Selected Papers: Toward Quantum Information Technology

Quantum Information Technology
Yoshiro Hirayama†
Abstract
Quantum mechanics and physics have become important sources for developing ultimately securecommunication in the form of quantum cryptography and an extremely powerful computer called aquantum computer. Cutting-edge nanofabrication technologies, highly accurate measurements, and a
deep understanding of quantum information science are necessary to control quantum systems arbitrarily and to advance the research on quantum information technology (QIT) toward quantum cryptography and quantum computers. This paper summarizes the present status and future expectations of QIT
and describes some recent activities in NTT Laboratories.

1. Introduction
To transfer, store, and process information, we haveused electrons and photons up to now. In current communication systems, they are manipulated by using
several materials, including metals, semiconductors,
and optical fibers. However, all these methods use theclassical properties of electrons and photons.
Recent research on information technology has discovered a new and powerful source for information
processing: quantum mechanics. Quantum mechanics was first developed at the beginning of the twentieth century and it has dominated many areas ofphysics, especially photonics and solid-state physics.
Now, it has become an important source for guaranteeing “ultimately secure communication” that prevents all efforts of eavesdroppers and for making an
extremely powerful “dream computer”, which has
vast parallelism based on quantum superposition [1].
Naturally the role of quantum mechanics has
expanded to solid-state systems. Following Moore’slaw, the integration density of semiconductor circuits
has increased dramatically every year, and semiconductor devices have developed from a single transistor to super computers. The size of structures inside
semiconductor circuits is approximately 100 nm at† NTT Basic Research Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: hirayama@will.brl.ntt.co.jp

10present and it is decreasing rapidly. That means the
structure size should become 20 to 50 nm by 2010.
This scale is already close to that of the wavelengths
of electrons in semiconductors. Therefore, quantum
mechanical effects are expected to be significant inthese future systems. Although it is not clear whether
quantum effects will have good or bad effects onthese cutting-edge supercomputers (which are stillclassical computers), using the quantum mechanicalprinciple in computing systems has produced a newcomputer type, called the quantum computer.
The quantum computer will be more powerful thanany existing or future classical computer because thesuperposition principle allows an extraordinarily
large number of computations to be performed simultaneously. It promises to have tremendous potential
for efficiently solving some of the most difficult problems in computational science, such as integer factorization and discrete logarithms, which are intractableon any present or future conventional (classical)
computer. Other examples of the potentially great
impact are quantum modeling and simulation, as firstmentioned by Feynman [2]. Although many physicalphenomena are now successfully described in quantum mechanics, we have simulated quantum mechanical systems by using a classical computer. Therefore,
it might not be surprising if a quantum simulation ismore effective than a classical one for representing
quantum mechanical systems.
Quantum mechanics is also revolutionizing comNTT Technical Review

munication systems. Any eavesdropping action
leaves traces when quantum mechanics is used for
information transfer processes. Therefore, in a quantum cryptography scheme where we utilize the uncertainty principle of quantum mechanics, we can confirm that communication has run completely securely
without any eavesdropping. As discussed below, we
can implement this quantum cryptography by placing
information on a series of photon pulses. In otherwords, quantum cryptography is based on the quantum properties of a single photon not on photon-photon interaction. Therefore, quantum cryptography
will be in actual use much earlier than the quantum
computer, which uses the entanglement of many
quantum particles. A test kit for quantum cryptography is already commercially available [3]. However,many issues remain to be solved before quantum
cryptography becomes practical.
Taking into account the huge impact that QIT will
have on future communication systems, we haveaccelerated research toward “ultimately secure communication” and “the dream computer” in NTT, especially in NTT Basic Research Laboratories. In thisintroductory paper, I summarize the status of quantum cryptography and quantum computers including
developments in NTT. Our activities lead the field insome aspects of QIT as discussed in more detail in thefollowing papers.
2. Quantum cryptography
Research for quantum cryptography has been progressing rapidly. Although using the macroscopic
coherence of laser beams has been proposed, we havedeveloped a more comprehensible method, whichuses the quantum features of a single photon, e.g., thecircular and linear polarization of a single photon [4],[5]. Figure 1 illustrates a typical setup for quantum
cryptography communication. The polarization characteristics of a single photon are used to share thecryptographic key between Alice and Bob. Becauseof the uncertainty principle of quantum mechanics,
any eavesdropping can be detected, so Alice and Bob
can share the key with the assurance of no eavesdropping. After the key has been successfully distributed,
Alice sends information to Bob over a conventional
communication line using the shared cryptographic
key. Quantum mechanical theory confirms that thepersons who know the key are limited to only Aliceand Bob. Therefore, the communication is guaranteed to be ultimately secure.
In this quantum cryptography scheme, importantVol. 1 No. 3 June 2003

Alice

Bob

Photonpulse
Polarizer

Analyzer

Eavesdropper?

