Continuous variables (CV) have become important across all facets of quantum information. From quantum sensing to quantum computing to quantum key distribution, the benefits of deterministic quantum state generation clearly make a compelling case to seek full CV-based quantum information solutions from top to bottom. Long range quantum networks have become of interest for their potential use in all three quantum information scenarios: second generation, distributed quantum sensors over quantum networks, multi-user QKD protocols across long range networks, and distributed quantum computing. However, a long range CV quantum network is impossible without overcoming two major roadblocks. First, to enable quantum state measurement or tomography at network nodes, coherent detection is required, which itself requires sending a powerful local oscillator across the network. Sending such a local oscillator across long distances presents a practical limitation: it cannot coexist on the same network infrastructure as with quantum signals. Second, very long range networks require robust quantum states and third-generation quantum repeaters, which themselves require a nonGaussian gate in the CV world. We will present our recent results on long range CV network generation made possible by feed forward phase recovery schemes for “locally” generated local oscillators. In addition, we will present our work on deterministic quantum network generation with highly accessible, cost effective, integrated sources of quantum entanglement. Finally, in order to enable all applications across true quantum networks, a non Gaussian quantum gate is required. We will outline our proposed cubic phase gate and our experimental progress towards achieving this goal.
Quantum Key Distribution (QKD) exploits the rules of quantum mechanics to generate and securely distribute a random
sequence of bits to two spatially separated clients. Typically a QKD system can support only a single pair of clients at a
time, and so a separate quantum link is required for every pair of users. We overcome this limitation with the design and
characterization of a multi-client entangled-photon QKD system with the capacity for up to 100 clients simultaneously.
The time-bin entangled QKD system includes a broadband down-conversion source with two unique features that enable
the multi-user capability. First, the photons are emitted across a very large portion of the telecom spectrum. Second, and
more importantly, the photons are strongly correlated in their energy degree of freedom. Using standard wavelength
division multiplexing (WDM) hardware, the photons can be routed to different parties on a quantum communication
network, while the strong spectral correlations ensure that each client is linked only to the client receiving the
conjugate wavelength. In this way, a single down-conversion source can support dozens of channels simultaneously--and
to the extent that the WDM hardware can send different spectral channels to different clients, the system can support
multiple client pairings. We will describe the design and characterization of the down-conversion source, as well as the
client stations, which must be tunable across the emission spectrum.
We present an experimental realization of a low-noise, phase-insensitive optical amplifier using a four-wave mixing
interaction in hot Rb vapor. Performance near the quantum limit for a range of amplifier gains, including near unity, can
be achieved. Such low-noise amplifiers are essential for so-called quantum cloning machines and are useful in quantum
information networks and protocols. We demonstrate that amplification and ''cloning'' of one half of a two-mode
squeezed state is possible while preserving entanglement. The inseparability criterion between the two original modes
remains satisfied for small to large gains, while the EPR criterion is satisfied for a smaller range. This amplification of
quantum correlations paves the way for optimal cloning of a bipartite entangled state.
Entangled multi-spatial-mode fields have interesting applications in quantum information, such as parallel quantum
information protocols, quantum computing, and quantum imaging. We study the use of a nondegenerate
four-wave mixing process in rubidium vapor at 795 nm to demonstrate generation of quantum-entangled images.
Owing to the lack of an optical resonator cavity, the four-wave mixing scheme generates inherently multi-spatialmode
output fields. We have verified the presence of entanglement between the multi-mode beams by analyzing
the amplitude difference and the phase sum noise using a dual homodyne detection scheme, measuring more
than 4 dB of squeezing in both cases. This paper will discuss the quantum properties of amplifiers based on
four-wave-mixing, along with the multi mode properties of such devices.