Quantum nondemolition measurement allows physicists to count photons without destroying them

first_img“A typical photon detector, like a CCD or photo-multiplier tube, absorbs photons,” Johnson said. “These detectors don’t work for microwaves because the energy of a microwave photon is too small to generate charges. However, with a setup similar to the one used in our paper, one could measure the photon state by transferring the photon energy into the qubit. This method would destroy exactly one photon every time. In contrast, our detector does not transfer any energy. Instead, we attempt to add energy to the qubit from an external source in such a way that the success or failure of these attempts reveals information about the cavity state. You might worry that this added energy might leak into the cavity and changed the photon number, but we have checked that this does not, in fact, happen.”Achieving QND measurements of photons, while challenging, could be very useful for the development of quantum information technologies, which require complete control of quantum measurements. As the physicists note in their study, recent progress in manipulating microwave photons in superconducting circuits has increased the demand for a QND detector that operates in the gigahertz frequency range (like the one demonstrated here). In addition, the physicists predict that further research could make it possible to observe quantum jumps of light in a circuit, among other things.“QND detection in general is interesting because it is the only way that quantum mechanics allows to extract information from a system without modifying its state, and then allowing feedback and manipulation of the same,” Johnson said. “The applications are interesting because if one could implement feedback of a quantum system, one could imagine using these systems for quantum simulation and quantum computation, harnessing quantum mechanics toward the goal of practical application.” Yale scientists bring quantum optics to a microchip The physicists performed a quantum non-demolition measurement, illustrated in this circuit schematic, that could detect single photons without destroying them. The technique allows repeated measurements to be made that give the same result. Image credit: B.R. Johnson, et al. ©2010 Macmillan Publishers Limited. (PhysOrg.com) — In a way, the quantum world seems to know when it’s being watched. When physicists make measurements on photons and other quantum-scale particles, the measurements always disturb the system in some way. Although an ideal disturbance should still enable physicists to make multiple measurements and get the same result twice, most real measurements cause a greater disturbance than this ideal minimum, and prohibit physicists from making repeated measurements. In a recent study, physicists have demonstrated a new way to make one of the ideal measurements – called quantum non-demolition (QND) measurements – allowing physicists to detect single particles repeatedly without destroying them. The concept of QND measurements has been around since the beginning of quantum mechanics, and physicists have demonstrated different QND measurement techniques since the ‘70s. In the latest technique, developed by a team of physicists from Yale University, Princeton University, and the University of Waterloo, the scientists have shown how to measure the number of photons inside a microwave cavity in a way that preserves the photon state 90% of the time; in other words, the method is 90% QND. The physicists explain that, unlike previously reported QND methods, the new technique is strongly selective to chosen photon number states, which could make it useful for applications such as monitoring the state of a photon-based memory in a quantum computer.In their experiments, the physicists wanted to find out how many photons were in a microwave cavity. To do this without disturbing the system, they coupled a superconducting qubit to a cavity. This cavity stored the photons long enough for them to be measured – or “interrogated” – by using a set of controlled-NOT (CNOT) operations to encode information about the cavity state onto the qubit state. Then the qubit and storage cavity were decoupled, and the qubit state was read out. Because the qubit state now depends on the number of photons in the cavity, measuring the qubit reveals the number of photons.