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The paper presents a new body RF coil design scheme for a low-field open MRI system. The RF coil is composed of four rectangular loops which are made of wide copper strips located near the surfaces of the bottom and top pole faces of the permanent magnet. The body RF coil has been designed by using the pseudo electric dipole radiation (PEDPR) method with the Metropolis algorithm. In the calculation of the RF fields via the finite difference time domain (FDTD) method, the computational time increases as the RF frequency becomes lower. Moreover, the computational process using the FDTD method takes a very long time when the RF coil is optimized. The optimization requires varying the configuration of the RF coil system and performing successive calculations of field strength and field homogeneity. When we perform these successive calculations, the computational time can be reduced by using the PEDPR method, where the segmented current elements of the RF coil are treated as pseudo electric dipole radiation sources. Because the RF coil is made of wide strips, the variation of the current density on the strip has been considered in the B1-field calculation. For each configuration of the RF coil system, the current distribution is calculated via circuit analysis, where each copper strip is considered as a parallel combination of current element lines. The preliminary field calculation study by the FDTD method verifies both the circuit analysis method for the current distribution and the PEDPR method for the radiation field strength. The optimization of the RF coil configuration is performed by the Simulated Annealing (SA) process using the Metropolis algorithm. Simulations have been performed for a 10 MHz RF frequency. The optimized RF coil has four rectangular loops of 37 cm × 100 cm with 6.5 cm wide strips which are separated vertically 49 cm and horizontally center-to-center 63 cm. In the 25 cm diameter of spherical volume (DSV), the design results show a good field inhomogeneity of the B1-field below 0.49 dB (5.8%).

We study complementarity in a two-terminal closed-loop Aharonov-Bohm interferometer with a quantum dot embedded in an arm of the ring and a nearby charge detector capacitively coupled to the quantum dot. We show that perfect charge detection does not give full information of the electronic path, so it does not completely suppress the interference. We propose that this property can be verified even in the limit of weak charge detection by investigating the dephasing of the first and the second harmonics of the Aharonov-Bohm oscillation. The first harmonic of the AB oscillation is reduced monotonically as a function of the charge detection strength. In contrast, the second harmonic interference remains unaected due to the fact that charge detection is not able to detect the electronic path responsible for the second harmonic. Our theory confirms that the particle-like behavior of an electron emerges from the path information itself rather than from the disturbance.

Quantum Key Distribution (QKD) protocols enable two parties to communicate in a secret manner. As it is necessary that there be a quantum channel between two users, n ( n - 1 ) / 2 quantum channels are required for a QKD system of n users. We here propose a Multi-user Quantum Key Distribution protocol that utilizes entanglement swapping and by which only n quantum channels are needed for a communication system of n users. Any two among n users of the system can communicate with each other, even though there is no direct quantum channel between them. This is possible because entanglement swapping entangles the quantum states of the two users, establishing a quantum connection between them. Hence n quantum channels are sufficient for communication between n users. Furthermore, Trent, who performs entanglement swapping, has no way to fraudulently intervene in or eavesdrop on the communication between two users.

We investigate the conditional evolution of a charge state coupled to a mesoscopic detector under continuous weak measurement. The state suffers relaxation into a particular state with a definite charge when electrons in a particular output lead are monitored in the detector. The process of the conditional relaxation is not restricted by the shot noise of the detector, unlike the case of the back-action dephasing. As a result, the relaxation of conditional evolution is much faster than the current-sensitive part of dephasing. Furthermore, the direction of the relaxation depends on the choice of the output lead. We propose that these properties can be verified in a two-path interferometer containing a quantum dot capacitively coupled to a detector. In this setup, the current-current correlation between the interferometer and the detector reveals characteristic features of conditional relaxation.

According to Bohr's complementarity principle, a particle possesses wave-like properties only when the different paths the particle may take are indistinguishable. In a canonical example of a two-path interferometer with a which-path detector, observation of interference and obtaining which-path information are mutually exclusive. Such duality has been demonstrated in optics with a pair of correlated photons and in solid-state devices with phase-coherent electrons. In the latter case, which-path information was provided by a charge detector embedded near one path of a two-path electron interferometer. Note that suppression of interference can always be understood either as obtaining path information or as unavoidable back action by the detector. The present study reports on dephasing of an Aharonov-Bohm (AB) ring interferometer via a coupled charge detector adjacent to the ring. In contrast to the two-path interferometer, charge detection in the ring does not always provide path information. Indeed, we found that the interference was suppressed only when path information could be acquired, even if only in principle. This demonstrates that dephasing does not always take place by coupling the `environment' to the interfering particle: path information of the particle must be available too. Moreover, this is valid regardless of the strength of environment-interferometer coupling, which refutes the general notion of the effect of strong interaction with the environment. In other words, it verifies that an acquisition of which-path information is more fundamental than the back-action in understanding quantum mechanical complementarity., 17 pages, 3 figures

One of the points at issue with closed-loop-type interferometers is beating in the Aharonov-Bohm (AB) oscillations. Recent observations suggest the possibility that the beating results from the Berry-phase pickup by the conducting electrons in materials with the strong spin-orbit interaction (SOI). In this study, we also observed beats in the AB oscillations in a gate-defined closed-loop interferometer fabricated on a GaAs/Al{sub 0.3}Ga{sub 0.7}As two-dimensional electron-gas heterostructure. Since this heterostructure has very small SOI, the picture of the Berry-phase pickup is ruled out. The observation of beats in this study, with the controllability of forming a single transverse subband mode in both arms of our gate-defined interferometer, also rules out the often-claimed multiple transverse subband effect. It is observed that nodes of the beats with an h/2e period exhibit a parabolic distribution for varying the side gate. These results are shown to be well interpreted, without resorting to the SOI effect, by the existence of two-dimensional multiple longitudinal modes in a single transverse subband. The Fourier spectrum of measured conductance, despite showing multiple h/e peaks with the magnetic-field dependence that are very similar to that from strong-SOI materials, can also be interpreted as the two-dimensional multiple-longitudinal-modes effect.

