Transfer of an unknown quantum state, quantum networks, and memory
Asoka Biswas and G. S. Agarwal
Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India
(Received 13 May 2003; revised manuscript received 19 May 2004; published 27 August 2004
)
We present a protocol for transfer of an unknown quantum state. The protocol is based on a two-mode cavity
interacting dispersively in a sequential manner with three-level atoms in the ⌳ configuration. We propose a
scheme for quantum networking using an atomic channel. We investigate the effect of cavity decoherence in
the entire process. Further, we demonstrate the possibility of an efficient quantum memory for arbitrary
superposition of two modes of a cavity containing one photon.
DOI: 10.1103/PhysRevA.70.022323 PACS number(s): 03.67.Hk, 42.50.Pq
I. INTRODUCTION
In the quantum information theory [1], transfer of infor-
mation in the form of a coherently prepared quantum state is
essential. One can transfer a quantum state either by the
method of teleportation [2] or through quantum networking.
The basic idea behind a quantum network is to transfer a
quantum state from one node to another node with the help
of a career (a quantum channel) such that it arrives intact. In
between, one has to perform a process of quantum state
transfer (QST) to transfer the state from one node to the
career and again from the career to the destination node.
There have been some proposals [3] for quantum networking
using cavity-QED, where two atoms trapped inside two spa-
tially separated cavities serve the purpose of two nodes. In
Ref. [3], the task was to transfer the state of one atom into
the other via the process of QST between the atom and pho-
ton, where the latter is used as a career. The photon carries
the information through either free space or an optical fiber
between the cavities, and the success depends on the proba-
bilistic detection of photons or adiabatic passage through the
cavities. We note that, though it may be difficult to beat the
communication with photons, it is always interesting to ex-
plore the alternatives. In fact, very recently, quantum net-
work using a linear XY chain of N interacting qubits was
proposed. In this proposal, the quantum state can be trans-
ferred from the first qubit to the Nth qubit within micro-
scopic distance by preengineering interqubit interactions [4].
Further, storage of quantum states is also an important
issue. There have been several proposals for quantum
memory. For example, recent proposals [5,6] have shown
how to transfer the field state into atomic coherence by the
adiabatic technique and again retrieve the same through the
method of adiabatic following [5] or using the teleportation
technique [6]. Quantum memory of the individual polariza-
tion state into a collective atomic ensemble has been pro-
posed [7]. Initially, an entangled state of two pairs of atomic
ensembles is prepared, where the single-photon polarization
state is stored through a process similar to teleportation.
Though the information can be transferred back to the pho-
ton state, the protocol only succeeds with a probability 1/4.
Decoherence-free memory of one qubit in a pair of trapped
ions has also been experimentally demonstrated [8]. Maître
et al. [9] have proposed a quantum memory, where the quan-
tum information on the superposition state of a two-level
atom was stored in a cavity as a superposition of zero- and
one-photon Fock states. The holding time of such memories
is generally limited by the cavity decay time.
In this paper, we propose a scheme for QST to transfer the
unknown state of one atom to another atom where the atoms
are not directly interacting with each other. Note that by
direct spin interaction of the S
ជ
1
·S
ជ
2
kind, the quantum state
could be transferred from one atom to another within a mi-
croscopic range. In the present scheme, we show how a simi-
lar kind of interaction between two atoms can be mediated
via a cavity. Thus the atomic state can be transferred from
one atom to another in the mesoscopic range.
We extend our idea of QST to a quantum network, where
we transfer the state of one cavity to another spatially sepa-
rated cavity. For this we use long-lived atoms as career, and
