152 CHINESE OPTICS LETTERS / Vol. 7, No. 2 / February 10, 2009
Multi-object quantum traveling ballot scheme
Yuan Li (
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∗
and Guihua Zeng (
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Laboratory of Coding and Communication Security, Department of Electronic Engineering,
Shanghai Jiao Tong University, Shanghai 200240
∗
E-mail: yuanli@sjtu.edu.cn
Received April 3, 2008
Based on quantum mechanics, a traveling ballot scheme with anonymity and secrecy is introduced to realize
voting. By searching th e objects in large amount of data bases, every voter may cast votes to his desired
candidates. Therefore, the proposed scheme may be applied to voting with a great deal of candidates, such
as network voting and so on. The security analysis of the present scheme is also performed.
OCIS codes: 060.0060, 260.0260, 270.0270.
doi: 10.3788/COL20090702.0152.
Quantum mechanics provides novel features to infor-
mation processing, extending the capabilities beyond
those classical applications only. As a resource in qua n-
tum communication, quantum entanglement is valu-
able for accomplishing many quantum computation
and other applications
[1,2]
. The most prominent re-
searches are investigated in some fields such as quantum
computation
[3,4]
, quantum teleportation
[5,6]
, and quan-
tum cryptography
[7]
, with the latter being the most ma-
ture experimentally. Based on the creation of a multipar-
tite entangled state in quantum computation, the quan-
tum gate tha t lies beyond the capabilities of linear optics,
can be implemented practically
[8]
. Recently, bec ause the
Heisenberg spin systems are natural candidates for simu-
lating the interactions between qubits, the entanglement
in Heisenberg spin chain has been studed.
Motivated by these development in quantum entangle-
ment, we investigate its application in quantum voting
with the relation between entanglement and quantum
phase transition. In some situations, ballot protocol is
always effective that peo ple want to vote their desired
objects among a great deal of candidates such as the elec-
tronic balloting or to select more prevalent books in net-
work. Reliable voting pr otocol should hence be a private,
secure, and verifiable scheme
[9,10]
. In quantum informa-
tion, the security and anonymity in quantum ballot pro-
tocol are based on the quantum mechanics. It requires
that the voters need quantum resources for a remote state
preparation to r e alize their votes and the resources can
be applied into practice such as network voting with the
development of quantum network
[11]
. Generally, two au-
thorities are addressed, one is called agent who prepares
ballot states and the other is called tallyman who counts
the votes. To the vote, ea ch party has to make a choice
between yes or no. After a ll votes have been made, the
vote tally can be determined by a c ollective measurement
of tallyman and read directly from the computatio n basis
states.
In this letter, we describe a system of quantum anony-
mous traveling voting protocol. One authority prepa res
an entangled state for the secrecy of ballots. After get-
ting the list of all candidates from the agent, voters first
search their desired objects if there exist a great deal of
ones to choose. Every voter may cast votes to his suit-
able objects which are mayb e more than o ne. Resorting
to the quantum search algorithm
[4]
, voters may find them
quickly.
In the voting scheme, assume there are K voters
V
1
, ··· , V
K
, N ballot objects B
0
, ··· , B
N−1
, and two au-
thorities, i.e., an a gent and a tallyman. In the anony-
mous traveling ballot scheme, the main idea is that the
voters cast their votes orderly with a traveling state. L e t
N
2
-dimension space H = H
V
⊗ H
T
be Hilbert space,
where H
V
and H
T
are N -dimension subspaces. Assume
{|0i, ··· , |N − 1i} is a set of computational orthonormal
basis s tates, i.e., hi|ji = δ
ij
, i, j ∈ {0, 1 , ··· , N − 1}. In
space H, the agent firstly prepares an entangled system
pair (p
v
, p
t
) which is expressed as
|Ai =
1
√
N
(
N−1
X
n=0
|n, N − n − 1i) = U |Ai
V
⊗ |Ai
T
, (1)
where p
v
∈ H
V
, p
t
∈ H
T
, |n, N − n − 1i = |ni
V
⊗ |N −
n − 1i
T
, and U is an entanglement generatio n op e rator.
Furthermore, subscript V is voting site and T is author-
ity site. The agent then sends p
v
to the first voter V
1
.
Having re c e ived the system sent from the agent, if V
1
does not cast any one of these candida tes , he sends p
v
to the next voter. Otherwise, V
1
will research his de-
sired candidate for casting vote. Let τ be the suitable
candidate whom he wants to vote. In terms of gener-
alized Grover algorithm
[3,4]
, |Ai
V
may be expressed as
|Ai
V
= sin φ|αi + cos φ|βi, where φ = arcsin(
1
√
N
), and
|αi = |τi, |βi =
1
√
N
X
n6=τ
|ni. (2)
Employing a sea rching operator Q on the s tate |Ai
V
for
times o f r = round(
π
2
√
N), V
1
may obtain the desired
state |τi with the possibility near to 1. For ascertaining
whether the found element is state |τi, V
1
may resort to
an ancilla state |qi in a register R
(1)
which is held by him-
self. With a Boolean function g(x) : {0, ··· , N − 1} →
{0, 1}, extremely V
1
can obtain his desired sta te. As fol-
lowing, he will cast his vote to the candidate τ. Denote an
phase shifting op e rator acted by V
k
as M
(k)
n
= exp(iθ
n
),
where 1 < k < K, 0 < n < N, θ
n
= 2nπ/N. The state
|τi after V
1
casting his vote becomes
|V
1
i = M
(1)
τ
|τi = exp(iθ
τ
)|τi. (3)
1671-7694/2009/020152-04
c
2009 Chinese Optics Letters