1,866 research outputs found
Minimally complex ion traps as modules for quantum communication and computing
Optically linked ion traps are promising as components of network-based
quantum technologies, including communication systems and modular computers.
Experimental results achieved to date indicate that the fidelity of operations
within each ion trap module will be far higher than the fidelity of operations
involving the links; fortunately internal storage and processing can
effectively upgrade the links through the process of purification. Here we
perform the most detailed analysis to date on this purification task, using a
protocol which is balanced to maximise fidelity while minimising the device
complexity and the time cost of the process. Moreover we 'compile down' the
quantum circuit to device-level operations including cooling and shutting
events. We find that a linear trap with only five ions (two of one species,
three of another) can support our protocol while incorporating desirable
features such as 'global control', i.e. laser control pulses need only target
an entire zone rather than differentiating one ion from its neighbour. To
evaluate the capabilities of such a module we consider its use both as a
universal communications node for quantum key distribution, and as the basic
repeating unit of a quantum computer. For the latter case we evaluate the
threshold for fault tolerant quantum computing using the surface code, finding
acceptable fidelities for the 'raw' entangling link as low as 83% (or under 75%
if an additional ion is available).Comment: 15 pages, 8 figure
Optimal approach to quantum communication using dynamic programming
Reliable preparation of entanglement between distant systems is an
outstanding problem in quantum information science and quantum communication.
In practice, this has to be accomplished via noisy channels (such as optical
fibers) that generally result in exponential attenuation of quantum signals at
large distances. A special class of quantum error correction protocols--quantum
repeater protocols--can be used to overcome such losses. In this work, we
introduce a method for systematically optimizing existing protocols and
developing new, more efficient protocols. Our approach makes use of a dynamic
programming-based searching algorithm, the complexity of which scales only
polynomially with the communication distance, letting us efficiently determine
near-optimal solutions. We find significant improvements in both the speed and
the final state fidelity for preparing long distance entangled states.Comment: 9 pages, 6 figure
Universal blind quantum computation
We present a protocol which allows a client to have a server carry out a
quantum computation for her such that the client's inputs, outputs and
computation remain perfectly private, and where she does not require any
quantum computational power or memory. The client only needs to be able to
prepare single qubits randomly chosen from a finite set and send them to the
server, who has the balance of the required quantum computational resources.
Our protocol is interactive: after the initial preparation of quantum states,
the client and server use two-way classical communication which enables the
client to drive the computation, giving single-qubit measurement instructions
to the server, depending on previous measurement outcomes. Our protocol works
for inputs and outputs that are either classical or quantum. We give an
authentication protocol that allows the client to detect an interfering server;
our scheme can also be made fault-tolerant.
We also generalize our result to the setting of a purely classical client who
communicates classically with two non-communicating entangled servers, in order
to perform a blind quantum computation. By incorporating the authentication
protocol, we show that any problem in BQP has an entangled two-prover
interactive proof with a purely classical verifier.
Our protocol is the first universal scheme which detects a cheating server,
as well as the first protocol which does not require any quantum computation
whatsoever on the client's side. The novelty of our approach is in using the
unique features of measurement-based quantum computing which allows us to
clearly distinguish between the quantum and classical aspects of a quantum
computation.Comment: 20 pages, 7 figures. This version contains detailed proofs of
authentication and fault tolerance. It also contains protocols for quantum
inputs and outputs and appendices not available in the published versio
Passing quantum correlations to qubits using any two-mode state
We draw an explicit connection between the statistical properties of an
entangled two-mode continuous variable (CV) resource and the amount of
entanglement that can be dynamically transferred to a pair of non-interacting
two-level systems. More specifically, we rigorously reformulate entanglement
transfer process by making use of covariance matrix formalism. When the
resource state is Gaussian, our method makes the approach to the transfer of
quantum correlations much more flexible than in previously considered schemes
and allows the straightforward inclusion of the effects of noise affecting the
CV system. Moreover, the proposed method reveals that the use of de-Gaussified
two-mode states is almost never advantageous for transferring entanglement with
respect to the full Gaussian picture, despite the entanglement in the
non-Gaussian resource can be much larger than in its Gaussian counterpart. We
can thus conclude that the entanglement-transfer map overthrows the "ordering"
relations valid at the level of CV resource states.Comment: 10 pages, 6 figures, RevTeX4, Accepted for publication in Physical
Review
Probabilistic Super Dense Coding
We explore the possibility of performing super dense coding with
non-maximally entangled states as a resource. Using this we find that one can
send two classical bits in a probabilistic manner by sending a qubit. We
generalize our scheme to higher dimensions and show that one can communicate
2log_2 d classical bits by sending a d-dimensional quantum state with a certain
probability of success. The success probability in super dense coding is
related to the success probability of distinguishing non-orthogonal states. The
optimal average success probabilities are explicitly calculated. We consider
the possibility of sending 2 log_2 d classical bits with a shared resource of a
higher dimensional entangled state (D X D, D > d). It is found that more
entanglement does not necessarily lead to higher success probability. This also
answers the question as to why we need log_2 d ebits to send 2 log_2 d
classical bits in a deterministic fashion.Comment: Latex file, no figures, 11 pages, Discussion changed in Section
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