Quantum channel

Classical channel

Fig. 1. Schematic diagram of quantum cryptography
communication.

factors determining the performance of the quantum
key distribution are a single-photon emitter anddetector. The device characteristics and performanceof the quantum key distribution system are summarized in Table 1. The emission efficiency of a singlephoton emitter and the dead time, jitter and detection
efficiency of a single-photon detector determine themaximum speed of quantum key distribution. The
dark current of the detector is an important factor for
determining the reliability of the system. Photon
number fluctuation is also very important and reducing it improves the maximum transmission distanceof the system. In a conventional system, a single-photon source is obtained by simply attenuating laser
light. In such systems, the photon number fluctuatesaccording to the Poisson distribution and there
always remains a possibility that one pulse includes
two or more photons. Developing technologies that
could lead to an ideal single-photon source is reallychallenging. Quantum dots in a microcavity and turnstile devices may be good candidates for a future single-photon emitters. Recently, NTT, in collaboration
with Stanford University, has demonstrated long-distance quantum key distribution using a quantum-dot
single-photon emitter [6].
A highly sensitive photon detector is also in strong
demand. The highly sensitive avalanche photodetector, the unicarrier photodetector, and the quantum dot
photodetector may all be potential candidates for asingle-photon detector. An efficient detector has been
11

Selected Papers

Table 1. Device characteristics and performance of quantum key distribution system.
DeviceCharacteristics

Performance of key
distribution system

Single-photon emitter

Photon number fluctuation
Emission efficiency
Higher temperature operation

Distance
Speed
Convenience

Single-photon detector

Dark countDead time
Jitter
Detection efficiency
Higher temperature operation

Reliability
Speed
Speed
Speed
Convenience

Wavelengths: 1.5 µm for an optical fiber system
0.8 µm for a short distance system, satellite systemdeveloped for the 0.8-µm wavelength band, but notfor the 1.5-µm band, which is necessary for quantum
cryptography using optical-fiber communication systems. Meanwhile, the 0.8-µm band is useful as a test
bed for quantum cryptography. This band is also
applicable to short-distance secure communication
systems and satellite communication.
3. Quantum computer
Research on the quantum computer has also progressed very rapidly. The Advanced Research andDevelopment Activity (ARDA) predicts in its quantum computer roadmap that by 2012 we will be
developing a suite of viable quantum computer technologies of sufficient complexity to function as quantum computer test beds, where architectural and algorithmic issues can be explored. This will require on
the order of 50 physical quantum bits (qubits).
Although the report claims that this development is
not a prediction but a desired outcome, it suggeststhat the development in the quantum computer field
will be fast and very competitive.
Developing a quantum computer is a basic endeavor in science today and a fault-tolerant quantum computer on a small scale could be made within the next
decade. A quantum computer consists of qubits. In
contrast to classical bits, 0〉 or 1〉, qubits can represent a superposition of 0〉 and 1〉, i.e., α0〉 + β1〉(where α2 + β 2 =1). Therefore, a large number of
qubits can represent a vast superposition and thishuge parallelism makes it possible to solve some of
the most difficult problems. For example, the timerequired for integer factorization increases exponentially as a function of the number of digits in a classical computer. However, in a quantum computer,
Shor’s algorithm [7] turns the integer factorization
into an easy problem, where the processing timeincreases approximately linearly with the number of
12

