Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2001-12-11
2004-09-07
Jackson, Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S027000, C257S040000, C438S099000
Reexamination Certificate
active
06787794
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a quantum computer.
BACKGROUND ART
Quantum computation involves manipulation of data in the form of quantum bits or “qubits”. Whereas in classical computation a bit of information is used to represent only one of two possible logical states, namely “1” or “0”, in quantum computation, a qubit can represent both logical states simultaneously as a superposition of quantum states. This property gives rise to powerful computational parallelism. Algorithms which exploit this parallelism have been developed, for example for efficiently factorising large integers. An overview of quantum computation is found “Quantum Computation” by David Deutsh and Artur Ekert in Physics World, pp. 47-52, March 1998 and in “Quantum Computation: An Introduction” by Adriano Barenco, pp. 143-183 of “Introduction to Quantum Computation and Information” ed. Hoi-Kwong Lo, Tim Spiller and Sandu Popescu (World Scientific Publishing, 1998).
In a classical computer, a bit of information is usually represented by a voltage level. Therefore, “0” can be represented by a relatively low voltage level, say 0 volts, and “1” can be characterised by a relatively high voltage level, say 5 volts.
In a quantum computer, a qubit can be represented in a number of ways, for example using left and right polarisation states of a photon, spin-up and spin-down states of an electron and ground and excited states of quantum dot. The qubit is defined by a basis consisting of two states, which are denoted |0> and |1>. Thus, the state of the qubit can be represented as:
|&psgr;>=
a
|0
>+b
|1> (1)
where a and b are complex number coefficients. The qubit stores information as a combination of 0 and 1 using different values of a and b. However, a measurement of the qubit will cause it to project onto the |0> or |1> state and return the result 0 or 1 accordingly. The probabilities of returning these values are |a|
2
and |b|
2
respectively. In this way, the system comprised of one qubit can store two binary values, 0 and 1, at the same time, although recovery of any stored information is restricted.
A system comprised of two qubits can store up to four binary values simultaneously as a result of superposition. Therefore, a system comprising a pair of qubits, labelled A and B, is defined by a basis of four states which can be written as |0>
A
|0>
B
, |0>
A
|1>
B
, |1>
A
|0>
B
and |1>
A
|1>
B
. In the same way that a single qubit can store information as superposition of |0> and |1>, a pair of qubits can store information as superposition of basis states |0>
A
|0>
B
, |0>
A
|1>
B
, |1>
A
|0>
B
and |1>
A
|1>
B
. For example, the two qubits may be prepared such that:
|&psgr;>
AB
=2
−½
(|0>
A
|0>
B
+|0>
A
|1>
B
+|1>
A
|0>
B
+|1>
A
|1>
B
) (2)
Thus, four binary values 00, 01, 10 and 11 are encoded simultaneously. In this case, the two qubits exist independently of one another, such that the result of a measurement qubit A is independent of the result of a measurement of qubit B.
However, if the two qubits are entangled, then the two measurements will become correlated. Entanglement allows qubits to be prepared such that:
|&psgr;>
AB
=2
−½
(|0>
A
|0>
B
+|1>
A
|1>
B
) (3)
Thus, binary values 00 and 11 are encoded simultaneously. However, if qubit A is measured and a result 0 is returned, then the outcome of a subsequent measurement of qubit B will, with certainty, also be 0.
A system comprised of three qubits is defined by basis of eight states which can store eight binary numbers, 000, 001, . . . , 111 simultaneously.
In general, a system of m qubits has a basis of 2
m
states and can be used to represent numbers from 0 to 2
m
−1. Thus, a quantum computer has a clear advantage over its classical counterpart in that it that it can store 2
m
numbers simultaneously, whereas a classical computer with an m-bit input register can only store one of these numbers at a time.
It is the ability to store many numbers simultaneously using superposition of quantum states which makes quantum parallel processing possible. Using a single computational step it is possible to perform the same mathematical operation on 2
m
different numbers at the same time and produce a superposition of corresponding output states. To achieve the same result in a classical computer, the computational step would need to be repeated 2
m
times or require 2
m
different processors.
Despite the power of quantum parallel processing, there is a drawback that only one state can be measured. However, some processes, such as sorting or searching of a database, may require only a single-valued solution. Thus, a system in which a mathematical operation has been performed on a plurality of numbers simultaneously may still benefit from parallelism provided that the desired value is the most probable outcome when the system is measured. An example of a quantum algorithm which operates in this way is described in “A Fast Quantum Mechanical Algorithm for Database Search” by Lov Grover, pp. 212-219, Proceedings of the 28
th
Annual ACM Symposium on the Theory of Computing (Philadelphia, May 1996).
Ideally, the qubits in the quantum computer should be identical, while also being individually tuneable in energy. Several systems have been proposed which fulfil the requirements of having identical qubits which are individually controllable. For example, experimental quantum computers have been implemented using atomic beams, trapped ions and bulk nuclear magnetic resonance. Examples of these systems are described in “Quantum computers, Error-Correction and Networking: Quantum Optical approaches” by Thomas Pellizari, pp. 270-310 and “Quantum Computation with Nuclear Magnetic Resonance” by Isaac Chuang pp. 311-339 of “Introduction to Quantum Computation and Information” ibid. However, these systems are difficult to fabricate and have the added disadvantage that their architecture cannot be easily scaled-up to accommodate a large number of qubits, i.e. more than about 10 qubits.
Quantum computers may also be implemented using solid-state systems employing semiconductor nanostructures and/or Josephson junctions. One such device is described in “Coherent control of macroscopic quantum states in a single-Cooper-pair box” by Y. Nakamura, Yu. A. Pashkin and J. S. Tsai, Nature, volume 398, p 786 (1999). Another device is described in our EP application 01304745.1. The advantage of such solid state systems is that they are better suited to being scaled and so provide quantum computers of practical utility. However, in semiconductor-based systems, the qubits are individually fabricated using lithographic methods. As a result, the qubits are slightly different from one another, even though they are intended to be identical.
The present invention seeks to provide an improved quantum computer. The present invention also seeks to provide a quantum computer in which the qubits are substantially identical to one another and easy to fabricate.
SUMMARY OF THE INVENTION
According to the present invention there is provided a quantum computer having at least one qubit comprising a system which exhibits first and second eigenstates, said system being one of a plurality of substantially identical systems and a structure for moveably anchoring said system to a predetermined position.
The system may occur naturally and may comprise a molecule. The molecule may be pyramidal, such as ammonia or cyanamide.
The structure for anchoring the system to the predetermined position may comprise a cage for the system, such as an endohedral molecule. The en
Cain Paul
Ferguson Andrew
Williams David
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Jackson Jerome
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