Two‐terminal devices generally work as “leaky resonators” in coherent transport, which mixes up the quantum phase information from all parts of the devices, e.g., quantum dots (QDs) embedded in them. With the aid of appropriate theoretical modeling, however, we can extract important information on the phase from the total conductance. As typical examples, we present here experiments in a side‐coupled QD, and a QD embedded in an Aharonov‐Bohm (AB) ring. In the former, kinetic degrees of freedoms transverse and longitudinal to a quantum wire give rise to dramatic change in the interference effect. In the latter, “phase shift locking to π/2” appears as a plateau structure in the conductance. Specialized theoretical models give reasonable explanations to these effects, bringing important information on the phase of the electron wavefunctions in the QDs.

We have studied the Fano effect in a few-electron quantum dot side-coupled to a quantum wire. The conductance of the wire, which shows an ordinal staircase-like quantization without the dot, is modified through the interference (the Fano effect) and the charging effects. These effects are utilized to verify the exhaustion of electrons in the dot. The "addition energy spectrum" of the dot shows a shell structure, indicating that the electron confinement potential is fairly circular. A rapid sign inversion of the Fano parameter on the first conductance plateau with the change of the wire gate voltage has been observed, and explained by introducing a finite width of dot-wire coupling., 11 pages, 7 figures

We report on a study of complementarity in a two-terminal "closed-loop" Aharonov-Bohm interferometer. In this interferometer, the simple picture of two-path interference cannot be applied. We introduce a nearby quantum point contact to detect the electron in a quantum dot inserted in the interferometer. We found that charge detection reduces but does not completely suppress the interference even in the limit of perfect detection. We attribute this phenomenon to the unique nature of the closed-loop interferometer. That is, the closed-loop interferometer cannot be simply regarded as a two-path interferometer because of multiple reflections of electrons. As a result, there exist indistinguishable paths of the electron in the interferometer and the interference survives even in the limit of perfect charge detection. This implies that charge detection is not equivalent to path detection in a closed-loop interferometer. We also discuss the phase rigidity of the transmission probability for a two-terminal conductor in the presence of a detector., Comment: 4 pages with 4 figures

We investigate properties of charge detection and detection-induced dephasing of a charge qubit interacting with an electron beam collider composed of a quantum point contact. We predict that the interference of the qubit is fully restored when the two inputs are identically biased so that all the electrons suffer two-electron collision, unlike the case without collision. This phenomenon is related to Fermi statistics and illustrates the peculiar nonlocality of dephasing. We also describe the detection properties of entangled electron pairs. For singlet pairs, the sensitivity of charge detection is enhanced which originates from the bunching behavior of electrons. This demonstrates that control of exchange statistics of particles can help sensitive charge detection.

We analyze the properties of charge detection and dephasing of a charge qubit through a resonant tunneling detector. The resonant tunneling detector is modeled as a one-dimensional double barrier delta-function potential with its barrier height being sensitive to the charge state of the qubit. The dephasing and the measurement rates show quasi-periodic oscillations as a function of the energy of the incident electron. We show that there exists a phase-sensitive contribution to dephasing, even for a detector with perfect mirror symmetry, which is not observed in a non-resonant detector. We demonstrate that this behavior originates from the abrupt phase change in the reection amplitude, which is not sensitive to the transmission probability, but retains the charge state information.

We investigate various aspects of the Kondo singlet in a quantum dot (QD) electrostatically coupled to a mesoscopic detector. The two subsystems are represented by an entangled state between the Kondo singlet and the charge-dependent detector state. We show that the phase-coherence of the Kondo singlet is destroyed in a way that is sensitive to the charge-state information restored both in the magnitude and in the phase of the scattering coefficients of the detector. We also introduce the notion of the `conditional evolution' of the Kondo singlet under projective measurement on the detector. Our study reveals that the state of the composite system is disentangled upon this measurement. The Kondo singlet evolves into a particular state with a fixed number of electrons in the quantum dot. Its relaxation time is shown to be sensitive only to the QD-charge dependence of the transmission probability in the detector, which implies that the phase information is erased in this conditional evolution process. We discuss implications of our observations in view of the possible experimental realization., Comment: Focus issue on "Interference in Mesoscopic Systems" of New J. Phys

We study the charge detection and controlled dephasing of a mesoscopic system via a quantum dot detector (QDD), in which the mesoscopic system and the QDD are capacitively coupled. The QDD is considered to exhibit coherent resonant tunnelling via a single level. It is found that dephasing rate is proportional to the square of the conductance of the QDD for the Breit–Wigner model, showing that dephasing is completely different from the shot noise of the detector. Measurement rate, on the other hand, shows a dip near the resonance of the QDD. Our findings are unique especially for a symmetric detector in the following aspects: The dephasing rate is maximum at the resonance of the QDD in which detector conductance is insensitive to the charge state of the mesoscopic system. As a result, the efficiency of the detector shows a dip and vanishes at the resonance, in contrast to the single-channel symmetric nonresonant detector that always has a maximum efficiency. We find that this difference originates from a v...