Strong suppression of shot noise in a feedback-controlled single-electron transistor (original) (raw)
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The effect of large quantum fluctuation on the noise of a single-electron transistor
Physica E: Low-dimensional Systems and Nanostructures, 2003
We theoretically investigate the noise of a single-electron transistor in the regime of large quantum fluctuation of charge out of equilibrium. We show that the charge noise is suppressed due to the charge renormalization caused by the quantum fluctuation. However the fluctuation is not strong enough to wash out the charge quantization. We find that the renormalization effect reduces the performance of a single-electron electrometer.
Quantum measurements performed with a single-electron transistor
Physical Review B, 1998
Low-capacitance Josephson junction systems as well as coupled quantum dots, in a parameter range where single charges can be controlled, provide physical realizations of quantum bits, discussed in connection with quantum computing. The necessary manipulation of the quantum states can be controlled by applied gate voltages. In addition, the state of the system has to be read out. Here we suggest to measure the quantum state by coupling a single-electron transistor to the q-bit. As long as no transport voltage is applied, the transistor influences the quantum dynamics of the q-bit only weakly. We have analyzed the time evolution of the density matrix of the transistor and q-bit when a voltage is turned on. For values of the capacitances and temperatures which can be realized by modern nano-techniques the process constitutes a quantum measurement process. I. INTRODUCTION Recent proposals 1-4 suggested to use nanoscale devices, such as low-capacitance Josephson junctions or coupled quantum dots as quantum bits (q-bits), which are the basic elements of quantum computers. The two logical states are different charge states of the system 1-3. Applied gate voltages allow the necessary controlled manipulations (single-bit and two-bit operations) of the quantum states. In addition to these manipulations, a read-out device is required to perform quantum measurements of the resulting state of the q-bit. We suggest to use single-electron transistors for this purpose. The requirements to perform, on one hand, quantum manipulations and, on the other hand, a quantum measurement appear to contradict each other. During the manipulations the dephasing should be minimized, while a quantum measurement should dephase the state of the q-bit as fast as possible. The option to couple the measuring device to the q-bit only when needed is hard to achieve in mesoscopic systems. The alternative, which we discuss here, is to keep the measuring device permanently coupled to the q-bit in a state of equilibrium during the quantum operations. The measurement is performed by driving the measuring device out of equilibrium, in a way which dephases the quantum state of the q-bit. Similar nonequilibrium dephasing processes have recently been considered by a number of authors 5-8. For definiteness we discuss in this paper the measurement process performed by a single-electron tunneling (SET) transistor coupled capacitively to a Josephson junction q-bit; however, this type of measurements may be performed for any quantum system with two different charge states. We describe the measuring process by considering the timeevolution of the density matrix of the coupled system. We show that the process is characterized by three different time scales: the dephasing time, the time of measurement, which may be longer than the dephasing time, and the mixing time, i. e. the time after which all the information about the initial quantum state is lost due to the transitions induced by the measurement. Thus, we arrive at a new criterion for a "good" quantum measurement: the mixing time should be longer than the time of measurement.
Measurement of the shot noise in a single-electron transistor
We have systematically measured the shot noise in a single electron transistor (SET) as a function of bias and gate voltages. By embedding a SET in a resonance circuit we have been able to measure its shot noise at the resonance frequency 464 MHz, where the 1/f noise is negligible. We can extract the Fano factor which varies between 0.5 and 1 depending on the amount of Coulomb blockade in the SET, in very good agreement with the theory. PACS numbers: 07.50.Hp, 73.23.Hk In both electronic and photonic devices the measured signal does not contain all the information about the state and dynamics of a system. In most cases there is additional information in the fluctuations of the signal [1]. For example the shot noise in electronic circuits contains information about the charge of charge carriers. This was used by Saminadayar et al. to demonstrate the fractional charge of the quasiparticles in a fractional quantum Hall system [2]. The shot noise can also reveal correlations of the charge carriers or photons. Bosons display bunching [3] whereas fermions display anti-bunching due to the Pauli principle [4, 5].
