Selected Papers

Quantum Information Technology basedon Single Electron Dynamics
Toshimasa Fujisawa†
Abstract
Quantum information technology promises powerful information processing based on the control ofthe dynamics of individual particles. Research on single electron dynamics aiming at the coherent electrical manipulation of single electron charge and spin in semiconductor quantum dots is expected to provide practical quantum information technology. A single electron charge quantum bit (qubit), which has
recently been realized in a double quantum dot, is advantageous for flexible control of the quantum state
by a high-speed electrical signal, while a single electron spin qubit is expected to have a sufficiently longdecoherence time. This paper reviews our research on single electron dynamics for quantum information
processing.

1. Introduction
Quantum information technology seeks to manipulate quantum states by coherent time evolution, in
which the bases of the quantum state are regarded asdigital information. By analogy to a bit, the unit ofclassical digital information, the unit of quantum
information is a quantum bit (qubit), which can basically be realized in any single two-level system. Various types of information processing based on qubits
have been proposed and demonstrated [1]. Quantum
information technology provides various advantages
that could never be obtained in the conventional classical approach. One would be secure telecommunications through quantum cryptography because an
unknown single quantum state can be neither duplicated nor determined completely. Another example isa quantum non-demolition measurement scheme,
which would improve the accuracy of a quantum
measurement by avoiding back action. Then there is
the quantum computer, a programmable interferometer in which only one or a few desired answers can be
efficiently obtained from an extremely large numberof candidates. Recently, quantum information technology has become attractive for very specific tasks.
† NTT Basic Research Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: fujisawa@will.brl.ntt.co.jp

Vol. 1 No. 3 June 2003

Although the potential of quantum computing isfascinating, only a few small-scale quantum computers have been built. Practical quantum computing
hardware with a large number of qubits is stronglydesired. Recently, coherent control of quantum statesin a nanostructure has been attracting growing interest for flexible control of many qubits [2]. Single
electron dynamics, which describes the dynamical
response of a single-electron in nanostructures, is oneof the best candidates for useful quantum information
technologies [2], [3]. Here, we review our research
activities and strategies on single electron dynamics
and discuss its prospects for future quantum information technology.
2. Concept of quantum computation
In quantum information technology, a quantum
state is a carrier of information. It is transformed froman initial input state to a final output state by applying
an external field, e.g., an electromagnetic field, for acertain period. The fundamental transformation of
the quantum state is called a quantum logic gate, andit should be a unitary transformation to keep coherency. The quantum state is changed by applying a seriesof quantum logic gates for the start of quantum computation, when classical digital information is input
to the quantum state, until the end of the computation,
when the output states are finally measured as classi41

Selected Papers

cal information. One may not measure the intermediate state, because that would cause the quantum state
to collapse. Quantum computing requires that a highdegree of quantum coherence be maintained for along enough time to complete the computation. One
can design an algorithm for quantum computation insuch a way that a series of data processing is performed simultaneously in a parallel fashion (quantumparallelism). Quantum computation is therefore
expected to provide extremely efficient calculations
for specific problems that cannot be solved efficiently with conventional classical computers.
To construct a quantum computer, a bunch of single
quantum states must be (i) prepared physically, (ii)initialized, (iii) manipulated coherently, (iv) preserved for a long enough time, and (v) measured individually. Each of these requirements needs furtherdevelopment. Below, we briefly summarize ourstrategies for realizing quantum computers using single-electron dynamics.
(a)

3. Single-electron dynamics
A quantum dot is a small conductive island (lessthan 100 nm in diameter) that contains a tunable number of electrons occupying discrete orbitals. Figure 1
shows a scanning electron micrograph of representative samples containing from one to four quantum
dots (schematically illustrated by circles). Since theelectron filling in a quantum dot resembles that innormal atoms, quantum dots are often referred to as
artificial atoms. For example, as is true for someatoms, a quantum dot containing a few electrons
exhibits magnetic properties depending on the electron filling. Actually, quantum dots with a few electrons show spin-polarized states even at zero magnetic field. The great advantage of artificial atoms (quantum dots) is that the element of the artificial atom (H,He, Li, and so on) can be changed just by changing
external voltages.
NTT Basic Research Laboratories have been study(b)300 nm
SDIsd

Vsd

VL
VlVR
VrVC
DrainQuantum dot

(c)

Gate

Heterostructure
SourceFig. 1. Scanning electron micrographs of quantum dot devices. (a) Double quantum dot, schematically shown by twocircles, fabricated by dry etching and fine metal gate patterning. (b) Two sets of double quantum dots (prototypedevice). (c) A vertical single quantum dot, in which the source and drain electrodes are located above and below
the quantum dot.