“Our method takes advantage of the ability to engineer interactions between cavities and qubits in superconducting circuits to make the qubit energies strongly depend on the number of photons in the cavity,” coauthor Blake Johnson of Yale University told PhysOrg.com. “We have made this effect large enough to build a new qubit-photon logic gate which allows us to perform conditional qubit operations based on the cavity state. This type of logic gate is not only applicable to photon readout, but also to some proposals for engineering interactions between photons by using a qubit as a mediator.”In the new design, the photon read out time is faster than the photon decay time. This timing difference allows the physicists to measure any qubit state several times during the lifetime of photons in the storage cavity. A single interrogation process takes about 550 nanoseconds, which includes the 50-nanosecond to initialize the qubit state. As expected with a high-quality QND method, the results of repeated interrogations are essentially indistinguishable from the first. In contrast, as Johnson explained, a typical quantum measurement would destroy one photon every time, so that repeated interrogations would give different results. Citation: Quantum non-demolition measurement allows physicists to count photons without destroying them (2010, July 9) retrieved 18 August 2019 from https://phys.org/news/2010-07-quantum-non-demolition-physicists-photons.htmlcenter_img Copyright 2010 PhysOrg.com. All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com. Explore further More information: B.R. Johnson, et al. “Quantum non-demolition detection of single microwave photons in a circuit.” Nature Physics. Advance Online Publication. DOI:10.1038/NPHYS1710 This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.last_img read more

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Atomically precise bottomup synthesis of πextended 5 triangulene

center_img Structural characterization of single π-extended [5]triangulene synthesized on Cu(111) and Au(111) surfaces. (A and D) Large-scale STM images of [5]triangulene molecules (A) on Cu(111) and (D) on Au(111) [(A) Vs = −1 V and I = 1 nA; scale bar, 5 nm; (D) Vs = 1 V and I = 0.2 nA; scale bar, 1.5 nm]. (B and E) Zoom-in STM images of a single [5]triangulene (B) on Cu(111) and (E) on Au(111) [(B) Vs = −0.8 V and I = 1 nA; (E) Vs = −0.8 V and I = 1 nA; scale bar, 4 Å]. (C and F) nc-AFM images of a single [5]triangulene (C) on Cu(111) and (F) on Au(111) acquired using a CO-functionalized tip [(C) ∆z = 0.15 Å, Vs = 30 mV, I = 0.3 nA; (F) ∆z = 0.15 Å, Vs = 10 mV, I = 0.5 nA; scale bar, 4 Å]. fcc, face-centered cubic; hcp, hexagonal close-packed. Credit: Science Advances, doi: 10.1126/sciadv.aav7717. In this way, Jie Su and colleagues demonstrated a feasible bottom-up approach to synthesize atomically precise unsubstituted [5]triangulene on metallic surfaces. They used nc-AFM imaging to ambiguously confirm the zigzag edge topology of the molecule and used STM measurements to resolve the edge localized electronic states. The successful synthesis of π-extended triangulenes will allow scientists to investigate magnetism and spin transport properties at the level of the single-molecule. The scientists envision that the synthetic process will open a new avenue to engineer larger, triangular zigzag edged graphene quantum dots with atomic precision for spin and quantum transport applications. It is therefore of great interest to continue generating similar systems with diverse sizes and spin numbers to uncover their properties on a variety of substrates using spin-polarized STM studies. More information: Jie Su et al. Atomically precise bottom-up synthesis of π-extended [5]triangulene, Science Advances (2019). DOI: 10.1126/sciadv.aav7717 Manuel Melle-Franco. When 1 + 1 is odd, Nature Nanotechnology (2017). DOI: 10.1038/nnano.2017.9 Yasushi Morita et al. Synthetic organic spin chemistry for structurally well-defined open-shell graphene fragments, Nature Chemistry (2011). DOI: 10.1038/nchem.985 Pascal Ruffieux et al. On-surface synthesis of graphene nanoribbons with zigzag edge topology, Nature (2016). DOI: 10.1038/nature17151 Journal information: Science Advances To gain further insights into the [5]triangulene electronic structure, Su et al. performed spin-polarized density functional theory (DFT) calculations. The energy ordering of these electron states were consistent with previous calculations of similar graphene molecular systems. Additionally, the calculations also revealed a total magnetic moment of 3.58 μb for [5]triangulene on the Au substrate, suggesting that its magnetic ground state could be retained on the Au (111) surface. The DFT (density functional theory) provided reliable information on the ground-state energy ordering and spatial shape of molecular orbitals. Su et al. observed the frontier molecular orbitals (highest-energy occupied and lowest-energy unoccupied molecular orbitals) to contain four pairs of orbitals with corresponding wave function plots. Su et al also used the GW method of many-body perturbation to calculate the quasiparticle energies of a free [5]triangulene, where the quasiparticle gap was predicted to be 2.81 eV. They then experimentally determined the energy gap of Au-supported [5]triangulene to be ~1.7 eV consistent with previous studies of GNRs and other molecular systems of comparable size. All observations