagating microwave photons and characterize its performance using a single-photon source. To this aim, we implement a cavity-assisted conditional phase gate between the incoming photon and a superconducting artificial atom. By reading out the state of this atom in a single shot, we reach an external (internal) photon-detection fidelity of 50% (71%), limited by transmission efficiency between the source and the detector (75%) and the coherence properties of the qubit. By characterizing the coherence and average number of photons in the field reflected off the detector, we demonstrate its quantum nondemolition nature. We envisage applications in generating heralded remote entanglement between qubits and for realizing logic gates between propagating microwave photons. DOI: 10.1103/PhysRevX.8.021003 Subject Areas: Quantum Physics, Quantum InformationSingle-photon detectors [1] for itinerant fields are a key element in remote entanglement protocols [2], in linear optics quantum computation [3,4], and, in general, in characterizing correlation properties of radiation fields [5]. While such detectors are well established at optical frequencies, their microwave equivalents are still under development, partly because of the much lower photon energy in this frequency band [6]. At microwave frequencies, itinerant fields are typically recorded with linear detection schemes [7], analogous to optical homodyne detection. Such detection can now be realized with high efficiency by employing near-quantum-limited parametric amplifiers [8], and furthermore, it allows for a full tomographic characterization of radiation fields [9]. However, protocols such as entanglement heralding require the intrinsic nonlinearity of a single-photon detector in order to yield high-purity states despite losses between the source and the detector. Such a component has therefore raised interest in the community, leading to a variety of theoretical proposals [10][11][12][13][14][15][16][17][18][19][20], as well as initial experimental demonstrations, in the last decade [21][22][23][24][25][26][27].The first microwave photodetection experiment with superconducting circuits that did not require photons to be stored in high-quality factor cavities [21-23] was based on current biased Josephson junctions [24], but it was destructive and involved a long dead time. Later, systems involving absorption into artificial atoms (and thus destruction) of traveling photons [25,26] were implemented. Very recently, a quantum nondemolition (QND) detection scheme based on a photon-qubit entangling gate, similar in spirit to this work, was implemented using a strong dispersive shift in a 3D cavity [27]. Projective measurements of coherent input states into single-photon Fock states were realized in that work [27].Here, we demonstrate single-shot QND detection of itinerant single ph...
“…Our device does not internally lose photons upon detection and is built with separate detection and readout lines, which provides easy access to the reflected radiation field. This allows one to take advantage of the nondemolition nature of the detector and use the device as a mediator of photon-photon interactions for allphotonic quantum computation [32][33][34][35][36]. Other applications include heralded entanglement [26] with a high rate, without the need to shape the photons or to perform Bell state analysis [13].…”
Single-photon detection is an essential component in many experiments in quantum optics, but it remains challenging in the microwave domain. We realize a quantum nondemolition detector for propagating microwave photons and characterize its performance using a single-photon source. To this aim, we implement a cavity-assisted conditional phase gate between the incoming photon and a superconducting artificial atom. By reading out the state of this atom in a single shot, we reach an external (internal) photon-detection fidelity of 50% (71%), limited by transmission efficiency between the source and the detector (75%) and the coherence properties of the qubit. By characterizing the coherence and average number of photons in the field reflected off the detector, we demonstrate its quantum nondemolition nature. We envisage applications in generating heralded remote entanglement between qubits and for realizing logic gates between propagating microwave photons. DOI: 10.1103/PhysRevX.8.021003 Subject Areas: Quantum Physics, Quantum InformationSingle-photon detectors [1] for itinerant fields are a key element in remote entanglement protocols [2], in linear optics quantum computation [3,4], and, in general, in characterizing correlation properties of radiation fields [5]. While such detectors are well established at optical frequencies, their microwave equivalents are still under development, partly because of the much lower photon energy in this frequency band [6]. At microwave frequencies, itinerant fields are typically recorded with linear detection schemes [7], analogous to optical homodyne detection. Such detection can now be realized with high efficiency by employing near-quantum-limited parametric amplifiers [8], and furthermore, it allows for a full tomographic characterization of radiation fields [9]. However, protocols such as entanglement heralding require the intrinsic nonlinearity of a single-photon detector in order to yield high-purity states despite losses between the source and the detector. Such a component has therefore raised interest in the community, leading to a variety of theoretical proposals [10][11][12][13][14][15][16][17][18][19][20], as well as initial experimental demonstrations, in the last decade [21][22][23][24][25][26][27].The first microwave photodetection experiment with superconducting circuits that did not require photons to be stored in high-quality factor cavities [21-23] was based on current biased Josephson junctions [24], but it was destructive and involved a long dead time. Later, systems involving absorption into artificial atoms (and thus destruction) of traveling photons [25,26] were implemented. Very recently, a quantum nondemolition (QND) detection scheme based on a photon-qubit entangling gate, similar in spirit to this work, was implemented using a strong dispersive shift in a 3D cavity [27]. Projective measurements of coherent input states into single-photon Fock states were realized in that work [27].Here, we demonstrate single-shot QND detection of itinerant single ph...
“…As shown in Eq. ( 3), the distance of the two pointer states from interference output depends on the relative phase θ RT , which experimentally can be tuned via a cryogenic phase shifter [32][33][34][35]. Here to demonstrate the idea, we vary θ RT by placing the readout cavity at different positions of the transmission line.…”
Section: Resultsmentioning
confidence: 99%
“…For the single shot qubit state measurement, we have observed a significant suppression on both measurement error and total readout error when using cavity interference as the readout signal instead of the output from T . In order to realize simultaneous improvement for multiple cavities coupled to a common transmission line, one could take the distance between adjacent cavities along the transmission line as λ/2, where λ corresponds to the mean value of the resonant frequencies of the cavities, and use a cryogenic phase shifter to realize constructive interference [32][33][34][35]. As a general scheme to effectively extract the cavity photons, our method can be combined with other readout optimization methods to further improve the performance of superconducting qubit state readout.…”
High fidelity single shot qubit state readout is essential for many quantum information processing protocols. In superconducting quantum circuit, the qubit state is usually determined by detecting the dispersive frequency shift of a microwave cavity from either transmission or reflection. In this paper, we demonstrate the use of constructive interference between the transmitted and reflected signal to optimize the qubit state readout, with which we find a better resolved state discrimination and an improved qubit readout fidelity. As a simple and convenient approach, our scheme can be combined with other qubit readout methods based on the discrimination of cavity photon states to further improve the qubit state readout.
“…In the current implementation, even for a fixed resource TMS state, S rp and γ rp can be changed at the expense of a reduced purity μ . By adding a phase shifter 39 on her side, Alice could prepare squeezed states with arbitrary γ rp while keeping S rp and μ constant.Fig. 3Purity of remotely prepared states in phase space.…”
Quantum communication protocols based on nonclassical correlations can be more efficient than known classical methods and offer intrinsic security over direct state transfer. In particular, remote state preparation aims at the creation of a desired and known quantum state at a remote location using classical communication and quantum entanglement. We present an experimental realization of deterministic continuous-variable remote state preparation in the microwave regime over a distance of 35 cm. By employing propagating two-mode squeezed microwave states and feedforward, we achieve the remote preparation of squeezed states with up to 1.6 dB of squeezing below the vacuum level. Finally, security of remote state preparation is investigated by using the concept of the one-time pad and measuring the von Neumann entropies. We find nearly identical values for the entropy of the remotely prepared state and the respective conditional entropy given the classically communicated information and, thus, demonstrate close-to-perfect security.
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