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ing the dynamical response of a single electron in aquantum dot, which is called single-electron dynamics. For instance, a single electron can be injected intoor extracted from a quantum dot by applying a highspeed electrical signal with time accuracy better than
1 ns. The injection and extraction can be detected asa change in the charge on the dot with time accuracy
better than 1 µs. Moreover, an electron in a quantum
dot can be excited by applying microwaves, whichcorresponds to optical excitation in atoms. This
research promises to provide novel semi-classicalelectronic devices that can precisely control and measure an electronic current. The more challenging andeffective application of this research though is inquantum information technology, in which single
electron charge and spin is controlled quantum
mechanically. Single electron dynamics in quantum
dots is expected to satisfy the requirements for quantum information technology and lead to useful quantum computers.
3.1 Charge quantum bit in a double quantum dot
Consider a double quantum dot, in which two quantum dots are separated by a tunneling barrier. The
double quantum dot can be regarded as diatomic molecule: the two dots are coupled electrostatically (corresponding to an ionic bond) and quantum mechanically (corresponding to a covalent bond). The coupling strengths can also be controlled by externalvoltages.
Figure 1(a) shows a typical double quantum dot
device fabricated in an AlGaAs/GaAs modulation
doped heterostructure. The upper and lower dark(a)regions are etched, and the adjacent region is depleted of conductive electrons. The bright vertical lines
are metal gate electrodes used to deplete the underlying region. The resultant conductive islands (doublequantum dot) are schematically shown by circles.
The double quantum dot is attached to the source anddrain electrodes and can be investigated by measuring the tunneling current.
Now, consider the situation where an excess electron is injected into the double quantum dot (See Fig.
2(a)). In the classical picture, the electron occupies
either the left or right dot at any given time. This isanalogous to the logical 0 and 1 of a classical bit. In
the quantum mechanical picture, however, an electron behaves as if it occupies the left and right dots
simultaneously (superposition of 0 and 1), but theprobability of finding the electron in one of the dots
is determined quantum mechanically. In quantum
information technology, this probability is the quantum information itself, and should be manipulated orpreserved during the calculation.
We have succeeded in controlling the quantum stateof a double quantum dot by applying a high-speed
electrical pulse (from 100 ps to 2 ns) to one of the electrodes [4]. The electron initially injected into the left
dot moves back and forth between the two dots during
the pulse, which corresponds to a sinusoidal oscillation in probability between 0% and 100%. The probability can be controlled by tailoring the pulse waveform and is obtained from the tunneling current under
the repetition of many pulses. A specific pulse waveform that changes the probability from 0% to 100%,or from 100% to 0% can be used as a NOT gate, which
(b)

11e0

e0

1e-Probability

e-e0
TimeFig. 2. (a) Quantum state of a single electron in a double quantum dot. When an excess electron is injected, it canoccupy the left dot (0), right dot (1), or both dots simultaneously (0+1). (b) The probability of finding the electron in
the right dot.

Vol. 1 No. 3 June 2003

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is the most fundamental quantum logic gate.
The next experiment would be two-qubit operation
on two sets of double quantum dots (See Fig. 1(b) and
Fig. 3(a)). The two double dots are coupled electrostatically to correlate the electron occupation in one double quantum dot with that in the other. For instance,
the NOT gate operation at one double quantum dot canbe performed only when the electron is located in theleft dot of the other double quantum dot. This conditional NOT operation (controlled NOT gate) is another fundamental gate operation in quantum informationtechnology. If it is performed coherently, the controlled-NOT gate should work for any superposition
state as well. One can design any quantum algorithm
by combining a few types of fundamental quantum
logic gates (a universal set of logic gates).
Another important requirement for quantum information technology is the single-shot measurement, inwhich the electron occupation in the double quantum
dot is determined by a single measurement. In thiscase, the measurement outcome is either 0 or 1,
whose probability can be controlled by quantum
logic gates. The electron occupation in a double
quantum dot can be measured with a single electrontransistor (SET), which can be fabricated near thedouble quantum dot by the same technique. We aredeveloping a high-speed version of the SET, operated(a) Charge qubit

One-qubit
operationwith a radio frequency signal (RF-SET), and expect it
to work as a single-shot measurement device [5].
So far, basic technologies (NOT gate, controlledNOT gate, and single shot measurement) for a charge
qubit have been summarized (Fig. 3(a)). However, aserious problem for the success of quantum information technology is decoherence, the loss of the quantum information: the whole computation must to befinished within this decoherence time. For a charge
qubit in a double quantum dot, the decoherence time
is not very long, about 1 ns, at present. Solving thisproblem requires further investigation and refinement.
3.2 Spin qubit in a single quantum dot
The spin degree of freedom, which originates fromthe spinning of a charged particle, is an alternative
way to construct a qubit. While the charge qubit is an
artificial qubit, the electron spin is a natural one. The
spin decoherence time of conductive electrons in bulk
GaAs crystal can be longer than 100 ns, and electron
spin bound to a donor in silicon shows a decoherencetime of about 300 µs. Electron-spin based quantum
computation is motivated by this long decoherence
time. Coherent manipulation of electron spins has
been studied in many systems. The easiest way is touse electron spin resonance, which involves applying
a microwave magnetic field under a static magnetic(b) Spin qubite-e-Tunneling couplingθ, φElectron spin resonanceeTwo-qubit
operationeExchange coupling
Dipole coupling

Single-shotmeasurement

e-RF-SET

Spin-dependent
tunneling

RF-SETFig. 3. Summary of quantum information technologies, one- and two-qubit operations and single-shot measurement, for(a) a charge qubit in a double quantum dot and (b) a spin qubit in a single quantum dot.

