* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Packard Poster-2 - Northwestern University Mesoscopic Physics
Renormalization wikipedia , lookup
Particle in a box wikipedia , lookup
Quantum dot cellular automaton wikipedia , lookup
Path integral formulation wikipedia , lookup
Hydrogen atom wikipedia , lookup
Quantum dot wikipedia , lookup
Bell test experiments wikipedia , lookup
Quantum fiction wikipedia , lookup
Double-slit experiment wikipedia , lookup
Symmetry in quantum mechanics wikipedia , lookup
Measurement in quantum mechanics wikipedia , lookup
Theoretical and experimental justification for the Schrödinger equation wikipedia , lookup
Wave–particle duality wikipedia , lookup
Many-worlds interpretation wikipedia , lookup
Atomic orbital wikipedia , lookup
Quantum electrodynamics wikipedia , lookup
Quantum computing wikipedia , lookup
Aharonov–Bohm effect wikipedia , lookup
Interpretations of quantum mechanics wikipedia , lookup
Bell's theorem wikipedia , lookup
Quantum group wikipedia , lookup
Quantum machine learning wikipedia , lookup
Orchestrated objective reduction wikipedia , lookup
History of quantum field theory wikipedia , lookup
Canonical quantization wikipedia , lookup
Quantum key distribution wikipedia , lookup
Quantum teleportation wikipedia , lookup
Quantum entanglement wikipedia , lookup
EPR paradox wikipedia , lookup
Electron configuration wikipedia , lookup
Quantum state wikipedia , lookup
Coherent Nonlocal Effects in Superconducting Nanostructures Paul Cadden-Zimansky, Jian Wei and Venkat Chandrasekhar Department of Physics and Astronomy, Northwestern University, Evanston, IL Motivation Microscopic objects that have become quantum mechanically entangled exhibit novel behavior that violates many of our classical intuitions. The exploitation of entangled quantum objects is at the heart of a number of recently developed subfields in physics – quantum computation, quantum cryptography, quantum information, etc. Perhaps the simplest entangled object is two electrons of opposite spin bound in a singlet state. Nonlocal quantum coherence between normal probes placed on a superconductor is predicted to occur through two microscopic processes. In crossed Andreev reflection (a) the electrons forming a Cooper pair in the superconductor break up, with each electron entering a different probe. This entanglement occurs in many materials which are cooled to low enough temperatures to become superconductors (S). In this phase transition singlet Cooper pairs of electrons are naturally created. Though the constituent electrons of these pairs form a single quantum object, they are spatially separated by a coherence length x which can extend several hundred nanometers. As this length scale is now easily accessible to modern nanolithographic techniques, we ask the question: is it possible to use the Cooper pairs in a superconductor to quantum mechanically couple two normal metal (N) probes placed on it? In particular, can the quantum phase of electrons in one probe be coherently communicated to the other, without any current being passed between the probes? 1 . Nonlocal Coherence Experiment To demonstrate that a nonlocal signal between two probes can communicate information about the quantum phase of the electrons, a phasedependent current I(F) from one probe into the superconductor needs to be established. The nonlocal voltage VN can then be monitored as the phase is tuned on a second probe located less than a superconducting coherence length from the first. To create the current, one of the normal probes is embedded in a hybrid normal metalsuperconducting loop known as an Andreev interferometer. The phase of electrons around this loop are tuned by threading a magnetic flux F through it. As this phase is altered, shifting quantum interference effects are observed, such as symmetric, periodic oscillations in the resistance of the interferometer. These oscillations are periodic in the Fo=h/2e superconducting quantum of flux. By creating a nonequilibrium distribution of electrons in the normal arm of the interferometer, such as by sending a small DC current into its center along with an AC measurement current, one can produce the phase-tunable I(F) current. The voltages generated by this current are monitored on probes at the top and bottom corners of the loop as well as the nonlocal probes just off it. 4 Nonlocal Signals Crossed Andreev Reflection & Elastic Cotunneling Injecting a current from a gold normal metal lead (I+) into an aluminum superconductor (I-) the nonlocal voltages on spatially separated normal probes (V1-6) are measured relative to the superconductor potential (V-). Just below the 0.6 K superconducting transition, peaks in the nonlocal resistance are observed due to single electron excitations in the superconductor (charge imbalance). These excitations are frozen out at the lowest temperature revealing a remnant nonlocal resistance that decays rapidly as the distance to each nonlocal probe is increased. The decay length of this lowtemperature, zero-bias resistance is several hundred nanometers, comparable to the superconducting coherence length. 1 mm Cadden-Zimansky et al., Physical Review Letters (2006) In elastic cotunneling (b) the spatially extended Cooper pair mediates a long-range tunneling of electrons from one probe to another. These two processes should only occur if the normal metal probe separation is on the order of x, and can be observed when electrons are injected from one probe into the superconductor and a nonlocal voltage is monitored on the second probe. 2 Observation of Nonlocal Phase Coherence Phase coherent signals are observed both at the corners of the loop and also nonlocally. The amplitude of the nonlocal signals are reduced sixfold from those measured on the corners, consistent with rapid decay over the superconducting coherence length. 3 Coexistence of Normal Current and Supercurrent One paradox regarding the supercurrent traveling around the hybrid loop is that the loop still has a finite resistance. This paradox can be resolved by showing that a normal metal can simultaneously support a resistive normal current and a resistanceless supercurrent. 1 mm Measurements of an SNS wire are made with two different sets of probes. Superconducting probes are used to measure the resistance of the whole wire while normal probes are used to measure a part of the normal section at its center. At low enough temperatures a supercurrent across the whole wire shows no resistance while the normal part is still resistive. The fact that the oscillations at the top and bottom of the loop are of opposite polarities despite the symmetry of the device about its horizontal axis indicates that the sign of the voltages are determined by a flux-induced supercurrent that circulates around the interferometer loop. 5 The apparent drop in the normal part resistance when the supercurrent is established is due to the fact that the measurement current injected at point A now has two paths to exit the wire at point B: the usual path along the normal wire and a second path that uses the resistanceless channel from one superconductor to the other. 6