make use of the QST process to transfer the state of the
cavity to an atom and again to the target cavity. Our protocol
for quantum networking provides a deterministic way to
transfer the quantum state between the cavities. This protocol
does not require any kind of probability arguments based on
the outcome of a measurement. Further, we propose the re-
alization of a quantum memory of arbitrary superposition of
two modes of a cavity which contains only one photon. This
superposition state can be stored in the long-lived states of
the neutral atoms and retrieved in another two-mode cavity
later, deterministically. Our proposal relies on the techno-
logical advances and realizations as described in Ref. [10].
The structure of the paper is as follows. In Sec. II, we
describe the model and provide the relevant equations. In
Sec. III, we discuss how transfer of an unknown quantum
state can be performed between two atoms. We provide an
estimate of possible decoherence in this process due to cavity
decay. In Sec. IV, we extend our scheme to quantum net-
works and quantum memory.
II. MODEL CONFIGURATION
To describe how the QST protocol works, we consider a
three-level atom in the ⌳ configuration interacting with a
two-mode cavity (see Fig. 1). The modes with annihilation
operators a and b interact with the 兩e典↔ 兩g典 and 兩e典↔兩f典
transitions, respectively. The Hamiltonian under the rotating
wave approximation can be written as
PHYSICAL REVIEW A 70, 022323 (2004)
1050-2947/2004/70(2)/022323(5)/$22.50 ©2004 The American Physical Society70 022323-1
H = ប关
eg
兩e典具e兩 +
fg
兩f典具f兩 +
1
a
†
a +
2
b
†
b + 兵g
1
兩e典具g兩a
+ g
2
兩e典具f兩b + H.c.其兴, 共1兲
where
lg
共l苸 e, f兲 is the atomic transition frequency,
i
共i
苸 1,2兲 is the frequency of the cavity modes a and b, and g
i
is the atom-cavity coupling constant. We assume g
i
to be
real.
We work under the two-photon resonance condition and
assume large single-photon detuning. After adiabatically
eliminating the excited level 兩e典 in the large detuning do-
main, we derive an effective Hamiltonian describing the sys-
tem of Fig. 1,
H
eff
=−
បg
2
⌬
关兩g典具g兩a
†
a + 兩f典具f兩b
†
b兴
−
បg
2
⌬
关兩g典具f兩a
†
b + 兩f典具g兩ab
†
兴, 共2兲
where ⌬ =
eg,f
−
1,2
is the common one-photon detuning of
the cavity modes and g
1
=g
2
=g共Ⰶ⌬兲. The condition g
1
=g
2
can be satisfied by proper choice as we can choose appropri-
ate transitions in atomic systems, frequencies, etc. Note that
if one considers the levels 兩g典 and 兩f典 as Zeeman sublevels,
then these conditions are automatically satisfied. In that case,
we may consider the two modes of the cavity as two or-
thogonal polarization states of a photon. Now note that the
first two terms in Eq. (2) represent the self-energy terms and
the last two terms give the interaction leading to a transition
from the initial state to the final state. The probability ampli-
tudes of relevant basis states 兩g典兩n,
典 and 兩f典兩n−1,
+1典 in
the state vector
兩
共t兲典 = d
g
共t兲兩g,n,
典 + d
f
共t兲兩f,n −1,
+1典共3兲
are given by
d
g
共t兲 =
冑
nXY
n +
+1
+ d
g
共0兲,
d
f
共t兲 =
冑
+1XY
n +
+1
+ d
f
共0兲, 共4兲
where X=
冑
nd
g
共0兲+
冑
+1d
f
共0兲, Y =exp关ig
2
共n+
+1兲t/⌬兴
−1, and n and
are the respective photon numbers in the
modes a and b. We note that the effective interaction (2) can
be seen as an interaction between two qubits defined via the
atomic variables and field variables
S
+
= 兩f典具g兩, S
−
= 兩g典具f兩, S
z
=
1
2
共兩f典具f兩 − 兩g典具g兩兲;
R
+
= a
†
b, R
−
= ab
†
, R
z
=
1
2
共a
†
a − b
†
b兲. 共5兲
In the single-photon space, the field operators R
±
, R
z
satisfy
spin-1/2 algebra and thus the interaction (2) can be written
as an interaction between two qubits,
H
eff
⬅ −
បg
2
⌬
共R
+
S
−
+ R
−
S
+
−2R
z
S
z
兲. 共6兲
In view of the above form of the effective interaction, we
conclude that our system of Fig. 1 can be used for a number
of quantum logic operations.
III. QUANTUM STATE TRANSFER PROTOCOL
We next demonstrate how the dynamics of an atom in a
two-mode cavity can be used to implement the QST proto-
col. Hereafter, we will use the term “
pulse” to denote an
equivalent traversal time T of the atom through the cavity
such that 2g
2
T/⌬=
. The time T could be controlled by
selecting the atomic velocity.
We assume that the atom A is initially in an unknown
state,
兩i典
A
=
␣
兩g典
A
+