Intensity feedback effects on quantum-limited noise
Journal of The Optical Society of America B-optical Physics, 1995
We examine the quantum-limited behavior of an electro-optical intensity feedback loop and present a simple theory and experimental data showing excellent agreement. We show that, although the light incident upon the in-loop detector may be sub-Poissonian, this light has unique properties different from those of a freely propagating beam of intensity squeezed light. We support this by presenting the results of homodyne measurements of the phase noise of light extracted from the loop. The utility of the in-loop field is discussed, and it is shown that in all cases in which linear optical components are used, no advantage in signal-tonoise ratio is gained by taking measurements by using this light rather than a coherent source. We also discuss effects seen in the extracted or out-of-loop light. We demonstrate the existence of, and derive an expression for, an optimum gain for suppressing low-level classical noise. Conversely, in the high-gain limit, we demonstrate that the extra noise seen in the out-of-loop photocurrent that is due to the feedback process is expressible purely in terms of the mean photocurrents involved. Lastly, we introduce the novel concept of using an intensity feedback loop in conjunction with a squeezed source and show that the feedback loop has the capability of electronically transferring squeezing from one light beam to another.
2003
By using the Schwinger-Keldysh approach, we evaluate the current noise and the charge noise of the single-electron transistor (SET) in the regime of large charge fluctuations caused by large tunneling conductance. Our result interpolates between previous theories; the "orthodox" theory and the "co-tunneling theory". We find that the life-time broadening effect suppresses the Fano factor below the value estimated by the previous theories. We also show that the large tunnel conductance does not reduce the energy sensitivity so much. Our results demonstrate quantitatively that SET electrometer can be used as the high-sensitivity and high-speed device for quantum measurements.
Renormalized dynamics in charge qubit measurements by a single electron transistor
2010
We investigate charge qubit measurements using a single electron transistor, with focus on the backaction-induced renormalization of qubit parameters. It is revealed the renormalized dynamics leads to a number of intriguing features in the detector's noise spectra, and therefore needs to be accounted for to properly understand the measurement result. Noticeably, the level renormalization gives rise to a strongly enhanced signal-to-noise ratio, which can even exceed the universal upper bound imposed quantum mechanically on linear-response detectors.
Applied Physics Letters, 2005
The radio frequency single electron transistor (rf-SET) possesses key requirements necessary for reading out a solid state quantum computer. This work explores the use of the rf-SET as a singleshot readout device in the presence of 1/f and telegraph charge noise. For a typical spectrum of 1/f noise we find that high fidelity, single-shot measurements are possible for signals ∆q > 0.01e. For the case of telegraph noise, we present a cross-correlation measurement technique that uses two rf-SETs to suppress the effect of random switching events on readout. We demonstrate this technique by monitoring the charge state of a metal double dot system on microsecond time-scales. Such a scheme will be advantageous in achieving high readout fidelity in a solid state quantum computer.
Single electron devices for simulating read-out in a solid state quantum computer
Surface Science, 2003
The radio frequency single electron transistor (rf-SET) is a prime candidate for reading out the final state of a qubit in a solid state quantum computer. Such a measurement requires the detection of sub-electron charge motion in the presence of random charging-decharging events associated with traps in the neighboring material system and SET tunnel junctions. Here we present a detection scheme together with experimental data from two dc-SETs where the signals from each are cross-correlated both temporally and spatially to suppress uncorrelated events arising from charge traps in the substrate and oxide. The technique is demonstrated using two dc-SETs that can detect the charge-state of two coupled metal dots, simulating charge transfer and read-out in a two-qubit system. In an effort towards operating the correlated detection scheme on microsecond timescales , we have demonstrated an ability to detect controlled single electron transfer using an rf-SET.
2002
By using the Schwinger-Keldysh approach, we evaluate the current noise and the charge noise of the single-electron transistor (SET) in the regime of large charge fluctuations caused by large tunneling conductance. Our result interpolates between previous theories; the "orthodox" theory and the "co-tunneling theory". We find that the life-time broadening effect suppresses the Fano factor below the value estimated by the previous theories. We also show that the large tunnel conductance does not reduce the energy sensitivity so much. Our results demonstrate quantitatively that SET electrometer can be used as the high-sensitivity and high-speed device for quantum measurements.