indicated a magnetic ground state of [5]triangulene on Au (111), which the scientists also validated with the DFT calculations. In a recent report on Science Advances, Jie Su and colleagues at the interdisciplinary departments of chemistry, advanced 2-D materials, physics and engineering developed bottom-up synthesis of π-extended [5]triangulene with atomic precision using surface-assisted cyclodehydrogenation of a molecular precursor on metallic surfaces. Using atomic force microscopy (AFM) measurements, Su et al. resolved the ZTGM-like skeleton containing 15 fused benzene rings. Then, using scanning tunneling spectroscopy (STM) measurements they revealed the edge-localized electronic states. Coupled with supporting density functional theory calculations, Su et al. showed that [5]triangulenes synthesized on gold [Au (111)] retained an open-shell π-conjugated character with magnetic ground states.In synthetic organic chemistry, when triangular motifs are clipped along the zigzag orientation of graphene, scientists can create an entire family of zigzag-edged triangular graphene molecules. Such molecules are predicted to have multiple, unpaired π-electrons (Pi-electrons) and high-spin ground states with large net spin that scaled linearly with the number of carbon atoms of the zigzag edges. Scientists therefore consider ZTGMs as promising candidates for molecular spintronic devices. The direct chemical synthesis of unsubstituted ZTGMs is a long-standing challenge due to their high chemical instability. Researchers had recently adopted a tip-assisted approach to synthesize unsubstituted [3]triangulene with detailed structural and electrical properties, but the method could only manipulate a single target molecule at a time. The strategy was therefore only useful for specific applications due to a lack of scalability. Su et al. used large-scale STM images to reveal well-separated triangle-shaped molecules after annealing to the precursor-decorated Cu (111) and Au (111) surfaces. They recorded the magnified STM images with a metallic tip to show that individual molecules adopted triangular/planar configurations on both substrates. At the edge of these molecules, the research team observed characteristic nodal features resembling the zigzag edges or termini of graphene nanoribbons (GNRs). When they conducted noncontact AFM (nc-AFM) measurements to accurately determine the chemistry of reaction products, the bright areas represented a high-frequency shift with higher electron density. As a result, they clearly resolved the zigzag-edged topology of 15 fused benzene rings, where the experimental results were in excellent agreement with those simulated using a numerical model in a previous study . The observed molecular morphology therefore corresponded to the expected [5]triangulene.The freestanding [5]triangulene contained four unpaired π-electrons as theoretically predicted. To unveil the peculiar electronic properties of the molecule, Su et al. performed scanning tunneling spectroscopy (STS) measurements of single [5]triangulene grown on the weakly interacting Au (111) substrates using a metallic tip. To capture the spatial distribution of the observed electron states, the scientists completed differential conductance (dI/dV) mapping on a single [5]triangulene molecule at different sample biases. On examination, the differential conductance map revealed five bright lobes located at the edge of the [5]triangulene, represented by a characteristic nodal map. The observed characteristic feature was similar to the nodal pattern of spin-polarized electronic states seen with zigzag termini and zigzag edge of GNRs. Electronic structure of [5]triangulene. (A to D) Experimental dI/dV maps recorded at different energy positions [−2.2 V for (A), −0.62 V for (B), 1.07 V for (C), and 2.2 V for (D); scale bar, 4 Å]. (E to H) Simulated dI/dV maps of [5]triangulene acquired at different energy positions corresponding to different sets of orbitals: (E) ψ2↓ and ψ3↓, (F) ψ4↑ to ψ7↑, (G) ψ4↓ to ψ7↓ (note: the weight of ψ5↓ is set to 0.7; refer to fig. S8 for more details), and (H) ψ8↑ and ψ9↑. Scale bar, 4 Å. (I) Calculated spin-polarized molecular orbital energies of an isolated [5]triangulene. Blue and red refers to spin-up and spin-down states, respectively. (J) DFT-calculated wave functions of four pairs of spin-polarized orbitals [ψ4 ↑ ( ↓ ), ψ5 ↑ ( ↓ ), ψ6 ↑ ( ↓ ), and ψ7 ↑ ( ↓ )]. Red and blue colors indicate the wave functions with positive or negative values, respectively. Credit: Science Advances, doi: 10.1126/sciadv.aav7717 Citation: Atomically precise bottom-up synthesis of π-extended [5] triangulene (2019, July 31) retrieved 18 August 2019 from https://phys.org/news/2019-07-atomically-precise-bottom-up-synthesis-extended.html This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.last_img read more

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