兩f典
A
, 共7兲
where
␣
and

are unknown arbitrary coefficients. The state
兩i典
A
of atom A is to be transferred to another atom B which is
elsewhere. Preparing the cavity in a state 兩0,1典 (i.e., initially
one photon in the b mode), we send the atom A through the
cavity for a certain time which is equivalent to a
pulse.
After atom A comes out of the cavity, atom B in state
兩i
⬘
典 =
␣
⬘
兩g典 +

⬘
兩f典共8兲
is sent through the cavity. Here
␣
⬘
and

⬘
are arbitrary co-
efficients and need not be known. Atom B also experiences a
pulse during the interaction with the cavity. The entire
process can be described as follows:
兩i典
A
兩0,1典
↓
pulse on atom A
兩g典
A
共
␣
兩0,1典 −

兩1,0典兲
↓ B atom enters
兩g典
A
兩i
⬘
典
B
共
␣
兩0,1典 −

兩1,0典兲
↓
pulse on atom B
兩g典
A
兩i典
B
共
␣
⬘
兩0,1典 −

⬘
兩1,0典兲. 共9兲
FIG. 1. Three-level atomic configuration with levels 兩g典, 兩e典, and
兩f典 interacting with two orthogonal modes of the cavity, described
by operators a and b. Here g
1
and g
2
represent the atom-cavity
coupling of the a and b modes with the corresponding transitions
and ⌬ is the common one-photon detuning.
A. BISWAS AND G. S. AGARWAL PHYSICAL REVIEW A 70, 022323 (2004)
022323-2
If one prepares the cavity initially in state 兩1,0典, then
following a similar sequence to the above, the final state will
be −兩f典
A
兩i典
B
共
␣
⬘
兩0,1典−

⬘
兩1,0典兲. Note that atom B has already
acquired the state 兩i典 of atom A, i.e., the state 兩i典 is transferred
from atom A to atom B.
More generally, our QST protocol can be written as
兩i典
A
兩i
⬘
典
B
共
␥
兩0,1典 +
␦
兩1,0典兲
cav
→
U共
兲
共
␥
兩g典 −
␦
兩f典兲
A
兩i典
B
兩
典
cav
,
共10兲
where U共
兲=U
A
共
兲U
B
共
兲, U
k
共
兲 (k苸 A,B) 关=exp
兵−iH
eff
T/ប其兴 denotes the
-pulse operation on the atom k,
and
兩
典
cav
=
␣
⬘
兩0,1典
cav
−

