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Quantum 2.0 Conference and Exhibition

Maximally Entangled State Generation Between Atomic Ensembles Using Adaptive Quantum Nondemolition Measurements

Manish Chaudhary, Tim Byrnes

Published Jan 1, 2023• 0 citations
Publisher

Abstract

We develop a scheme to prepare a macroscopic maximally entangled state (MMES) between two atomic ensembles. The quantum state of system is evolved using a sequence of quantum nondemolition (QND) measurements followed by adaptive unitaries in z and x spin basis alternately. This procedure is repeated such that the state converges deterministically towards the MMES.

DOI
10.1364/quantum.2023.qtu3a.5

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We develop a scheme to prepare a macroscopic maximally entangled state (MMES) between two atomic ensembles. The quantum state of system is evolved using a sequence of quantum nondemolition (QND) measurements followed by adaptive unitaries in z and x spin basis alternately. This procedure is repeated such that the state converges deterministically towards the MMES.
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We develop a scheme to prepare a macroscopic maximally entangled state between two atomic ensembles using adaptive quantum nondemolition (QND) measurements. The quantum state of the system is evolved using a sequence of QND measurements followed by adaptive unitaries such that the desired measurement outcome is obtained with asymptotically unit probability. This procedure is repeated in zzz- and xxx-spin bases alternately such that the state converges deterministically towards the maximally entangled state. Up to a local spin-basis rotation, the maximally entangled state has zero total spin angular momentum, i.e., it is a singlet state. Our protocol does not perform postselection and works beyond the Holstein-Primakoff regime for the atomic spin degrees of freedom, producing genuine macroscopic entanglement.
We develop a measurement operator formalism to handle quantum nondemolition (QND) measurement induced entanglement generation between two atomic gases. We first derive how the QND entangling scheme reduces to a positive operator-valued measure, and consider its limiting case when it can be used to construct a projection operator that collapses the state to a total spin projection state. We then analyze how a stroboscopic sequence of such projections made in the xxx and zzz basis evolves the initial wave function. Such a sequence of QND projections can enhance the entanglement between the atomic ensembles and makes the state converge towards a highly entangled state. We show several mathematical identities which greatly simplify the state evolution in the projection sequence and allow one to derive the exact state in a highly efficient manner. Our formalism does not use the Holstein-Primakoff approximation as is conventionally done, and treats the spins of the atomic gases in an exact way.
We develop a measurement operator formalism to handle quantum nondemolition (QND) measurement induced entanglement generation between two atomic gases. We first derive how the QND entangling scheme reduces to a positive operator valued measure (POVM), and consider its limiting case when it is a projection operator that collapses the state to a total spin projection state. We then analyze how a stroboscopic sequence of such projections made in the x and z basis evolves the initial wavefunction. We show that for this projection sequence there exists a unique basis where the state evolution can be simplified. This allows one to derive the exact state evolution in a highly efficient manner. Our formalism does not use the Holstein-Primakoff approximation as is conventionally done, and treats the spins of the atomic gases in an exact way.
We propose a probabilistic scheme to prepare a maximally entangled state between a pair of two-level atoms coupled to a leaking cavity mode in circuit QED, without requiring precise time control of the system evolution and initial atomic state. We show that the steady state of this dissipative system is a mixture of two parts, where the atoms are either in their ground state or in a maximally entangled one. Then, by applying a weak probe field to the cavity mode, we are able to discriminate those states without disturbing the atomic system, i.e., to perform a quantum nondemolition measurement via the cavity transmission. In this scheme, one has maximum cavity transmission only when the atomic system is in an entangled state, so that a single click in the detector is enough to be sure that the atoms are in a maximally entangled state. Our scheme relies on an interference effect as it happens in an electromagnetically induced transparency phenomenon so that it works even in the limit of a cavity decay rate much stronger than the atom-field coupling.