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field.
However, in contrast to the countless studies on theensemble of spins in many materials, little work has
been done on the manipulation of single-electronspin. A quantum dot containing a single electron spin
provides flexible quantum information storage (SeeFig. 1(c) and Fig. 3(b)). We have developed a novel
electrical pump and probe experiment for a quantum
dot, and investigated inelastic spin relaxation time ina quantum dot containing a few electrons (Fig. 1(c))[6]. We found that the energy relaxation time, whichis the time necessarily to complete the quantum computation including the last measurement, can belonger than 200 µs for a realistic quantum dot structure. This result is very encouraging.
The NOT gate operation, which reverses the spin
direction from up (0) to down (1) or vice versa can be
performed using microwave irradiation (electron spinresonance). To address each electron spin (qubit) inmany quantum dots, single-spin manipulation andmeasurement techniques are essential. The effective
g-factor of each electron spin can be made different
for different quantum dots through g-factor engineering, or a moderate magnetic field gradient is applied
to the quantum dots, so that each qubit is addressedby a corresponding microwave frequency. However, atypical one-qubit operation using electron spin resonance requires a relatively long period of time, 100 nsfor instance, because of the weak magnetic dipoleinteration. Alternative approaches, i.e., using theoptical Stark effect in a specific band structure or
exchange coupling among three electron spins constituting one qubit, are suitable for much faster operations.
Two-qubit operation, which is required to constructa universal set of logic gates, is expected to beachieved by connecting two quantum dots with a tunable tunneling barrier. The exchange coupling between
the two spins can be used to swap the two spins(SWAP gate). The controlled-NOT gate can be constructed by combining a NOT gate and a SWAP gate.
Single-shot spin measurement is a challenging
technique for quantum information technology. One
proposal is based on the spin-dependent tunnelingbetween two quantum dots combined with an RFSET. When each of the dots possesses one electron
spin before the measurement, tunneling from one dot
to the other is allowed, if the two electron spins canmake a spin pair (spin singlet state). This spin-dependent tunneling could be measured with an RF-SET ina short time.

Vol. 1 No. 3 June 2003

4. Future prospects
This paper has outlined our research activities andstrategies on single electron dynamics for future
quantum information technology. It was once
believed that quantum mechanics can only appear inlimited cases, such as in atoms, but this has beenshown to be untrue, and many kinds of quantum
mechanical behavior have been designed andobserved in many different materials and devices.
Recent nanofabrication techniques allow us to fabricate much smaller structures than our sub-micrometer double quantum dot. The combination of nanofabrication techniques and high-speed measurement
techniques will lead to progress in quantum information technology. Another thing we should note is thatsingle-electron dynamics is also attractive for novelsemi-classical devices, such as a very sensitive electrometer and logic devices with ultimately low power
consumption. The ideas arising from our research on
quantum information technology might also improvethe characteristics of these devices. We are confident
that quantum information technology will offer valuable applications, even if they are very small scale.
References
[1]
[2]

[3]
[4]

[5][6]M. A. Nielsen and I. L. Chuang, “Quantum Computation and Quantum Information,” (Cambridge, 2000).
H. Takayanagi, H. Ando, and T. Fujisawa, “From Quantum Effects toQuantum Circuits, Toward the Quantum Computer,” NTT REVIEW,
Vol. 12, No. 1, pp. 17-25, 2000.
T. Fujisawa, “Single electron dynamics,” to be published in Encyclopedia of Nanoscience and Nanotechnology.
T. Hayashi, T. Fujisawa, H. D. Cheong, Y. H. Jeong, and Y. Hirayama, “Coherent manipulation of an electric dipole in a double quantumdot,” to be submitted.
H. D. Cheong, T. Fujisawa, T. Hayashi, Y. Hirayama, and Y. H.
Jeong, “Impedance analysis of a radio-frequency single-electron transistor,” Appl. Phys. Lett., Vol. 81, p. 3257, 2002.
T. Fujisawa, D. G. Austing, Y. Tokura, Y. Hirayama, and S. Tarucha,“Allowed and forbidden transitions in artificial hydrogen and heliumatoms,” Nature, Vol. 419, p. 278, 2002.

Toshimasa Fujisawa
Distinguished Technical Member, SeniorResearch Scientist, Physical Science ResearchLaboratory, NTT Basic Research Laboratories.
He received the Ph.D. degree in engineeringfrom Tokyo Institute of Technology, Japan, in
1991. Since 1991, he has been working on electron transport in semiconductor nanostructures in
NTT Basic Research Laboratories. Recently hehas been studying single electron dynamics inquantum dots. He was a visiting researcher at
Delft University of Technology from Dec. 1997
to Aug. 1998.

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