arXiv subjects

Andrew D. Greentree

Publications and source records attributed to Andrew D. Greentree.

At least 109 records · Page 6Linked to original sources

arXiv2006

Spectroscopy of a Cooper-Pair box in the Autler-Townes configuration

A theoretical spectroscopic analysis of a microwave driven superconducting charge qubit (Cooper-pair box coupled) to an RLC oscillator model is performed. By treating the oscillator as a probe through the backreaction effect of the qubit on the oscillator circuit, we extract frequency splitting features analogous to the Autler-Townes effect from quantum optics, thereby extending the analogies between superconducting and quantum optical phenomenology. These features are found in a frequency band that avoids the need for high frequency measurement systems and therefore may be of use in qubit characterization and coupling schemes. In addition we find this frequency band can be adjusted to suit an experimental frequency regime by changing the oscillator frequency.

cond-mat.supr-con↗

arXiv2006

Quantum phase transitions of light

Recently, condensed matter and atomic experiments have reached a length-scale and temperature regime where new quantum collective phenomena emerge. Finding such physics in systems of photons, however, is problematic, as photons typically do not interact with each other and can be created or destroyed at will. Here, we introduce a physical system of photons that exhibits strongly correlated dynamics on a meso-scale. By adding photons to a two-dimensional array of coupled optical cavities each containing a single two-level atom in the photon-blockade regime, we form dressed states, or polaritons, that are both long-lived and strongly interacting. Our zero temperature results predict that this photonic system will undergo a characteristic Mott insulator (excitations localised on each site) to superfluid (excitations delocalised across the lattice) quantum phase transition. Each cavity's impressive photon out-coupling potential may lead to actual devices based on these quantum many-body effects, as well as observable, tunable quantum simulators. We explicitly show that such phenomena may be observable in micro-machined diamond containing nitrogen-vacancy colour centres and superconducting microwave strip-line resonators.

cond-mat.other↗

arXiv2006

A multiplexed single electron transistor for application in scalable solid-state quantum computing

Single Electron Transistors (SETs) are nanoscale electrometers of unprecedented sensitivity, and as such have been proposed as read-out devices in a number of quantum computer architectures. We show that the functionality of a standard SET can be multiplexed so as to operate as both read-out device and control gate for a solid-state qubit. Multiplexing in this way may be critical in lowering overall gate densities in scalable quantum computer architectures.

cond-mat.mes-hall↗

arXiv2006

Coalitions in the quantum Minority game: classical cheats and quantum bullies

In a one-off Minority game, when a group of players agree to collaborate they gain an advantage over the remaining players. We consider the advantage obtained in a quantum Minority game by a coalition sharing an initially entangled state versus that obtained by a coalition that uses classical communication to arrive at an optimal group strategy. In a model of the quantum Minority game where the final measurement basis is randomized, quantum coalitions outperform classical ones when carried out by up to four players, but an unrestricted amount of classical communication is better for larger coalition sizes.

quant-ph↗

arXiv2006

Identifying a Two-State Hamiltonian in the Presence of Decoherence

Mapping the system evolution of a two-state system allows the determination of the effective system Hamiltonian directly. We show how this can be achieved even if the system is decohering appreciably over the observation time. A method to include various decoherence models is given and the limits of this technique are explored. This technique is applicable both to the problem of calibrating a control Hamiltonian for quantum computing applications and for precision experiments in two-state quantum systems. For simple models of decoherence, this method can be applied even when the decoherence time is comparable to the oscillation period of the system.

quant-ph↗

arXiv2006

Coherent Population Trapping in Diamond N-V Centers at Zero Magnetic Field

Coherent population trapping at zero magnetic field was observed for nitrogen-vacancy centers in diamond under optical excitation. This was measured as a reduction in photoluminescence when the detuning between two excitation lasers matched the 2.88 GHz crystal-field splitting of the color center ground states. This behavior is highly sensitive to strain, which modifies the excited states, and was unexpected following recent experiments demonstrating optical readout of single nitrogen-vacancy electron spins based on cycling transitions. These results demonstrate for the first time that three-level Lambda configurations suitable for proposed quantum information applications can be realized simultaneously for all four orientations of nitrogen-vacancy centers at zero magnetic field.