digits.
The necessary conditions for a practical quantum
computer were first shown by DiVincenzo [8]. His
criteria are:
1. a scalable physical system of well-characterized
qubits;2. the ability to initialize the state of the qubits to asimple fiducial state;
3. long (relative) decoherence times;
4. a universal set of quantum gates (for example arotation gate and a controlled NOT (CNOT)gate); and5. a qubit-specific measurement capability.
The first condition means a well-defined quantum
superposition, i.e., qubit formation, and the possibility to increase the number of qubits. The physicalresource requirements, including gate operation steps,
must scale linearly, not exponentially, with the number of qubits if the approach is to be a candidate for alarge-scale quantum computer. The second condition
describes the preparation of the initialization stage formany qubits, such as that of the 000 – – – 0〉 state.
The third condition is essential for a practical quantum computer. All gate operations should be done
maintaining the coherence of the system. This meansthat the decoherence time should be much longer than
the gate operation time. The fourth condition is a universal set of gate operations for implementing several kinds of quantum calculation. Any kind of quantum calculation can be implemented by using only
two gate operations: a rotation gate of one qubit anda CNOT gate for two qubits. The fifth condition isnecessary for a successful readout of the calculated
results.
Table 2 summarizes the statuses of proposed quantum computer systems from the viewpoint of DiVincenzo’s criteria. Nuclear spin resonance (NMR) has
been studied widely for the structural analysis of molecules and a coherent pulse technique has been develNTT Technical Review

Selected Papers

Table 2. QC roadmap: development status.
Single qubit
operation

Two-qubit
quantum gate
operation#1
Scalability#2
Initialization#3
Long
decoherence#4
Set ofquantum gates#5
Measurements

Solution NMR◎

Ion/atom trap

◎

◎

?

○

◎

◎

○

◎

○

◎

○

◎

Optical

◎

◎

?

○

○

◎

○

○

e-helium

○

○

○

◎

○

○

○

Solid state

◎

○

○

◎

○

○

○

◎ = A potential approach has been successful with sufficient proof of principle.
○ = A potential approach has been proposed, but the principle has not been sufficiently proved.
? = No viable approach has been proposed.

oped. Therefore, at present, solution NMR is the leading technique for realizing a quantum computer system [9]. Shor’s factorization algorithm has been
implemented with a seven-qubit molecule using solution NMR techniques and has successfully identified
the factors of 15 as 3 and 5 [10]. However, a chemical synthesis of special macromolecules is necessary
for large qubit systems. Furthermore, solution NMR
utilizes an advantage of solutions, and the energy separation of the nuclear spins of the 0〉 and 1〉 states ismuch smaller than the solution temperature, so theinitialization of many qubits is very difficult. Therefore, this system may be inappropriate for a largescale quantum computer, although it will be usefulfor testing some basic features of quantum computation.
An ion/atom trap is also an ideal test bed for quantum coherent control. However, the scalability of thissystem is also questionable. Optical systems areinteresting because photons can easily form entangled pairs, which are used to confirm the principal
aspects of entanglement [11]. However, the nonlinearity of the photon-photon interaction is not strong
enough to achieve a viable two-qubit operation.
Although implementing a quantum computer byusing only linear optical systems has been proposed[12], its scalability may remain a problem. Althoughusing electrons on the surface of liquid helium [13] isa new and interesting idea for implementing many
qubits, solid-state qubits are considered the most suitable for future scalable quantum computers.
For many kinds of solid-state qubit systems, the initialization is rather easy and operations for one- andtwo-qubits have already been demonstrated. The
most serious and challenging problem is how to get aVol. 1 No. 3 June 2003

long decoherence time in a solid-state system where
decoherence is caused by several interactions, such as
those with phonons and background charge fluctuation. Once the decoherence problem has been
removed, the number of qubits in a solid-state system
will increase, like the development from a single transistor to present-day large-scale integration. NTT isalso focusing on developing solid-state qubits
because they are expected to have scalability andbecause NTT Laboratories have excellent backgrounds for semiconductor and material science.
The present statuses of solid-state qubits are summarized in Table 3. Trial solid-state qubits can beroughly divided into three categories: superconductor
qubits, quantum-dot qubits, and nuclear-spin qubits.
For a superconductor qubit, we can use a charge(Cooper pair) [14] or flux [15] to implement a quantum two-level system. One-qubit operation wasdemonstrated earlier and two-qubit operation was
achived recently [16]. A superconducting system has
a macroscopic coherence with a superconducting gap
so we can expect a rather long decoherence time. A
cutting-edge experiment for a superconductor flux
qubit has been done in NTT including single-shotreadout [17].
Semiconductor quantum dots are extremely interesting nanostructures for realizing quantum two-levelsystems. A small quantum dot at a low temperature
can provide an ideal system where a single electron or
exciton is precisely controlled. Quantum-dot charge
qubits have been demonstrated in NTT using coherent control of a single electron in a coupled quantumdot system [18], [19]. This is the first demonstrationof a fully electrical semiconductor qubit, and the electrical method has the advantage of local controllabil13

Selected Papers

Table 3. QC roadmap: status of solid-state QC.
Preparation of
qubit states

Readout of
qubit states

Coherent
oscillationLong
decoherence

Two-qubit
quantum gate
operation

Multiple-qubit
quantum gate
operation

Superconductor
charge qubit

◎

◎

◎

?

