38,814 research outputs found
Coherent quantum state storage and transfer between two phase qubits via a resonant cavity
A network of quantum-mechanical systems showing long lived phase coherence of
its quantum states could be used for processing quantum information. As with
classical information processing, a quantum processor requires information bits
(qubits) that can be independently addressed and read out, long-term memory
elements to store arbitrary quantum states, and the ability to transfer quantum
information through a coherent communication bus accessible to a large number
of qubits. Superconducting qubits made with scalable microfabrication
techniques are a promising candidate for the realization of a large scale
quantum information processor. Although these systems have successfully passed
tests of coherent coupling for up to four qubits, communication of individual
quantum states between qubits via a quantum bus has not yet been demonstrated.
Here, we perform an experiment demonstrating the ability to coherently transfer
quantum states between two superconducting Josephson phase qubits through a
rudimentary quantum bus formed by a single, on chip, superconducting
transmission line resonant cavity of length 7 mm. After preparing an initial
quantum state with the first qubit, this quantum information is transferred and
stored as a nonclassical photon state of the resonant cavity, then retrieved at
a later time by the second qubit connected to the opposite end of the cavity.
Beyond simple communication, these results suggest that a high quality factor
superconducting cavity could also function as a long term memory element. The
basic architecture presented here is scalable, offering the possibility for the
coherent communication between a large number of superconducting qubits.Comment: 17 pages, 4 figures (to appear in Nature
Quantum receiver beyond the standard quantum limit of coherent optical communication
The most efficient modern optical communication is known as coherent
communication and its standard quantum limit (SQL) is almost reachable with
current technology. Though it has been predicted for a long time that this SQL
could be overcome via quantum mechanically optimized receivers, such a
performance has not been experimentally realized so far. Here we demonstrate
the first unconditional evidence surpassing the SQL of coherent optical
communication. We implement a quantum receiver with a simple linear optics
configuration and achieve more than 90% of the total detection efficiency of
the system. Such an efficient quantum receiver will provide a new way of
extending the distance of amplification-free channels, as well as of realizing
quantum information protocols based on coherent states and the loophole-free
test of quantum mechanics.Comment: 5 pages, 3 figure
Continuous variable quantum key distribution with two-mode squeezed states
Quantum key distribution (QKD) enables two remote parties to grow a shared
key which they can use for unconditionally secure communication [1]. The
applicable distance of a QKD protocol depends on the loss and the excess noise
of the connecting quantum channel [2-10]. Several QKD schemes based on coherent
states and continuous variable (CV) measurements are resilient to high loss in
the channel, but strongly affected by small amounts of channel excess noise
[2-6]. Here we propose and experimentally address a CV QKD protocol which uses
fragile squeezed states combined with a large coherent modulation to greatly
enhance the robustness to channel noise. As a proof of principle we
experimentally demonstrate that the resulting QKD protocol can tolerate more
noise than the benchmark set by the ideal CV coherent state protocol. Our
scheme represents a very promising avenue for extending the distance for which
secure communication is possible.Comment: 8 pages, 5 figure
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