PROPOSAL OF A UNIVERSAL QUANTUM COPYING MACHINE IN CAVITY QED

PROPOSAL OF A UNIVERSAL QUANTUM COPYING MACHINE IN CAVITY QED • Joanna Gonzalez • Miguel Orszag • Sergio Dagach • Facultad de Física • Pontificia Universidad Católica de Chile Quantum Optics II COZUMEL-MEXICO

The No-Cloning Theorem (Wooters and Zurek,Nature299,802(1982)) showed that it is not possible to construct a device that will produce an exact copy of an arbitrary quantum state. This Theorem is an unexpected quantum effect due to the linearity of Quantum Mechanics, as opposed to Classical Physics, where the copying Process presents no difficulties, and this represents the most significant difference between Classical and Quantum Information. Thus, an operation like:

Thus, an operation like: Is not possible, with: =INPUT QUBIT =initial state of cloner =final state of cloner =Blank copy Because of this Theorem, scientist ignored the subject up to 1996 when Buzek and Hillery (V.Buzek,M.Hillery,Phys.Rev.A,54,1844(1996) proposed the Universal Quantum Copying Machine(UQCM)-that produced two imperfect copies from an original qubit, the quality of which was independent of the input state.

The quality of the copy is measured through the FIDELITY UNIVERSAL QUANTUM COPYING MACHINE BASIS

In the present work, we propose a protocol that produces 2 copies from an input state, with Fidelity In the context of Cavity QED, in which the information is encoded in the electronic levels of Rb atoms, that interact with two Nb high Q cavities. SOME PREVIOUS BACKGROUND TO THE PROPOSAL Consider a two level atom that is prepared in a superposition state , using the Microwave pulses in a Ramsey Zone, with frequency Near the e(excited)-g(ground) transition. It generates superpositions Depends on the interaction time Is prop. detuning

On the other hand, the atom-field interaction is described by the Jaynes Cummings Hamiltonian Coupling constant

The atom-field state evolves like For example, for

Now, consider an external Classical pulse, interacting with the atom We use the dressed state basis that diagonalizes the J-C Hamiltonian: , The Energies of the dressed states are

In the limit Consider the external field in resonance with the (+,1)-(-,0) Transition, that is Where f(t) is some smooth function of time to represent the pulse shape, with(in the dispersive case)

The above Hamiltonian has been studied by several authors (Domokos et al;Giovannetti et al) and arrive to the conclusion that For a suitable pulse, a C-NOT gate can be achieved, where the photon Number (0 or 1) is the control and the atom the target The mechanism of the above C-NOT gate that forbids, for example the (g,0>--(e,0> transition is the Stark Effect, caused by one photon in the cavity. In order to resolve these two transitions, we have to make sure that

Where Is the frequency difference between these two transitions.

The exchange IS POSSIBLE

C-NOT GATE N=0 ATOMIC STATE IS NOT CHANGED N=1 ATOMIC STATE IS EXCHANGED CONTROL TARGET

UQCM PROPOSED PROTOCOL

ATOM 1 A1, initially at is prepared in a superposition, via a Ramsey Field A1 interacts with the cavity Ca(initially in )through a Rotation, so State swapping.The excitation of atom 1 is transferred to the cavity a

ATOM 2 IT CONTAINS THE INFORMATION TO BE CLONED This state can be prepared in the same fashion as the atom 1, for example with a Ramsey Field. Then we apply a Classical pulse, as described before, generating a C-NOT gate ,nothing happens with 0 photons C-not

A3 and A4 are the atoms carrying the two copies(IDENTICAL)

FINAL STATE

DISCUSSION Experimental numbers(Haroche et al) An interaction time of Marginally satisfies the earlier requirement. The whole scheme should With the flight time of 100 Take about 700 Which is reasonable in a cavity with a Relaxation time of 16ms.They achieved a resolution required to Distinguish between 1 or 0 photons

DISCUSSION OF THE C-NOT GATE The complete Hamiltonian in the Interaction Picture is: Since the external pulse is resonant with the (+,1)-(-,0) Transition,this imposes a condition on

Also, we notice that we have introduced exponential factors In both terms of the Hamiltonian just to mimic the passage Time and duration of the pulse, referred to as and Respectively. We have done this in order to solve Schrodingers equation With continuous functions.

Assuming We have to solve the following set of differential equations

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PROPOSAL OF A UNIVERSAL QUANTUM COPYING MACHINE IN CAVITY QED