⬘
兩1,0典
cav
. 共11兲
Our protocol has interesting features: (a) the initial states of
the atoms can be arbitrary, and (b) the field state can also be
an arbitrary superposition of 兩0,1典 and 兩1,0典. Note that in the
case of a two-level atom interacting with a resonant single-
mode cavity, the QST protocol from one atom to another
atom has difficulties associated with a relative phase which
can be changed either by using a conditional phase shift
which is essentially a two-qubit operation [see Eq. (3.8) of
Ref. [10]] or by applying a resonant microwave field to the
atomic qubit.
We note that if the initial state of the atom B is 兩g典 (or 兩f典)
and the cavity is initially in state 兩0,1典 (or 兩1,0典), then we
can not only transfer the state of atom A to B, but we also
can interchange the states between them. However, the QST
protocol described here cannot be interpreted as a SWAP
gate. As in the usual version of a quantum gate, atoms A and
B must interact with the field simultaneously. We also note
that, in the process of coherence transfer between two atoms
using, for example, the scheme of Ref. [11], the atoms must
be addressed by the pulses simultaneously, which is basically
a local interaction. In the present protocol, the atoms interact
with the
pulse in a sequential manner. This is essentially a
nonlocal process.
Extending the idea of QST described above to a number
of atoms, we can transfer the state of any atom to the con-
secutive atom. This means that if we consider a sequel of
atoms, then the state of any atom can be transferred to the
consecutive atom which will pass the cavity after the former
leaves the cavity. The procedure of transfer of atomic states
to consecutive atoms has been shown schematically in Fig. 2.
Here the atoms A, B, C, etc. are sent through another iden-
tical bimodal cavity in initial state 兩0,1典. After passing
through this cavity, atom C is again prepared in state 兩i典.
Thus, using a second cavity in this way, we can transfer the
state of the first atom A to a third atom C. Clearly, if we used
n number of cavities in this sequence, we could transfer the
state of atom A to the 共n+1兲th atom in the sequence.
Effects of decoherence: Fidelity of the QST protocol
Decoherence is a strong limiting factor in the realization
of any quantum computational protocol. The interaction of
the atom and the cavity with the environment causes them to
decay and results in decoherence. Thus, one has to consider
the effect of decoherence to examine with how much effi-
ciency the desired outcome can be produced. These calcula-
tions can be done in the density-matrix framework using the
following Liouville equation:
˙
=−
i
ប
关H
eff
,
兴 −
a
共a
†
a
−2a
a
†
+
a
†
a兲
−
b
共b
†
b
−2b
b
†
+
b
†
b兲, 共12兲
where
a
and
b
are the decay constants of the two modes
and H
eff
is given by Eq. (2).
In the present case, to investigate the effect of decoher-
ence, let us consider a possible scheme. We consider 兩g典 and
兩f典 to be the Rydberg levels as in Haroche’s experiments. In
that case, we can use a bimodal microwave cavity like the
one used by Haroche’s group. We use parameters similar to
those in the experiments by Haroche and his co-workers. If
the cavity coupling constant g is 2
⫻50 kHz and the cavity
decay constant
a
=
b
=
for each mode is 2
⫻100 Hz,
then
/g=0.002. Further, for ⌬ =10g, we calculate the cavity
interaction time to be 50
s for a
pulse, which is consis-
tent with the interaction time possible to achieve in a micro-
wave experiment. One sends the atoms with a velocity
⬃10
2
cm s
−1
through a few-cm-long cavity to achieve this
interaction time. Using these parameters, we calculate the
fidelity F that the first step of the evolution (9) occurs. The
variation of F共T兲 with the decay constant
is shown in Fig.
3(a), where T is the interaction time of the atom with the
cavity. Note that the probability that the state of atom A is
transferred to the cavity remains more than 90% for
=0.002g. We next show [see Fig. 3(b)] the variation of the
fidelity F共2T+
兲 of the entire process (9) to occur with the
time delay
between the atoms A and B for
=0.002g.Itis
clear that the probability that the atom B acquires the desired
state remains above 80% even at g
=20共⬅
⬇63
s兲.
IV. EXTENSIONS OF QUANTUM STATE
TRANSFER PROTOCOL
A. Quantum networks
Now we show how the above QST protocol can be made
useful in preparing a quantum network, in which long-lived
atomic states are used to communicate between the two
nodes of the network. We assume that there are two identical
two-mode cavities C
1
and C
2
, which are considered as two
nodes of the network. Let us consider that the cavity C
1
is
initially in a state 兩0,1典. To prepare this cavity in a superpo-
sition state,
FIG. 2. Schematic diagram for the QST protocol for a number of
atoms interacting with the two-mode cavity in a sequential manner
for a time T=⌬
/2g
2
.
TRANSFER OF AN UNKNOWN QUANTUM STATE,… PHYSICAL REVIEW A 70, 022323 (2004)
022323-3
兩E典
cav
=
␣
兩0,1典
cav
−