cond-mat.mes-hall↗

arXiv2005

Quantum gate for Q switching in monolithic photonic bandgap cavities containing two-level atoms

Photonic bandgap cavities are prime solid-state systems to investigate light-matter interactions in the strong coupling regime. However, as the cavity is defined by the geometry of the periodic dielectric pattern, cavity control in a monolithic structure can be problematic. Thus, either the state coherence is limited by the read-out channel, or in a high Q cavity, it is nearly decoupled from the external world, making measurement of the state extremely challenging. We present here a method for ameliorating these difficulties by using a coupled cavity arrangement, where one cavity acts as a switch for the other cavity, tuned by control of the atomic transition.

quant-ph↗

arXiv2005

Quantum information transport to multiple receivers

The importance of transporting quantum information and entanglement with high fidelity cannot be overemphasized. We present a scheme based on adiabatic passage that allows for transportation of a qubit, operator measurements and entanglement, using a 1-D array of quantum sites with a single sender (Alice) and multiple receivers (Bobs). Alice need not know which Bob is the receiver, and if several Bobs try to receive the signal, they obtain a superposition state which can be used to realize two-qubit operator measurements for the generation of maximally entangled states.

quant-ph↗

arXiv2005

Coherent electronic transfer in quantum dot systems using adiabatic passage

We describe a scheme for using an all-electrical, rapid, adiabatic population transfer between two spatially separated dots in a triple-quantum dot system. The electron spends no time in the middle dot and does not change its energy during the transfer process. Although a coherent population transfer method, this scheme may well prove useful in incoherent electronic computation (for example quantum-dot cellular automata) where it may provide a coherent advantage to an otherwise incoherent device. It can also be thought of as a limiting case of type II quantum computing, where sufficient coherence exists for a single gate operation, but not for the preservation of superpositions after the operation. We extend our analysis to the case of many intervening dots and address the issue of transporting quantum information through a multi-dot system.

cond-mat.other↗

arXiv2005

Electrical readout of a spin qubit without double occupancy

We identify a novel mechanism to read out a single solid-state electron spin using an all-electrical spin-to-charge conversion in a closed system. Our scheme uses three donors and two electron spins, one spin is the qubit, the other is a reference. The population in the third, originally ionized, donor is monitored with an electrometer. Energy-dependent tunnelling of the reference spin to the ionized donor is used to determine the state of the qubit. In contrast to previous methods [e.g., B.E. Kane, Nature (London), \textbf{393}, 133 (1998)] we avoid double electron occupancy of any site within the system, thereby eliminating the possibility of unwanted electron loss from the system. The single spin readout scheme described here is applicable to both electron and nuclear spin based quantum computer architectures.

cond-mat.other↗

arXiv2005

Optically induced spin to charge transduction in donor spin read-out

The proposed read-out configuration D+D- for the Kane Si:P architecture[Nature 393, 133 (1998)] depends on spin-dependent electron tunneling between donors, induced adiabatically by surface gates. However, previous work has shown that since the doubly occupied donor state is so shallow the dwell-time of the read-out state is less than the required time for measurement using a single electron transistor (SET). We propose and analyse single-spin read-out using optically induced spin to charge transduction, and show that the top gate biases, required for qubit selection, are significantly less than those demanded by the Kane scheme, thereby increasing the D+D- lifetime. Implications for singlet-triplet discrimination for electron spin qubits are also discussed.

cond-mat.other↗

arXiv2005

Robust Charge-based Qubit Encoding

We propose a simple encoding of charge-based quantum dot qubits which protects against fluctuating electric fields by charge symmetry. We analyse the reduction of coupling to noise due to nearby charge traps and present single qubit gates. The relative advantage of the encoding increases with lower charge trap density.