○

?

Superconductor
flux qubit

◎

◎

◎

?

?

?

Quantum-dot
charge qubit

◎

○

◎

?

?

?

Quantum-dot
spin qubit

○

○

○

?

?

?

Quantum-dot
exciton qubit

◎

○

◎

?

○

?

Nuclear spin in
solid-state

○

○

○

◎

?

?

◎ = Sufficient experimental demonstration○ = Preliminary experimental demonstration, but further work is required? = No experimental demonstration

ity of the qubits. Using electron spin is also a plausible method for semiconductor qubits, although electrically controlling the electron spin is not easy.
Coherent oscillation of electron spin has been studied
widely in optical pump and probe experiments [20],and a gate control of coherent oscillation has been
demonstrated recently [21].
A semiconductor quantum dot also operates as aquantum two-level system, whether an exciton is
located in the dot or not. All-optical qubits have beenrealized by using this type of coherent control of theexciton population with ultrafast optical pulses [22],[23]. NTT currently leads this field and details are
discussed in the following paper [24].
Nuclear spin has a very long decoherence time andit is one ideal system for future quantum computers[25], [26]. A fully electrical control of the nuclearspin polarization in semiconductors has been shown
experimentally [27], and coherent oscillation has
been detected electrically [28]. However, ideal control of nuclear spin, i.e., manipulating a single nuclear
spin, is very challenging and, over a certain time period, we should develop a method that coherently controls a group of nuclear spins.
Figure 2 shows how the physical number of qubits
has increased every year. Several qubits have been
demonstrated in solution NMR and in ion/atom traps.
One- or two-qubit operations have been demonstrated in many solid-state systems within the last two
years. In this sense, the twenty-first century is thedawn of the solid-state qubit. It is noteworthy that theslope in Fig. 2 shows that the number of bits has
increased by nearly 2.7 times over three years, which
14

corresponds to Moore’s law for logic circuits. The
increase in solution NMR qubits and ion/atom trap
qubits really does fall on this line except for a small
shift in the horizontal direction. Although the numberof solid-state qubits is too small to agree with theslope, the optimistic expectations of ARDA (10 physical qubits in 2007 and 50 qubits in 2012) do fall on
it. This means that quantum computer development isreally a rapidly growing field and highly competitive,
comparable with that of cutting-edge Si integrated
circuits. The goals set by ARDA are ambitious, butthey may be attainable through the collective effortsof NTT Laboratories in cooperation with outsideorganizations.
On the other hand, a practical quantum computerwith about 10,000 qubits will only be demonstrated
after 2025, even with this rapid progress. This clearly
indicates the need to take a long-term view for quantum computer research. At present, a diverse range ofexperimental approaches from a variety of scientificdisciplines are pursuing different routes to meet therequirements for a scalable quantum computing system. It is too soon to attempt to identify which ones
may be successful. This is because the ultimate technology may not have been invented yet. We should
remember the history of semiconductor devices
where the first successful transistor was made of Ge,
but Si is now the established material. Therefore, weneed to be careful in comparing different materials,
including superconductors and semiconductors. In
addition, several qubit gate designs exist. In someelectrical and optical manipulations, the quantum
logic gate operation is intrinsically limited to operaNTT Technical Review

Selected Papers

104

Practical quantum computer
Solution NMR (IBM)Ion/atom trap

Number of qubits

103

Quantum computer
Moore’s law ?