兩1,0典
cav
, 共13兲
we send an atom A in state 兩i典 through the cavity (see Fig. 4)
such that the atom A experiences a
pulse. Now our goal is
to transfer this cavity state 兩E典
cav
to the other node C
2
. For
that we send a second atom B through the cavity C
1
after A
comes out of it. We see that the atom B is prepared in state 兩i典
through the evolution (9). This atom is now sent through the
second node C
2
which is initially in state 兩0,1典. In this way,
the state 兩E典
cav
of node C
1
is transferred to the node C
2
.
Extending the above idea to a number of distant nodes
(cavities), we thus can transfer the state 兩E典
cav
from one node
to another node of the proposed quantum network via a
quantum channel (atom). For example, to send this state
兩E典
cav
from C
2
to another node (say, C
3
), we can send a third
atom C through these two nodes subsequently.
We emphasize that our protocol of quantum networking is
distinct from the teleportation protocol of Davidovich et al.
[12]. Their protocol depends on the Bell state measurements,
whereas in our protocol no Bell measurement is ever made.
We further note that the present scheme can be used to
spread entanglement between two distant cavities. For this,
one first sends an atom A in state 兩g典 through the first cavity
C
1
prepared initially in the state 兩1,0典 such that the atom
experiences a
/2 pulse 共2g
2
T/⌬=
/2兲. This would prepare
the atom and the cavity in the following entangled state:
兩⌿典
AC
1
=
1
冑
2
e
i
/2
共兩g典
A
兩1,0典
1
+ 兩f典
A
兩0,1典
1
兲. 共14兲
Next the atom passes through a second cavity C
2
initially in
the state 兩0,1典 and experiences a
pulse. Thus, at the end of
this process, the two cavities are prepared in an entangled
state of two modes as
兩⌿典
C
1
C
2
=
1
冑
2
e
i
/2
关兩1,0典
1
兩0,1典
2
− 兩0,1典
1
兩1,0典
2
兴. 共15兲
Clearly one can spread entanglement between the atom and
the cavity to another distant cavity. Note that in our proposal,
entanglement is created between the modes of the two dif-
ferent cavities. The entanglement between two modes of a
single cavity has been produced in [13].
B. Storage and retrieval of an arbitrary superposition state
of two modes of a cavity
We now discuss how the present
-pulse technique can be
used to prepare an efficient quantum memory for arbitrary
superposition of two cavity modes, where there is only one
photon present in either mode. Let us consider a two-mode
cavity which is in a superposition state of two modes [see
Eq. (13)],
兩E典
cav
=
␣
兩0,1典
cav
−

兩1,0典
cav
, 共16兲
where
␣
and

are known coefficients. Now we send an atom
in state (8) through the cavity. Applying a
pulse on it, we
can map the superposition of 兩E典
cav
into the state of the atom.
This procedure can be written as
兩i
⬘
典兩E典
cav
→ − 兩i典兩
典
cav
, 共17兲
where 兩i典=
␣
兩g典+

兩f典 and 兩
典
cav
is given by Eq. (11). Be-
cause, the states 兩g典 and 兩f典 of the atom are radiatively long-
lived, information about the state of the cavity can be stored
inside the atom for a sufficiently long time. To retrieve this
information into the cavity, we prepare a second cavity in
either of the states 兩0,1典 or 兩1,0典 and send the atom in state
兩i典 through the cavity. Upon applying a
pulse, the cavity
can again be prepared in the superposition state as before.
The retrieval of superposition can be shown as
FIG. 4. Schematic diagram for the quantum network between
distant cavities via the atomic channel. Description of the figure is
in the text.
FIG. 3. (a) Variation of the fidelity F共T兲 of mapping the state of
the atom A in the cavity C
1
with
/g. We have assumed that the
cavity decay rates are the same for both the modes and ⌬=10g. (b)
Variation of the fidelity F calculated at time 2T+
, with the time
delay
between the atoms for
=0.002g and ⌬=10g.
A. BISWAS AND G. S. AGARWAL PHYSICAL REVIEW A 70, 022323 (2004)
022323-4
兩i典兩0,1典
cav
→ 兩g典兩E典
cav
, 兩i典兩1,0典
cav
→ − 兩f典兩E典
cav
. 共18兲
We should mention here that the quantum memory pro-
posed here for the cavity state is expected to work better
since the information is being stored inside the long-lived
atomic states 兩g典 and 兩f典. However, the transfer time of the
cavity state to the atom is limited by the cavity holding time
and the atom must stop interacting with the cavity before it
decays. We also note that if the two modes are degenerate
and correspond to two states of circular polarizations, then
Eq. (16) can be viewed as a superposition of two polarization
states of a photon. In such a case, our proposal corresponds
to storage and retrieval of the polarization states of a photon.
V. CONCLUSION
In conclusion, we have presented a protocol for the trans-
fer of a quantum state from one atom to another atom. This
protocol can be extended to a number of atoms passing
through sequential cavities and thus one can set up a quan-
tum network. We have further shown how an efficient quan-
tum memory of arbitrary superposition of two cavity modes
can be built up. Our proposals have certain advantages as we
work with long-lived states of atoms. We provide a proper
estimate of the efficiency of the state transfer protocol
against cavity decoherence.
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