quant-ph↗

arXiv2005

Identifying an Experimental Two-State Hamiltonian to Arbitrary Accuracy

Precision control of a quantum system requires accurate determination of the effective system Hamiltonian. We develop a method for estimating the Hamiltonian parameters for some unknown two-state system and providing uncertainty bounds on these parameters. This method requires only one measurement basis and the ability to initialise the system in some arbitrary state which is not an eigenstate of the Hamiltonian in question. The scaling of the uncertainty is studied for large numbers of measurements and found to be proportional to one on the square-root of the number of measurements.

quant-ph↗

arXiv2005

Quantum-Dot Cellular Automata using Buried Dopants

The use of buried dopants to construct quantum-dot cellular automata is investigated as an alternative to conventional electronic devices for information transport and elementary computation. This provides a limit in terms of miniaturisation for this type of system as each potential well is formed by a single dopant atom. As an example, phosphorous donors in silicon are found to have good energy level separation with incoherent switching times of the order of microseconds. However, we also illustrate the possibility of ultra-fast quantum coherent switching via adiabatic evolution. The switching speeds are numerically calculated and found to be 10's of picoseconds or less for a single cell. The effect of decoherence is also simulated in the form of a dephasing process and limits are estimated for operation with finite dephasing. The advantages and limitations of this scheme over the more conventional quantum-dot based scheme are discussed. The use of a buried donor cellular automata system is also discussed as an architecture for testing several aspects of buried donor based quantum computing schemes.

cond-mat.mes-hall↗

arXiv2005

Analysis and Geometric Optimization of Single Electron Transistors for Read-Out in Solid-State Quantum Computing

The single electron transistor (SET) offers unparalled opportunities as a nano-scale electrometer, capable of measuring sub-electron charge variations. SETs have been proposed for read-out schema in solid-state quantum computing where quantum information processing outcomes depend on the location of a single electron on nearby quantum dots. In this paper we investigate various geometries of a SET in order to maximize the device's sensitivity to charge transfer between quantum dots. Through the use of finite element modeling we model the materials and geometries of an Al/Al2O3 SET measuring the state of quantum dots in the Si substrate beneath. The investigation is motivated by the quest to build a scalable quantum computer, though the methodology used is primarily that of circuit theory. As such we provide useful techniques for any electronic device operating at the classical/quantum interface.

cond-mat.mes-hall↗

arXiv2004

Single-Shot Readout with the Radio Frequency Single Electron Transistor in the Presence of Charge Noise

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 single-shot 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.

cond-mat.mes-hall↗

arXiv2004

Charge shelving and bias spectroscopy for the readout of a charge-qubit on the basis of superposition states

Charge-based qubits have been proposed as fundamental elements for quantum computers. One commonly proposed readout device is the single-electron transistor (SET). SETs can distinguish between localized charge states, but lack the sensitivity to directly distinguish superposition states, which have greatly enhanced coherence times compared with position states. We propose introducing a third dot, and exploiting energy dependent tunnelling from the qubit into this dot (bias spectroscopy) for pseudo-spin to charge conversion and superposition basis readout. We introduce an adiabatic fast passage-style charge pumping technique which enables efficient and robust readout via charge shelving, avoiding problems due to finite SET measurement time.

cond-mat.other↗

arXiv2003

Current suppression in a double-island single-electron transistor for detection of degenerate charge configurations of a floating double-dot

We have investigated a double-island single-electron transistor (DISET) coupled to a floating metal double-dot (DD). Low-temperature transport measurements were used to map out the charge configurations of both the DISET and the DD. A suppression of the current through the DISET was observed whenever the charge configurations of the DISET and the DD were energetically co-degenerate. This effect was used to distinguish between degenerate and non-degenerate charge configurations of the double-dot. We also show that this detection scheme reduces the susceptibility of the DISET to interference from random charge noise.

cond-mat↗