Quantum
computerbased onsolid-statesystems

Research on solid-state qubits
Superconductor charge qubit(1999 NEC, 2000 Saclay, -----2003 2-qubits NEC)

2

10

101

Limit of ion/atom trap

Superconductor flux qubit(2003 Delft Univ. Technol., NTT)

Limit of solution NMR

Quantum-dot exciton qubit(2001 NTT, Univ. Michigan
2002 2-qubits Univ. Michigan)
Quantum-dot charge qubit (2003 NTT)

100

Expectation of ARDA
2000

2010
Year

2020

2030

Fig. 2. Road map of quantum computers. Only some examples are plotted here.

tions between nearest-neighbor qubits. This type of
logic operation, however, would allow parallel operation within a quantum computer. On the other hand,
other approaches are capable of acting as logic gates
between widely separated qubits, but they are limited
to serial operations. Therefore, for both materials andimplementation methods, we should develop various
different systems in parallel for the time being.
4. Conclusion
Quantum mechanics was an important aspect ofscience in the twentieth century and a lot of physics,
especially solid-state physics, was explained using it.
Quantum mechanics will now become the key to ultimate communication and computer technologies.
Quantum cryptography and quantum computers are
fascinating examples of quantum information technology (QIT). Although we do not know if quantum
computers will offer computational advantages overconventional computers for general-purpose computation, they are clearly extremely powerful tools for
performing certain calculations.
A quantum computer consists of qubits. The development of solid-state qubits to build the first quantum
computer has been slow, due to decoherence, which
implies the breakdown of the quantum informationstoring state within a qubit. But new designs based onsuperconductors, quantum dots, and nuclear spin systems have improved the quality of the individualVol. 1 No. 3 June 2003

qubit, and the next step, to connect qubits together,has been taken. On the other hand, the quantum features of photons have been used to realize ultimately
secure communication in the form of quantum cryptography.
NTT has studied QIT and its advanced activities areshown in the following papers. In particular, NTT
Laboratories have accumulated knowledge and technologies about nanostructures. They have been used todevelop solid-state qubit systems, based on superconductors and quantum dots, as well as single-photon
sources and highly sensitive photon detectors. These
are firm foundations for developing QIT, but considerable efforts will be needed to make practical quantum
cryptography for ultimately secure electronic banking
and commercial systems and to develop a practicalquantum computer for executing real quantum algorithms. Putting this potential into practice will require
engineering and control of quantum-mechanical systems on a scale far beyond anything achieved in any
physical laboratory so far. However, the activities in
NTT Laboratories should provide enough potential torealize these goals. Collaboration with outside organizations is also important to enhance fundamental
research.
Finally, I would like to add that the development of
new quantum algorithms that have attractive applications is also important for quantum computer
research. Quantum bit commitment, quantum teleportation, and other quantum information technolo15

Selected Papers

gies are also core long-term interests. They are also
being studied at NTT.
Acknowledgments
The studies on QIT in NTT Basic Research Laboratories have partly been supported by CREST-JST
and NEDO projects. Collaboration with many outside organizations through these programs is helping
our activities. I would like to thank H. Takayanagi, T.
Fujisawa, H. Kamada, K. Shimizu, and Y. Tokura for
their discussions about quantum information processing including recent experimental results. I also
would like to thank M. Morita and S. Ishihara for
their encouragement throughout these studies.
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D. Bouwmeester, A. Ekert, and A. Zeilinger “The Physics of Quantum Information,” Springer, 2000.
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Yoshiro Hirayama
Executive Manager, Physical Science Laboratory, Group Leader, Quantum Solid State
Physics Research Group and Group Leader, Photonic Nanostructure Research Group, NTT Basic
Research Laboratories.
He received the B.E., M.E., and Ph.D. degrees
in electrical engineering from the University ofTokyo in 1978, 1980 and 1983, respectively. He
joined NTT Musashino Electrical Communications Laboratories in 1983. He was a guest scientist in Max-Planck-Institüt für Festkörperforschung, Stuttgart, Germany during 19901991. Since 1983 he has engaged in the study ofsemiconductor nanostructure fabrication,nanoscale characterization, and mesoscopictransport of semiconductor quantum systems.
His current interests are transport properties ofsemiconductor layer and nanostructures especially putting emphasis on carrier interaction
phenomena. From 1998 to 2001, he was a
research coordinator of NEDO international joint
research project (NTDP-98). He has also been acoordinator of a CREST research team for interacting carrier electronics since 1998. He is amember of the Japan society of Applied Physics,
the Physical Society of Japan, and IEEE.

NTT Technical Review